Characterisation of engine oil

Abstract

Engine oil is configured to lubricate the combustion engine. A characterisation of evolution of engine oil during operation of a combustion engine is provided. The characterisation comprises variation between spectral analyses of more than one sample of engine oil. Each sample has been exposed to one of a plurality of operational conditions of the combustion engine. The variation between the spectral analyses of the more than one sample of engine oil are obtained by using multi-dimensional analysis to interrogate variation between the spectral analyses of the more than one sample. Spectral analyses of each sample are obtained by using spectrometry to analyse elemental composition and / or chemical bonds of the more than one sample of engine oil.

Description

[0001]CHARACTERISATION OF ENGINE OIL Field of the Disclosure The disclosure relates to the field of engine oil and engine health monitoring. Background It is known to use engine oil to lubricate combustion engines, protecting and prolonging the life of engine components. The engine oil may optionally perform additional functions, particularly in an event that additives are added to the engine oil. Functions of the engine oil may include cleaning (using detergents added to the engine oil), cooling, inhibiting corrosion, neutralising acids from combustion, and so on. The engine oil performance for both lubrication and any other functions evolves over time. In many instances, as it evolves the engine oil performance is acceptable for successful engine operation. However, if the engine oil is utilised for too long, the performance of the engine oil may deteriorate, possibly to the point that damage to the engine may occur. To overcome this, it is known to periodically change the engine oil. The time interval between engine oil changes may be called the service interval or oil change interval. It is also possible that the engine oil performance evolution is such that undesirable engine oil performance characteristics may develop at shorter time scales than the engine oil change interval. Oil sample analysis may be used to characterise some changes to the oil throughout the service period. Conventionally, the engine oil is analysed using analytical chemistry techniques. These techniques may include spectroscopy, such as Fourier-transform infrared (FTIR) spectroscopy, inductively coupled plasma atomic emission spectroscopy (ICP-AES) or Raman spectroscopy. The techniques may include Gas Chromatography, Gel permeation chromatography (GPC), or other performance characterisations such as viscometry. For example, in the case of FTIR spectroscopy the resulting spectrum is integrated over broad wavelength ranges to calculate values indicative of oxidation, nitration and sulfation of the engine oil. These values provide some indication of broad changes to the engine oil. Summary of the Disclosure Against this background, there is provided: a method of characterising evolution of engine oil during operation of a combustion engine, wherein the engine oil is configured to lubricate the combustion engine, the method comprising: obtaining more than one sample of engine oil, wherein each sample of engine oil has been exposed to one of a plurality of operational conditions of the combustion engine; using spectrometry to analyse elemental composition and / or chemical bonds of the more than one sample of engine oil to provide a spectral analysis of each sample; and using multi-dimensional analysis to interrogate variation between the spectral analyses of the more than one sample, wherein the variation characterises an evolution pathway of the engine oil. In this way, a particular change or changes to a spectrum of engine oil may be identified used to characterise evolution of engine oil. The engine oil may follow a different evolution pathway depending on how it is used; in other words, the engine oil may follow a different evolution pathway depending on the operational conditions to which it is exposed. The variation in the spectral analyses of different samples exposed to different operational conditions may provide a fingerprint of how the engine oil evolves for those operational conditions. An expected, or normal, engine oil evolution may be determined for normal operational conditions of an engine. Abnormal engine oil evolution may also be determined, for operational conditions of an engine experiencing a failure mode or other issue. By carrying out this method for different operational conditions, engine oil evolutions may be determined for different operational conditions. In particular, evolution of bulk engine oil that has evolved at more than one local interface may be determined by carrying out this method for more than one local operational condition. Advantageously, an expected engine oil evolution allows an actual engine oil evolution to be tracked, wherein any deviations from the expected engine oil evolution are indicative of abnormal engine oil evolution. Abnormal engine oil evolution allows for any deviation from an expected engine oil evolution to be identified, and linked to local engine states. In this way, whether the engine or engine oil requires maintenance may be determined before any issues result in damage or downtime. Chemical and physical properties of engine oil are a function of engine operating conditions. Engine operating conditions may, in turn, be a function of engine health. Advantageously, engine oil evolution may be used to identify health of the engine, a system of the engine, a subsystem of the engine, a component of the engine, or the engine oil itself. By obtaining an expected engine oil evolution for particular engine operational conditions by the method outlined herein, any deviation of actual engine oil evolution from expected engine oil evolution may be identified. A deviation of actual engine oil evolution may be linked to a particular local engine state or local engine operational condition, using the results of the method described herein. Any issue may, therefore, be precisely identified and any maintenance of the engine or the engine oil may be directed to a particular area of the engine. In an example, the method may comprise associating an operational condition to an evolution of the engine oil, such that means that analysis of a sample of engine oil may be compared to the characterisation used to identify health of at least one of the engine, a system of the engine, a subsystem of the engine, a component of the engine, or the engine oil itself. In another example, the method may not comprise associating an operational condition to an evolution of the engine oil, and in use analysis of at least one sample of engine oil may be correlated to the characterisation to identify health of at least one of the engine, a system of the engine, a subsystem of the engine, a component of the engine, or the engine oil itself. An operational condition may be a local operational condition of a component, sub-system or system of the engine. A fingerprint of that local operational condition isolates how the engine oil evolves when exposed to that local operational condition. In other words, an evolution pathway for the engine oil is isolated. Bulk engine oil may have been exposed to various local operational conditions. A fingerprint of a local operational condition may be identified in a spectrum or spectra of a sample of bulk engine oil, indicating that the bulk engine oil contains engine oil that has passed through the local operational condition. The method may be carried out to provide a characterisation of evolution of engine oil that has been exposed to a plurality of different operational conditions, each having its own fingerprint of changes to a spectrum. As a result, a spectrum of a sample of bulk engine oil may be scrutinised to identify more than one fingerprint, each resulting from a different local operational condition. From a sample of bulk oil, operation of a combustion engine may therefore be determined. Operation of the combustion engine may include how the engine has been used and the health of the engine. The expected engine oil evolution may comprise vectors of change, wherein the way in which the engine oil changes provides information as to the engine operation. By monitoring the way in which the engine oil changes, rather than only monitoring whether a parameter of the engine oil exceeds a limit, any change to the engine operation may be detected more quickly. The vectors of change may comprise multivariable vectors. Spectral analysis may be carried out on engine oil samples after they have been exposed to various different operational conditions, wherein exposing the engine oil samples to various different operational conditions may be achieved on or off engine. The spectral analysis shows how the engine oil has evolved, so the spectral analysis allows an engine oil evolution to be developed as a function of engine operation. In this way, engine oil evolution may be linked to operation of a combustion engine. The characterisation may include operational conditions, or the characterisation may not include operational conditions. In use, even if the characterisation does not include the operational conditions, the characterisation may be used to link engine oil evolution to operation of a combustion engine, for example via correlation of the characterisation with data collected from engine oil samples from combustion engines. In an event that the characterisation includes operational conditions, the association of variations in spectral analysis with operational conditions may be included in the characterisation of the engine oil evolution (for example by recording regions of interest associated with an operational condition to which a particular sample was exposed), or by correlation with data from other sources. Separately, engine oil evolution may be linked to engine oil performance. Advantageously, multi-dimensional analysis of spectral analyses permits a nuanced characterisation of engine oil evolution. The spectral analysis may comprise spectral data and / or a plurality of engine oil variables extracted from the spectral data. The multi- dimensional analysis allows spectra to be simplified, and / or the multi-dimensional analysis allows variation in a plurality of engine oil variables to be simplified. A combination of variations of different engine oil variables may be considered. For example, an increase in a first engine oil variable may occur alongside a decrease in a second engine oil variable when the engine oil is exposed to a first operational condition. However, an increase in the first engine oil variable may occur alongside an increase in the second engine oil variable when the engine oil is exposed to a second operational condition. Furthermore, more nuanced variations may be considered in addition to or instead of a change in magnitude of a variable. For example, by analysing spectral analyses of the engine oil samples, a change in peak width, height, position, skew, and / or other features may be considered. This may be achieved by inspecting variation in more than one direction or dimension. Associating nuanced variations in spectral analyses of engine oil allows for more accurate characterisation of engine oil evolution pathways for particular engine operational conditions. This may be used both to evaluate engine oil health and to evaluate local engine health. Using this method, a characterisation may be provided of at least one evolution pathway of engine oil. The method may be repeated for more than one volume of engine oil, each exposed to one of a plurality of operational conditions. By exposing different volumes of engine oil to different operational conditions, evolution of engine oil that has been exposed to the different operational conditions may be obtained. In an example, in an event that a test sample of engine oil is taken from an engine and analysed to provide a spectrum, the presence (or absence) of the fingerprint for the first operational condition in the spectrum may be indicative that the test sample has been exposed to (or has not been exposed to) the first operational condition. The first operational condition may be a local operational condition of a component, sub-system or system of the engine. A fingerprint of that local operational condition isolates how the engine oil evolves when exposed to that local operational condition. In other words, an evolution pathway for the engine oil is isolated. Bulk engine oil may have been exposed to various local operational conditions. A fingerprint of a local operational condition may be identified in a spectrum of a sample of bulk engine oil, indicating that the bulk engine oil contains engine oil that has passed through the local operational condition. The method may be carried out to provide a characterisation of evolution of engine oil that has been exposed to a plurality of different operational conditions, each having its own fingerprint of changes to a spectrum. As a result, a spectrum of a sample of bulk engine oil may be scrutinised to identify more than one fingerprint, each associated with a different local operational condition. From a sample of bulk oil, operation of a combustion engine may therefore be determined. Operation of the combustion engine may include how the engine has been used and the health of the engine. The variation may be expressed as at least one vector of change (wherein a vector of change may also be referred to as a component). The at least one vector of change may define an engine oil evolution pathway. The at least one vector of change may be indicative of a change elemental composition and / or chemical bonds in the first volume of engine oil. Each vector of change may be indicative of variation of more than one engine oil variable, wherein variation of each engine oil variable is represented as a certain proportion of the vector of change. An engine oil variable may be any property of engine oil, including but not limited to elemental composition and / or chemical bonds and / or other properties such as oxidation, sulfation and nitration. A given vector of change may be defined by the particular engine oil variables the particular proportions in which they are represented. A vector of change or set of vectors of change define the engine oil evolution pathway. In use, a magnitude of a vector of change may be indicative of how far the engine oil has progressed along said evolution pathway. In other words, a magnitude of a vector of change may be indicative of a current state of the engine oil. The method may be repeated for more than one volume of engine oil, exposed to more than one operational condition. A characterisation of engine oil evolution may comprise a plurality of sets of vectors of change. Each set of vectors may be obtained by carrying out the method for at least one sample of engine oil exposed to at least one operational condition. Each set of vectors may describe an evolution pathway of the engine oil. A particular set of vectors of change may be associated with normal engine oil evolution. Other sets of vectors of change may be associated with abnormal engine oil evolution. A set of vectors of change may comprise more than one orthogonal vector of change. Each vector of change may be indicative of a variation in more than one engine oil variable. Each vector of change may be obtained via dimension reduction. Where a volume of engine oil is exposed to an operational condition that results in abnormal engine oil evolution, the vectors of change associated with the volume of engine oil may be associated with at least one metric indicative of the operational condition to which the volume of engine oil was exposed. The spectral analysis of each sample may comprise a plurality of engine oil variables. The output of the multi-dimensional analysis may comprise one or more than one vector of change that is indicative of a difference in elemental composition and / or chemical bonds between the more than one sample of engine oil, wherein the multi-dimensional analysis may be carried out via dimension reduction techniques such that the number of vectors of change is fewer than the number of engine oil variables. Each vector of change may be orthogonal to the other vector(s) of change. Each operational condition may be identified by one or more than one engine variable. The samples may be exposed to an operational condition using an experimental rig or by using engine oil in a combustion engine and operating the combustion engine. In an event that the sample is exposed to an operational condition using an engine, the variation may be associated with metadata from the combustion engine. Exposing a sample of engine oil to an operational condition may be achieved by using the engine oil in a combustion test engine that is not used to drive equipment, wherein the combustion test engine is configured to replicate the operational condition. Exposing a sample of engine oil to an operational condition may be achieved by using the engine oil in a combustion engine that is used to drive equipment such that the operational condition is implemented. Exposing a sample of engine oil to an operational condition may be achieved by exposing the engine oil to an experimental condition, wherein the experimental condition corresponds to the operational condition of at least one component, sub-system or system of the combustion engine. At least a portion of the plurality of operational conditions may comprise local operational conditions. An operational condition of the plurality of operational conditions may be an operational condition of a component or sub-system or system of the combustion engine. The spectral analysis of each sample of engine oil may comprise at least one of: spectral data; and a plurality of engine oil variables extracted from spectral data. The output of the multi-dimensional analysis may comprise at least one vector of change that is indicative of a difference in elemental composition and / or chemical bonds between the more than one sample of engine oil. The multi-dimensional analysis may be carried out via dimension reduction. The variation(s) between the spectral analyses of the more than one sample may comprise at least one of: a change to a peak height of one or more peaks; a change to a location of one or more peaks; a change to a shape of one or more peaks; a change to a trough height of one or more troughs; a change to a location of one or more troughs; and a change to a shape of one or more troughs. The variation(s) between the spectral analyses of the more than one sample may comprise at least one vector of change indicative of variation between spectral analyses. Each vector of change may comprise a number indicative of variation between spectral analyses. Each vector of change may be indicative of a change in value of at least one engine oil variable, wherein the spectral analyses each comprise more than one engine oil variable. Each vector of change may be indicative of a change in value of at least one engine oil variable with respect to an engine variable. Each spectral analysis may comprise data indicative of a spectrum. Each vector of change may be indicative of a change to one or more of: a peak height of one or more peaks; a location of one or more peaks; a shape of one or more peaks; a trough height of one or more troughs; a location of one or more troughs; and a shape of one or more troughs. The method may further comprise associating data relating to the engine oil or to the use of the engine oil with the operational condition to which a sample was exposed. Optionally the data may comprise one or more of: a temperature to which the sample was exposed; chemistry of any contaminants to which the sample was exposed; concentration of any contaminants to which the sample was exposed; a surface area of an interface between the sample and any contaminants; a length of time for which the engine oil has been exposed to the operational conditions; a type or model of combustion engine; in an event that exposing the engine oil to the operational condition is achieved by using the engine oil in a combustion engine, data relating to engine operation and / or engine hardware and / or engine age. Each sample may be taken from a volume of engine oil that has been exposed to an operational condition. More than one sample may be taken from the same volume of engine oil, wherein in between taking each sample, the volume of engine oil is exposed to one of a plurality of operational conditions. More than one sample may be obtained from different volumes of engine oil. More than one sample may be exposed to operational conditions identified by the same engine variable, such that the more than one sample of engine oil are indicative of engine oil that has been used in a combustion engine operating: over different periods of time; and / or with different values of the engine variable. The plurality of operational conditions may comprise more than one operational condition of the same component, sub-system or system of the combustion engine. The plurality of operational conditions may comprise operational conditions of different component, sub-system or system of the combustion engine. Using spectrometry to analyse each sample may comprise using Fourier Transform Infrared (FTIR) spectrometry to analyse each sample. Using spectrometry to analyse each sample may comprise using inductively coupled plasma atomic emission spectroscopy (ICP-AES) to analyse each sample. Using spectrometry to analyse each sample may comprise using Raman spectroscopy to analyse each sample. Using spectrometry to analyse each sample may comprise using more than one spectrometer to analyse each sample. The multi-dimensional analysis may be carried out using Principle Component Analysis. The more than one sample may be further analysed to provide oil viscometrics information, wherein multi-dimensional analysis is used to interrogate variation between the spectral analyses and the viscometric information of the more than one sample. The viscometric information may be obtained as a function of engine oil temperature. The spectral analysis of a sample may be processed based on viscometric information and wherein multi-dimensional analysis is used to interrogate variation between the processed spectral analyses, wherein the processed spectral analyses comprise one or more of: a maximum of a peak of a spectrum as a function of viscosity at a specific temperature of the engine oil; a moment of a spectrum normalised to a change in viscosity at a specific condition; and a ratio of two or more peaks of a spectrum, normalised to a viscosity of the engine oil at a certain shear rate. There is also provided a characterisation of evolution of engine oil during operation of a combustion engine, wherein the engine oil is configured to lubricate the combustion engine and wherein the characterisation comprises variation between spectral analyses of more than one sample of engine oil, wherein: each sample has been exposed to one of a plurality of operational conditions of the combustion engine; and the variation between the spectral analyses of the more than one sample of engine oil are obtained by using multi-dimensional analysis to interrogate variation between the spectral analyses of the more than one sample; wherein spectral analyses of each sample are obtained by using spectrometry to analyse elemental composition and / or chemical bonds of the more than one sample of engine oil. In this way, a particular change or changes to spectra of engine oil may be used to characterise evolution of engine oil. The engine oil may follow a different evolution pathway depending on how it is used; in other words, the engine oil may follow a different evolution pathway depending on the operational conditions to which it is exposed. The variation in the spectral analyses of different samples exposed to different operational conditions may provide a fingerprint of how the engine oil evolves for those operational conditions. An expected, or normal, engine oil evolution may be determined for normal operational conditions of an engine. Abnormal engine oil evolution may also be determined, for operational conditions of an engine experiencing a failure mode or other issue. By carrying out this method for different operational conditions, engine oil evolutions may be determined for different operational conditions. In particular, evolution of bulk engine oil that has evolved at more than one local interface may be determined by obtaining and analysing samples for more than one local operational condition. Advantageously, an expected engine oil evolution allows an actual engine oil evolution to be tracked, wherein any deviations from the expected engine oil evolution are indicative of abnormal engine oil evolution. Abnormal engine oil evolution allows for any deviation from an expected engine oil evolution to be identified, and linked to local engine conditions. In this way, whether the engine or engine oil requires maintenance may be determined before any issues result in damage or downtime. Chemical and physical properties of engine oil are a function of engine operating conditions. Engine operating conditions may, in turn, be a function of engine health. Advantageously, engine oil evolution may be used to identify health of the engine, a system of the engine, a subsystem of the engine, a component of the engine, or the engine oil itself. By obtaining an expected engine oil evolution for particular engine operational conditions, any deviation of actual engine oil evolution from expected engine oil evolution may be identified. A deviation of actual engine oil evolution may be linked to a particular local engine state or local engine operational condition, using the characterisation described herein. Any issue may, therefore, be precisely identified and any maintenance of the engine or the engine oil may be directed to a particular area of the engine. In an example, characterisation may comprise an association of an operational condition with an evolution of the engine oil, such that means that analysis of a sample of engine oil may be compared to the characterisation used to identify health of at least one of the engine, a system of the engine, a subsystem of the engine, a component of the engine, or the engine oil itself. In another example, characterisation may not comprise an association of an operational condition with an evolution of the engine oil, and in use analysis of at least one sample of engine oil may be correlated to the characterisation to identify health of at least one of the engine, a system of the engine, a subsystem of the engine, a component of the engine, or the engine oil itself. An operational condition may be a local operational condition of a component, sub-system or system of the engine. A fingerprint of that local operational condition isolates how the engine oil evolves when exposed to that local operational condition. In other words, an evolution pathway for the engine oil is isolated. Bulk engine oil may have been exposed to various local operational conditions. A fingerprint of a local operational condition may be identified in a spectrum or spectra of a sample of bulk engine oil, indicating that the bulk engine oil contains engine oil that has passed through the local operational condition. A characterisation of evolution of engine oil may be provided for engine oil that has been exposed to a plurality of different operational conditions, each having its own fingerprint of changes to a spectrum. As a result, a spectrum of a sample of bulk engine oil may be scrutinised to identify more than one fingerprint, each resulting from a different local operational condition. From a sample of bulk oil, operation of a combustion engine may therefore be determined. Operation of the combustion engine may include how the engine has been used and the health of the engine. The expected engine oil evolution may comprise vectors of change, wherein the way in which the engine oil changes provides information as to the engine operation. By monitoring the way in which the engine oil changes, rather than only monitoring whether a parameter of the engine oil exceeds a limit, any change to the engine operation may be detected more quickly. Spectral analysis may be carried out on engine oil samples after they have been exposed to various different operational conditions, wherein exposing the engine oil samples to various different operational conditions may be achieved on or off engine. The spectral analysis shows how the engine oil has evolved, so the spectral analysis allows an engine oil evolution to be developed as a function of engine operation. In this way, engine oil evolution may be linked to operation of a combustion engine. The characterisation may include operational conditions, or the characterisation may not include operational conditions. In use, even if the characterisation does not include the operational conditions, the characterisation may be used to link engine oil evolution to operation of a combustion engine, for example via correlation of the characterisation with data collected from engine oil samples from combustion engines. In an event that the characterisation includes operational conditions, the association of variations in spectral analysis with operational conditions may be included in the characterisation of the engine oil evolution (for example by recording regions of interest associated with an operational condition to which a particular sample was exposed), or by correlation with data from other sources. Separately, engine oil evolution may be linked to engine oil performance. Advantageously, multi-dimensional analysis of spectral analyses permits a nuanced characterisation of engine oil evolution. The spectral analysis may comprise spectral data and / or a plurality of engine oil variables extracted from the spectral data. The multi- dimensional analysis allows spectra to be simplified, and / or the multi-dimensional analysis allows variation in a plurality of engine oil variables to be simplified. A combination of variations of different engine oil variables may be considered. For example, an increase in a first engine oil variable may occur alongside a decrease in a second engine oil variable when the engine oil is exposed to a first operational condition. However, an increase in the first engine oil variable may occur alongside an increase in the second engine oil variable when the engine oil is exposed to a second operational condition. Furthermore, more nuanced variations may be considered in addition to or instead of a change in magnitude of a variable. For example, a change in peak width, height, position, skew, and so on may be considered. This may be achieved by inspecting variation in more than one direction or dimension. Associating nuanced variations in spectral analyses of engine oil allows for more accurate characterisation of engine oil evolution pathways for particular engine operational conditions. This may be used both to evaluate engine oil health and to evaluate local engine health. A characterisation may be provided of at least one evolution pathway of engine oil. By exposing different volumes of engine oil to different operational conditions, evolution of engine oil that has been exposed to the different operational conditions may be obtained. The variation may be expressed as at least one vector of change (wherein a vector of change may also be referred to as a component). The at least one vector of change may define an engine oil evolution pathway. The at least one vector of change may be indicative of a change elemental composition and / or chemical bonds in the first volume of engine oil. Each vector of change may be indicative of variation of more than one engine oil variable, wherein variation of each engine oil variable is represented as a certain proportion of the vector of change. An engine oil variable may be any property of engine oil, including but not limited to elemental composition and / or chemical bonds and / or other properties such as oxidation, sulfation and nitration. A given vector of change may be defined by the particular engine oil variables the particular proportions in which they are represented. A vector of change or set of vectors of change define the engine oil evolution pathway. In use, a magnitude of a vector of change may be indicative of how far the engine oil has progressed along said evolution pathway. In other words, a magnitude of a vector of change may be indicative of a current state of the engine oil. A characterisation of engine oil evolution may comprise a plurality of sets of vectors of change. Each set of vectors may be obtained by exposing at least one sample of engine oil to at least one operational condition, obtaining spectral analysis for the at least one sample, and interrogating variation of spectral analysis for each sample to at least one other spectral analysis for another sample. Each set of vectors may describe an evolution pathway of the engine oil. A particular set of vectors of change may be associated with normal engine oil evolution. Other sets of vectors of change may be associated with abnormal engine oil evolution. A set of vectors of change may comprise more than one orthogonal vector of change. Each vector of change may be indicative of a variation in more than one engine oil variable. Each vector of change may be obtained via dimension reduction. Where a volume of engine oil is exposed to an operational condition that results in abnormal engine oil evolution, the vectors of change associated with the volume of engine oil may be associated with at least one metric indicative of the operational condition to which the volume of engine oil was exposed. The spectral analysis of each sample may comprise a plurality of engine oil variables. The output of the multi-dimensional analysis may comprise one or more than one vector of change that is indicative of a difference in elemental composition and / or chemical bonds between the more than one sample of engine oil, wherein the multi-dimensional analysis may be carried out via dimension reduction techniques such that the number of vectors of change is fewer than the number of engine oil variables. Each vector of change may be orthogonal to the other vector(s) of change. Each operational condition may be identified by one or more than one engine variable. The samples may be exposed to an operational condition using an experimental rig or by using engine oil in a combustion engine and operating the combustion engine. In an event that the sample is exposed to an operational condition using an engine, the variation may be associated with metadata from the combustion engine. Exposing a sample of engine oil to an operational condition may be achieved by using the engine oil in a combustion test engine that is not used to drive equipment, wherein the combustion test engine is configured to replicate the operational condition. Exposing a sample of engine oil to an operational condition may be achieved by using the engine oil in a combustion engine that is used to drive equipment such that the operational condition is implemented. Exposing a sample of engine oil to an operational condition may be achieved by exposing the engine oil to an experimental condition, wherein the experimental condition corresponds to the operational condition of at least one component, sub-system or system of the combustion engine. At least a portion of the plurality of operational conditions may comprise local operational conditions. An operational condition of the plurality of operational conditions may be an operational condition of a component or sub-system or system of the combustion engine. The spectral analysis of each sample of engine oil may comprise at least one of: spectral data; and a plurality of engine oil variables extracted from spectral data. The output of the multi-dimensional analysis may comprise at least one vector of change that is indicative of a difference in elemental composition and / or chemical bonds between the more than one sample of engine oil. The multi-dimensional analysis may be carried out via dimension reduction. The variation(s) between the spectral analyses of the more than one sample may comprise at least one of: a change to a peak height of one or more peaks; a change to a location of one or more peaks; a change to a shape of one or more peaks; a change to a trough height of one or more troughs; a change to a location of one or more troughs; and a change to a shape of one or more troughs. The variation(s) between the spectral analyses of the more than one sample may comprise at least one vector of change indicative of variation between spectral analyses. Each vector of change may comprise a number indicative of variation between spectral analyses. Each vector of change may be indicative of a change in value of at least one engine oil variable, wherein the spectral analyses each comprise more than one engine oil variable. Each vector of change may be indicative of a change in value of at least one engine oil variable with respect to an engine variable. Each spectral analysis may comprise data indicative of a spectrum. Each vector of change may be indicative of a change to one or more of: a peak height of one or more peaks; a location of one or more peaks; a shape of one or more peaks; a trough height of one or more troughs; a location of one or more troughs; and a shape of one or more troughs. The method may further comprise associating data relating to the engine oil or to the use of the engine oil with the operational condition to which a sample was exposed. Optionally the data may comprise one or more of: a temperature to which the sample was exposed; chemistry of any contaminants to which the sample was exposed; concentration of any contaminants to which the sample was exposed; a surface area of an interface between the sample and any contaminants; a length of time for which the engine oil has been exposed to the operational conditions; a type or model of combustion engine; in an event that exposing the engine oil to the operational condition is achieved by using the engine oil in a combustion engine, data relating to engine operation and / or engine hardware and / or engine age. Each sample may be taken from a volume of engine oil that has been exposed to an operational condition. More than one sample may be taken from the same volume of engine oil, wherein in between taking each sample, the volume of engine oil is exposed to one of a plurality of operational conditions. More than one sample may be obtained from different volumes of engine oil. More than one sample may be exposed to operational conditions identified by the same engine variable, such that the more than one sample of engine oil are indicative of engine oil that has been used in a combustion engine operating: over different periods of time; and / or with different values of the engine variable. The plurality of operational conditions may comprise more than one operational condition of the same component, sub-system or system of the combustion engine. The plurality of operational conditions may comprise operational conditions of different component, sub-system or system of the combustion engine. Using spectrometry to analyse each sample may comprise using Fourier Transform Infrared (FTIR) spectrometry to analyse each sample. Using spectrometry to analyse each sample may comprise using inductively coupled plasma atomic emission spectroscopy (ICP-AES) to analyse each sample. Using spectrometry to analyse each sample may comprise using Raman spectroscopy to analyse each sample. Using spectrometry to analyse each sample may comprise using more than one spectrometer to analyse each sample. The multi-dimensional analysis may be carried out using Principle Component Analysis. The more than one sample may be further analysed to provide oil viscometrics information, wherein multi-dimensional analysis is used to interrogate variation between the spectral analyses and the viscometric information of the more than one sample. The viscometric information may be obtained as a function of engine oil temperature. The spectral analysis of a sample may be processed based on viscometric information and wherein multi-dimensional analysis is used to interrogate variation between the processed spectral analyses, wherein the processed spectral analyses comprise one or more of: a maximum of a peak of a spectrum as a function of viscosity at a specific temperature of the engine oil; a moment of a spectrum normalised to a change in viscosity at a specific condition; and a ratio of two or more peaks of a spectrum, normalised to a viscosity of the engine oil at a certain shear rate. In a second aspect of the disclosure, there is also provided: a method of characterising evolution of engine oil during operation of a combustion engine, wherein the engine oil is configured to lubricate the combustion engine, the method comprising: obtaining more than one sample of engine oil, wherein each sample of engine oil has been exposed to one of a plurality of operational conditions of the combustion engine; using Fourier transform infrared spectrometry to analyse elemental composition and / or chemical bonds of the more than one sample of engine oil to provide a spectral analysis of each sample; and using multi-dimensional analysis to interrogate variation between the spectral analyses of the more than one sample, wherein the variation characterises an evolution pathway of the engine oil. There is also provided: a characterisation of evolution of engine oil during operation of a combustion engine, wherein the engine oil is configured to lubricate the combustion engine and wherein the characterisation comprises variation between spectral analyses of more than one sample of engine oil, wherein: each sample has been exposed to one of a plurality of operational conditions of the combustion engine; and the variation between the spectral analyses of the more than one sample of engine oil are obtained by using multi-dimensional analysis to interrogate variation between the spectral analyses of the more than one sample; wherein spectral analyses of each sample are obtained by using Fourier transform infrared spectrometry to analyse elemental composition and / or chemical bonds of the more than one sample of engine oil. A variation between the spectral analyses may comprise a difference in a feature of the Fourier-transform infrared (FTIR) spectrum. A feature of an FTIR spectrum may comprise a peak and / or a trough. A feature of an FTIR spectrum may correspond to a particular chemical bond of a molecule, such that a corresponding feature in different spectral analyses corresponds to the same chemical bond of the same molecule. FTIR spectrometry allows analysis of the chemical bonds of the molecules within the oil. Analysis of individual features of an FTIR spectrum allows analysis of the same chemical bond within different molecules. For example, a double chemical bond between carbon and oxygen may be found in two different molecules, and the changes of that chemical bond in the two molecules may have different implications for the evolution of the oil. Furthermore, analysis of individual features of an FTIR spectrum allows analysis of how a chemical bond changes during use of the engine oil. Changes to a chemical bond may result in a change to an individual feature of an FTIR spectrum. For example, FTIR spectra taken for two samples of the same volume of engine oil (wherein one of the two samples is taken from the volume of engine oil before the volume of engine oil has been exposed to an operational condition of a combustion engine, and the second of the two samples is taken from the volume of engine oil after the volume of engine oil has been exposed to an operational condition of a combustion engine) may show a change to one or more feature of the spectrum. A change to a feature of the spectrum may be indicative of a change to a particular chemical bond of a particular molecule of the engine oil. The particular change to the feature or features of an FTIR spectrum may be associated with the operational condition to which the volume of engine oil was exposed between the first sample being taken and the second sample being taken, such that in an event that a similar change to the same feature or features is seen in an FTIR spectrum of a second volume of engine oil, it may be inferred that the second volume of engine oil has been exposed to that operational condition. A particular operational condition may be associated with one or more changes to one or more features of an FTIR spectrum. A particular change to a feature of an FTIR spectrum may be indicative of a different operational condition depending on whether (or how) other features of the FTIR spectrum have changed. A change to a feature of an FTIR spectrum may comprise a change to a position and / or absorbance and / or shape of the feature. A variation between spectral analyses may comprise one or more of: a shift of a feature with respect to an axis; a scaling of a feature with respect to an axis; a change to an absorbance ratio of two or more features; a change to a number of features in a wavenumber range; a change to an integral of the spectrum within a wavenumber range; and a roughness over a wavenumber range. In a third aspect of the disclosure, there is also provided: a method of characterising evolution of engine oil during operation of a combustion engine, wherein the engine oil is configured to lubricate the combustion engine, the method comprising: obtaining more than one sample of engine oil, wherein each sample of engine oil has been exposed to one of a plurality of operational conditions of the combustion engine; using spectrometry to analyse elemental composition and / or chemical bonds of the more than one sample of engine oil to provide a spectral analysis of each sample; and using multi-dimensional analysis to interrogate variation between the spectral analyses of the more than one sample, wherein the variation comprises a difference between at least one of a position, shape and intensity of corresponding features of the spectral analyses of the more than one sample and wherein the variation characterises an evolution pathway of the engine oil. The variation may comprise one or more than one difference between more than one feature of the spectral analyses. A feature of a spectrum may comprise a peak and / or a trough. An intensity of the feature may comprise an absorbance. A difference between spectral analyses may comprise one or more of: a shift of a feature with respect to an axis; a scaling of a feature with respect to an axis; a change to an absorbance ratio of two or more features; a change to a number of features in a wavenumber range; a change to an integral of the spectrum within a wavenumber range; and a roughness over a wavenumber range. There is also provided: a characterisation of evolution of engine oil during operation of a combustion engine, wherein the engine oil is configured to lubricate the combustion engine and wherein the characterisation comprises variation between spectral analyses of more than one sample of engine oil, wherein: each sample has been exposed to one of a plurality of operational conditions of the combustion engine; the variation comprises a difference between at least one of a position, shape and intensity of corresponding features of the spectral analyses of the more than one sample; and the variation between the spectral analyses of the more than one sample of engine oil are obtained by using multi-dimensional analysis to interrogate variation between the spectral analyses of the more than one sample; wherein spectral analyses of each sample are obtained by using spectrometry to analyse elemental composition and / or chemical bonds of the more than one sample of engine oil. In a fourth aspect of the disclosure, there is also provided: a method of characterising evolution of engine oil during operation of a combustion engine, wherein the engine oil is configured to lubricate the combustion engine, the method comprising: obtaining more than one sample of engine oil, wherein each sample of engine oil has been exposed to one of a plurality of local operational conditions of the combustion engine; using spectrometry to analyse elemental composition and / or chemical bonds of the more than one sample of engine oil to provide a spectral analysis of each sample; and using multi-dimensional analysis to interrogate variation between the spectral analyses of the more than one sample, wherein the variation characterises an evolution pathway of the engine oil. There is also provided: a characterisation of evolution of engine oil during operation of a combustion engine, wherein the engine oil is configured to lubricate the combustion engine and wherein the characterisation comprises variation between spectral analyses of more than one sample of engine oil, wherein: each sample has been exposed to one of a plurality of local operational conditions of the combustion engine; and the variation between the spectral analyses of the more than one sample of engine oil are obtained by using multi-dimensional analysis to interrogate variation between the spectral analyses of the more than one sample; wherein spectral analyses of each sample are obtained by using spectrometry to analyse elemental composition and / or chemical bonds of the more than one sample of engine oil. In a fifth aspect of the disclosure, there is provided: a method of characterising evolution of engine oil during operation of a combustion engine, wherein the engine oil is configured to lubricate the combustion engine, the method comprising: a. using a first spectrometer to analyse a primary sample of a first volume of engine oil and provide a primary spectrum of the first volume of engine oil; b. exposing the first volume of engine oil to a first operational condition of the combustion engine; c. using the first spectrometer to analyse a secondary sample of the first volume of engine oil and provide a secondary spectrum of the first volume of engine oil; d. comparing the primary spectrum of the first volume of engine oil and the secondary spectrum of the first volume of engine oil to determine one or more differences between the primary spectrum of the first volume of engine oil and the secondary spectrum of the first volume of engine oil, wherein the one or more differences are indicative of a change in the first volume of engine oil; and e. associating the one or more differences with the first operational condition to provide a characterisation of evolution of engine oil that has been exposed to the first operational condition. The first operational condition may be an operational condition of a component or sub- system or system of the combustion engine. Exposing the first volume of engine oil to the first operational condition may be achieved by using the engine oil in a combustion engine. Exposing the first volume of engine oil to the first operational condition may be achieved by using the engine oil in a combustion test engine that is not used to drive equipment, wherein the combustion test engine is configured to replicate the first operational condition. Exposing the first volume of engine oil to the first operational condition may be achieved by using the engine oil in a combustion engine that is used to drive equipment such that the first operational condition is implemented. Exposing the first volume of engine oil to the first operational condition may be achieved by exposing the first volume of engine oil to a first experimental condition, wherein the first experimental condition corresponds to a first operational condition of one or more component, sub-system or system of the combustion engine. The one or more differences between the primary spectrum of the first volume of engine oil and the secondary spectrum of the first volume of engine oil may comprise one or more of: a change to a peak height of one or more peaks; a change to a location of one or more peaks; a change to a shape of one or more peaks; a change to a trough height of one or more troughs; a change to a location of one or more troughs; and a change to a shape of one or more troughs. Step e) may further comprise associating data relating to the engine oil or to the use of the engine oil with the first operational condition, wherein optionally the data may comprise one or more of: a length of time for which the engine oil has been exposed to the first operational conditions; a type or model of combustion engine; in an event that exposing the first volume of engine oil to the first operational condition is achieved by using the engine oil in a combustion engine, data relating to engine operation and / or engine hardware and / or engine age. The primary sample may be removed from the first volume of engine oil before exposing the first volume of engine oil to the first operational condition; and the secondary sample may be removed from the first volume of engine oil after exposing the first volume of engine oil to the first operational condition. The method may further comprise repeating steps (a) to (e) for a second volume of engine oil such that the method further comprises: f. using the first spectrometer to analyse a primary sample of a second volume of engine oil and provide a primary spectrum of the second volume of engine oil; g. exposing the second volume of engine oil to a second operational condition of the combustion engine; h. using the first spectrometer to analyse a secondary sample of the second volume of engine oil and provide a secondary spectrum of the second volume of engine oil; i. comparing the primary spectrum of the second volume of engine oil and the secondary spectrum of the second volume of engine oil to determine one or more differences between the primary spectrum of the second volume of engine oil and the secondary spectrum of the second volume of engine oil, wherein the one or more differences are indicative of a change in the second volume of engine oil; and j. associating the one or more differences with the second operational condition to provide a characterisation of evolution of engine oil that has been exposed to the second operational condition. The second volume of engine oil may be the first volume of engine oil after the first volume of engine oil has been exposed to the first operational condition. The second volume of engine oil may be separate to the first volume of engine oil. The first operational condition may be an operational condition of a first component or sub- system or system of the combustion engine; the second operational condition may be an operational condition of a second component or sub-system or system of the combustion engine; and the first component or sub-system or system of the combustion engine may be different to the second component or sub-system or system of the combustion engine. The second operational condition may be an operational condition of the same component, sub-system or system of the combustion engine as the first operational condition. The second operational condition may correspond to an engine condition over a different time period than the first operational condition. The method may comprise carrying out steps (a) to (e) for a plurality of volumes of engine oil, wherein each of the plurality of volumes of engine oil are exposed to one of a plurality of operational conditions and wherein the method provides a characterisation of the evolution of engine oil that has been exposed to each operational condition. The characterisation may comprise an association between certain changes in spectra of the engine oil and certain operational conditions. The method may further comprise: before exposing the first volume of engine oil to the first operational condition, using a second spectrometer to analyse a tertiary sample of the first volume of the engine oil and provide a tertiary spectrum of the first volume of engine oil; after exposing the first volume of engine oil to the first operational condition, using the second spectrometer to analyse a quaternary sample of the first volume of engine oil and provide a quaternary spectrum of the first volume of engine oil; comparing the tertiary spectrum of the first volume of engine oil and the quaternary spectrum of the first volume of engine oil to determine one or more differences between the tertiary spectrum of the first volume of engine oil and the quaternary spectrum of the first volume of engine oil, wherein the one or more differences are indicative of a change in the engine oil; and associating the one or more differences with the first operational condition. Either the first spectrometer or the second spectrometer may be a Fourier-transform infrared (FTIR) spectrometer; and / or either the first spectrometer or the second spectrometer may be an inductively coupled plasma atomic emission spectrometer. Comparison of the primary spectrum and the secondary spectrum may be achieved using one or more of: visual inspection; mathematical comparison; binary additive operations; simplification of spectra; and data science tools. Comparison of the primary spectrum and the secondary spectrum may be achieved using the Principal Component Analysis. There is also provided: a characterisation of evolution of engine oil during operation of a combustion engine, wherein the engine oil is configured to lubricate the combustion engine. The characterisation comprises one or more differences between a primary spectrum and a secondary spectrum of a first volume of the engine oil, wherein the one or more differences are indicative of a change in the first volume of engine oil. The characterisation further comprises an association of the one or more differences to a first operational condition of the combustion engine. The primary spectrum is obtained by using a first spectrometer to analyse a primary sample of the first volume of engine oil, wherein the primary sample is indicative of the first volume of engine oil prior to being exposed to the first operational condition. The secondary spectrum is obtained by using the first spectrometer to analyse a secondary sample of the first volume of engine oil, wherein the secondary sample is indicative of the first volume of engine oil after being exposed to the first operational condition. The first operational condition may be an operational condition of a component or sub- system or system of the combustion engine. Exposing the first volume of engine oil to the first operational condition may be achieved by: using the engine oil in a combustion engine; or exposing the first volume of engine oil to a first experimental condition, wherein the first experimental condition corresponds to a first operational condition of one or more component, sub-system or system of the combustion engine. The characterisation may comprise: one or more differences between a primary spectrum and a secondary spectrum of each of a plurality of volumes of the engine oil, wherein the one or more differences are indicative of a change in each volume of engine oil; and an association of the one or more differences to one of a plurality of operational conditions of the combustion engine; wherein each of the plurality of operational conditions is an operational condition of a component or sub-system or system of the combustion engine. In a sixth aspect of the disclosure, there is provided: a method of characterising evolution of engine oil during operation of a combustion engine, wherein the engine oil is configured to lubricate the combustion engine, the method comprising: a. using spectrometry to analyse elemental composition and / or chemical bonds of a primary sample of a first volume of engine oil and provide a primary spectral analysis of the first volume of engine oil; b. exposing the first volume of engine oil to a first operational condition of the combustion engine; c. using spectrometry to analyse elemental composition and / or chemical bonds of a secondary sample of the first volume of engine oil and provide a secondary spectral analysis of the first volume of engine oil; d. using multi-dimensional analysis to compare the primary spectral analysis of the first volume of engine oil and the secondary spectral analysis of the first volume of engine oil to determine a variation between the primary and secondary spectral analyses that is indicative of a change in at least one of the elemental composition and / or chemical bonds of the first volume engine oil, wherein the variation provides a characterisation of evolution of engine oil that has been exposed to the first operational condition. Advantageously, multi-dimensional analysis of spectral analyses permits a nuanced characterisation of engine oil evolution. The spectral analysis may comprise spectral data and / or a plurality of engine oil variables. The multi-dimensional analysis allows spectra to be simplified, and / or the multi-dimensional analysis allows variation in a plurality of engine oil variables to be considered. A combinations of variations of different engine oil variables to be considered, or simplified. For example, an increase in a first engine oil variable may occur alongside a decrease in a second engine oil variable when the engine oil is exposed to a first operational condition. However, an increase in the first engine oil variable may occur alongside an increase in the second engine oil variable when the engine oil is exposed to a second operational condition. Furthermore, more nuanced variations may be considered in addition to or instead of a change in magnitude of a variable. For example, a change in peak width, height, position, skew, and so on may be considered. Associating nuanced variations in spectral analyses of engine oil allows for more accurate characterisation of engine oil evolution pathways for particular engine operational conditions. This may be used both to evaluate engine oil health and to evaluate local engine health. Using this method, a characterisation may be provided of at least one evolution pathway of engine oil. The method may be repeated for more than one volume of engine oil, each exposed to one of a plurality of operational conditions. By exposing different volumes of engine oil to different operational conditions, evolution of engine oil that has been exposed to the different operational conditions may be obtained. The variation may be expressed as at least one vector of change (wherein a vector of change may also be referred to as a component). The at least one vector of change may define an engine oil evolution pathway. The at least one vector of change may be indicative of a change elemental composition and / or chemical bonds in the first volume of engine oil. Each vector of change may be indicative of variation of more than one engine oil variable, wherein variation of each engine oil variable is represented as a certain proportion of the vector of change. An engine oil variable may be any property of engine oil, including but not limited to elemental composition and / or chemical bonds and / or other properties such as oxidation, sulfation and nitration. A given vector of change may be defined by the particular engine oil variables the particular proportions in which they are represented. A vector of change or set of vectors of change define the engine oil evolution pathway. In use, a magnitude of a vector of change may be indicative of how far the engine oil has progressed along said evolution pathway. In other words, a magnitude of a vector of change may be indicative of a current state of the engine oil. The method may be repeated for more than one volume of engine oil, exposed to more than one operational condition. A characterisation of engine oil evolution may comprise a plurality of sets of vectors of change. Each set of vectors may be obtained by carrying out the method of the sixth aspect for at least one sample of engine oil exposed to at least one operational condition. Each set of vectors may describe an evolution pathway of the engine oil. A particular set of vectors of change may be associated with normal engine oil evolution. Other sets of vectors of change may be associated with abnormal engine oil evolution. A set of vectors of change may comprise more than one orthogonal vector of change. Each vector of change may be indicative of a variation in more than one engine oil variable. Each vector of change may be obtained via dimension reduction. Where a volume of engine oil is exposed to an operational condition that results in abnormal engine oil evolution, the vectors of change associated with the volume of engine oil may be associated with at least one metric indicative of the operational condition to which the volume of engine oil was exposed. The spectral analysis of each sample may comprise a plurality of engine oil variables. The output of the multi-dimensional analysis may comprise one or more than one component that is indicative of a difference in elemental composition and / or chemical bonds between the more than one sample of engine oil, wherein the multi-dimensional analysis may be carried out via dimension reduction techniques such that the number of vectors of change is fewer than the number of engine oil variables. Each component may be orthogonal to the other component(s). The samples may be exposed to an operational condition using an experimental rig or by using engine oil in a combustion engine and operating the combustion engine. In an event that the sample is exposed to an operational condition using an engine, the variation may be further associated with metadata from the combustion engine. The method of the sixth aspect of the disclosure may be carried out for more than one volume of engine oil, wherein for each volume of engine oil the method comprises: a. using spectrometry to analyse elemental composition and / or chemical bonds of a primary sample of a first volume of engine oil and provide a primary spectral analysis of the volume of engine oil; b. exposing the volume of engine oil to an operational condition of the combustion engine; c. using spectrometry to analyse elemental composition and / or chemical bonds of a secondary sample of the volume of engine oil and provide a secondary spectral analysis of the volume of engine oil; d. using multi-dimensional analysis to compare the primary spectral analysis and the secondary spectral analysis of to determine variation between the primary and secondary spectral analyses; and e. associating the variation with the operational condition to which the volume of engine oil was exposed; such that an association of variation with each operational condition provides a characterisation of evolution of engine oil that has been exposed to each operational condition. There is also provided a characterisation of evolution of engine oil during operation of a combustion engine, wherein the engine oil is configured to lubricate the combustion engine. The characterisation comprises variation between a primary spectral analysis and a secondary spectral analysis of a first volume of the engine oil, wherein the variation is indicative of a change in at least one of the elemental composition and / or chemical bonds of the first volume of engine oil. The variation is obtained by using multi-dimensional analysis to compare the primary spectral analysis and the secondary spectral analysis. The characterisation further comprises an association of the variation to a first operational condition of the combustion engine to provide a characterisation of evolution of engine oil that has been exposed to the first operational condition. The primary spectral analysis is obtained by using a first spectrometer to analyse elemental composition and / or chemical bonds of a primary sample of the first volume of engine oil, wherein the primary sample is indicative of the first volume of engine oil prior to being exposed to the first operational condition. The secondary spectral analysis is obtained by using the first spectrometer to analyse elemental composition and / or chemical bonds of a secondary sample of the first volume of engine oil, wherein the secondary sample is indicative of the first volume of engine oil after being exposed to the first operational condition. The primary and secondary spectral analyses may each comprise a plurality of engine oil variables. The multi-dimensional analysis of the variation between the primary and secondary spectral analyses may provide at least one component indicative of variation between the primary and secondary spectral analyses. A number of vectors of change for a sample may be fewer than the number of engine oil variables of the primary and secondary spectral analyses. For each sample, the characterisation may comprise at least one component indicative of variation between the primary and secondary spectral analyses. Brief Description of the Drawings A specific embodiment of the disclosure will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 shows a flowchart illustrating a method of characterising evolution of engine oil, according to an embodiment of the present disclosure. Figure 2 shows a flowchart illustrating a method of characterising evolution of engine oil, according to an embodiment of the present disclosure. Figures 3A – 3D show examples of changes to peaks of a spectrum. Figures 4A – 4D show examples of changes to troughs of a spectrum. Figure 5 shows an example of a spectrum used in a method characterising evolution of engine oil according to an embodiment of the present disclosure. Figure 6 shows a schematic illustration of examples of spectra indicative of engine undergoing a “normal” evolution pathway. Figure 7 shows a schematic illustration of examples of spectra indicative of engine undergoing an “abnormal” evolution pathway. Figure 8 shows a schematic illustration of examples of spectra indicative of engine undergoing a “normal” evolution pathway. Figure 9 shows a schematic illustration of examples of spectra indicative of engine undergoing an “abnormal” evolution pathway. Detailed Description During operation of a combustion engine (internal combustion engine), engine oil may lubricate the combustion engine. The engine oil may be configured to lubricate one or more components, sub-systems or systems of the combustion engine. The engine oil may be configured to travel to the components, systems or subsystems via a lubrication system. The engine oil may move through the various parts of the lubrication system or combustion engine at different mass flow rates. Furthermore, the engine oil may perform additional functions, particularly in an event that additives are added to the engine oil. Functions of the engine oil may include cleaning (using detergents added to the engine oil), cooling, inhibiting corrosion, neutralising acids from combustion, and so on. As used herein, “engine oil” may refer to both a base engine oil (or base stock) and any additives, if present. A local residence time of the engine oil in a particular component, system or subsystem may be the average time a particular volume of engine oil spends at or in a particular component, system or subsystem. In certain examples, an oil pump may pressurise the engine oil, such that the engine oil passes through a lubrication system to the components, sub-systems and systems of the combustion engine. In an example, the oil pump may pressurise the engine oil such that the engine oil passes into an inlet comprising a suction tube into an oil pan or sump. From the oil pan, the pressurised oil may flow to an oil cooler and then to oil filters, or straight to oil filters. The engine oil may then be routed to a main distribution volume of the engine. The main distribution volume may be referred to as an oil gallery or galleries. The engine oil may pass from the gallery to other components, systems or subsystems of the combustion engine. A portion of the engine oil may pass from the galleries to directly fed components, systems or subsystems via passages. The passages may be configured to physically connect the directly fed components, systems or subsystems to the main gallery or galleries. The passages may, for example, comprise tubes or pipes. The combustion engine may comprise a plurality of passages connecting the galleries to components, systems or subsystems. For example, one or more passages may connect the galleries to bearings. A bearing may result in a restriction of flow to the lubrication system. The mass flow rate of the engine oil through the components, systems or subsystems may vary. The local residence time of engine oil in a directly fed component, system or subsystem may be the volume of the component, system or subsystem divided by the mass flow rate. A portion of the engine oil may pass from the galleries to indirectly fed components, systems or subsystems, based on directed flow from the main oil gallery. Indirectly fed components, systems or subsystems may not be physically connected to the main gallery or galleries via passages. Possible examples of indirectly fed components, systems or subsystems may be a cylinder bore, a piston cooling gallery, valve guides and valve seats. For example, an amount of engine oil may reside in aerosol form inside the combustion engine. Some of this engine oil in aerosol form may pass to indirectly fed components, systems or subsystems. In another example, piston cooling jets may be fed from the main gallery and provide engine oil to indirectly fed components, systems or subsystems. Due to a mass flow of engine oil in the combustion engine, for example through and out of directly fed components, systems or subsystems, there may be engine oil in motion in the combustion engine, some of which may be passed to the indirectly fed components, systems or subsystems. As an example, engine oil may be used to cool a piston, wherein the piston is directly fed. The piston cooling gallery may be a roughly toroidal shaped volume in the piston, with an inlet and an outlet. A piston cooling jet may provide engine oil to the piston. In certain examples, the end of the jet (comprising, for example, a nozzle) may not be connected to the piston. The jet outlet may not provide perfect columnar flow. Engine oil exiting the jet may “broom” and spread out as it exits the jet. Any engine oil exiting the jet that does not enter the piston may be incident on the bottom of the piston or may land on walls of a cylinder bore, so lubricating an internal cylinder wall. The cylinder wall is, in this example, indirectly fed. The cylinder wall may be additionally or alternatively lubricated by engine oil that is expelled from bearings in the vicinity of the cylinder wall. The engine lubrication system provides engine oil to several locations within the engine. These locations may comprise specific component(s), sub-systems or systems that receive engine oil and introduce the engine oil into areas where the engine oil is to be used. The engine oil may exist within component interfaces or within an engine system or sub-system for a certain length of time. For example, the engine oil may be introduced to component tribological interfaces and / or the engine oil may be introduced to systems to be cooled by the engine oil. The engine oil may be introduced to a system or component to do mechanical work (such as hydraulic lash adjusters). The local engine oil comprises a particular quantity of oil in an area where the engine oil is in use. For example, the local engine oil may comprise a particular quantity of oil in the tribological interfaces (between surfaces), or in surfaces to be cooled, or in an area where the engine oil is doing mechanical work of or for a specific component(s), system and / or sub-system. Examples of local engine oil include engine oil in a main crankshaft bearing, engine oil on the cylinder bore, the engine oil film between the cylinder bore and piston ring or piston skirt, or the engine oil in the piston cooling gallery. Examples of engines that may use engine oil for lubrication and, optionally, other functions include an engine in a vehicle, an engine in a work machine or work equipment, a marine propulsion engine, a locomotive engine, an engine used in a stationary or mobile or towable configuration for electrical power generation, an engine used in a stationary or mobile or towable configuration to support petroleum industry functions, an engine used on a vehicle or marine vessel for auxiliary power, an industrial machine, or other engine applications During operation of the engine, the engine oil is exposed to various conditions within the engine. The engine oil evolves over time, such that changes occur in the engine oil. The changes to the engine oil may include one or more of physical changes, chemical changes and contamination. Physical changes might include changes to the engine oil viscosity, such as those due to permanent shear thinning, or other physical changes to the engine oil. Chemical changes might include chemical reactions in the engine oil, and / or chemical species being formed or consumed. Contamination increasing or decreasing, or deposits forming in the engine might result in changes to the engine oil. As noted, the term “engine oil” may refer to both a base engine oil (or base stock) and any additives, if present. The changes to the engine oil may comprise changes to the engine oil itself and / or changes to the additives, if additives are present. The engine oil performance evolves over time. The engine oil performance may be the performance of the engine oil in relation to one or more of its functions, including but not limited to lubrication. According to certain embodiments of the present disclosure, a method of characterising evolution of engine oil during operation of a combustion engine is provided, wherein the engine oil is configured to lubricate the combustion engine. The engine oil may be further configured to perform other functions, as described elsewhere herein. Engine oil passes through an engine, such that engine oil passes through or near to various engine subsystems or resides in various engine subsystems. An engine system, subsystem or component may expose engine oil that is in proximity to said system, subsystem or component to certain local conditions that result in chemical reactions of the engine oil, contaminants to appear (as a result of wear materials contaminating the engine oil, for example), or other changes to the engine oil to occur. In other words, the engine oil evolves in a certain manner due to being exposed to certain local conditions within the engine. An engine system, subsystem or component may expose engine oil to certain local conditions in an event that the engine oil passes through or by the system, subsystem or component, or in an event that the engine oil resides in the system, subsystem or component. Engine oil may evolve when exposed to consistent local conditions. In other words, a change to engine oil does not necessarily represent a change in local conditions to which the engine oil is exposed. However, evolution of the engine oil may depend on the local conditions to which the engine oil is exposed, so a change to the engine oil may represent a change in local conditions to which the engine oil is exposed. A change in performance of an engine system, subsystem or component may alter the local conditions to which the engine oil passing through or residing in the engine system, subsystem or component is exposed. Since evolution of the engine oil may depend on the local conditions to which the engine oil is exposed, changes to the performance of an engine system, subsystem or component may affect the evolution of engine oil passing through or residing in the engine system, subsystem or component. Certain changes to engine oil may represent changes in performance of the engine system, subsystem or component. Other changes to engine oil may not represent changes in performance of the engine system, subsystem or component. The engine oil may pass through different engine subsystems at different mass flow rates, such that the engine oil is in proximity to different engine subsystems for different lengths of time. Furthermore, the engine oil may follow different routes through the engine. A plurality of routes may be followed by the engine oil, with the engine oil splitting such that a given time the engine oil follows a plurality of different routes through the engine. For example, a quantity of the engine oil may follow a first route through the engine, while a sub-quantity of engine oil may diverge from this route. The sub-quantity of engine-oil may return to the first route or may mix with the rest of the engine oil in the oil pan or sump. Certain engine systems, subsystems or components may rely on the state or performance of the engine oil that is passing through or by said system, subsystem or component. For example, certain engine systems, subsystems or components may rely on rheological performance of the engine oil (wherein the engine oil forms films of oil to separate adjacent surfaces) or tribochemical performance of the engine oil (wherein the engine oil forms protective films on a surface, and performs other functions), and / or on other functions of the engine oil such as cleaning, cooling, inhibiting corrosion, and neutralising acids from combustion. The performance of an engine system, sub-system or component may be affected by the performance of the engine oil. Evolution of engine oil in a certain engine system, subsystem or component may affect other engine system(s), subsystem(s) or component(s) which rely on engine oil performance. In other words, oil evolution that is caused or driven by interaction of the engine oil with a first engine component, sub-system or system may alter the bulk oil chemistry of the engine oil, especially for abnormal oil evolution. The new bulk oil chemistry may result in changes of performance of the engine oil that impact the interaction of the engine oil with a second engine component, sub- system or system and that therefore impact the performance of the engine oil in the second engine component, sub-system or system. For example, the new bulk oil chemistry of the engine oil may result in changes in performance of the engine oil that results in abnormal performance of the second engine component, sub-system or system, such as increased wear or damage, to the second engine component, sub-system or system. Changes to the performance of the engine oil may affect the health of engine components, subsystems or systems that are reliant on the performance of the engine oil. The change to the health of the engine components, subsystems or systems may, in turn, change the conditions to which the engine oil is exposed in those components, subsystems or systems. The engine oil will continue to evolve based on those changed conditions, which may further affect the engine components, subsystems or systems, and so on. Changes to the performance of the engine oil due to exposure to certain conditions in a particular engine component, subsystem or system may affect the health of that engine component, subsystem or system and / or the health of a different engine component, subsystem or system. Evolution of the engine oil may result in decreased performance of the engine oil in some respects. Evolution of the engine oil may result in increased performance of the engine oil in other respects. In an example, engine oil that is further along an evolution pathway may have improved capability to control wear in regions of mixed friction regime, or improved load bearing capability in regions such as engine bearings. Health of an engine or engine components, subsystems or systems may be reflected by the performance or operation of the engine. Good engine health may be reflected by good or acceptable overall engine performance; reliable engine performance or a lack of reliability issues; and a lack of abnormal wear of engine components, wherein abnormal wear may comprise accelerated wear or a high magnitude of wear. Poor engine health may be reflected by fair to unacceptable overall engine performance or operational. For example, the engine may be functional but with performance that is below expected performance. The engine components may be suffering from abnormal wear. A system or component of the engine may be undergoing initial or intermediate phases of a component or system failure or performance degradation. Specific evolution pathways of the engine oil can be associated with proximity to specific engine components, subsystems or systems, or to specific operating conditions within those engine components, subsystems or systems. Engine oil may be considered to be in proximity to an engine subsystem when the engine oil is exposed to certain conditions by that engine subsystem. For example, the engine oil may pass through, by or near to said subsystem, or may reside near to the subsystem such that the engine oil is exposed to certain engine conditions as a result of its proximity to the subsystem. The evolution pathways may comprise changes to the engine oil. These changes may include one or more of physical changes, chemical changes and contamination. Physical changes might include changes to the engine oil viscosity, such as those due to permanent shear thinning, or other physical changes to the engine oil. Physical changes might be referred to as mechanical changes. Chemical changes might include chemical reactions in the engine oil, and / or chemical species being formed or consumed. Contamination increasing or decreasing, or deposits forming in the engine might result in changes to the engine oil. Engine oil evolves throughout a service interval in a diesel engine. As noted previously, these changes can include chemical changes and / or physical changes. The chemical changes may be due to chemical reactions taking place, chemical changes due to contamination, a combination of these. Physical changes may include changes to the oil viscosity due to temporary or permanent shear thinning. The causes of this engine oil evolution throughout the service period may be due to interaction of the engine oil with the environments within the combustion engine to which the engine oil is exposed. As noted, mechanisms by which the engine oil changes may include chemical evolution, contamination, and mechanical evolution. Engine oil may chemically react in high temperature environments, leading to chemical evolution. An example of this includes the oxidation cycle, in which oil is exposed to high temperatures within an oxygen rich environment in the engine causing oxidation of certain molecules. Contaminants may become mixed with the engine oil, changing the overall composition of the engine oil. The addition of contaminants may not react with the engine oil molecules (wherein the engine oil molecules comprise the molecules of the initial formulation of engine oil including any additives, either in their new state or in an evolved state) and may simply affect the overall composition of the engine oil. Or, the contaminants may react with the engine oil molecules. Contaminants can catalyse and / or inhibit chemical reactions, and / or mechanically affect the lubrication & wear of surfaces in motion. Contaminants in a combustion engine can include, for example, soot, fuel, water, coolant or NOxgases. Mechanical evolution may occur in an event that molecules of the engine oil temporarily or permanently change shape or shear. Mechanical evolution may result in changes to engine oil viscometrics and / or to the rheological response of the engine oil. In an example, mechanical changes to molecules may occur in an event that larger additives, such as Viscosity Index Improvers (VII) are sheared between two surfaces of the engine. Evolution factors such as chemical evolution, contamination and mechanical evolution can be interrogated by analytical chemistry techniques and by measuring properties such as viscosity. Spectroscopy (or spectrometry) is one such technique that can be used to evaluate how the engine oil has evolved. For example, spectroscopy may be used to measure certain aspects of the evolving chemical composition, by providing information as to evolution of engine oil chemistry and / or addition and subtraction of engine oil additives or engine contaminants through engine oil consumption. As such, spectrometry may provide a fingerprint of the engine oil. Fingerprints of engine oil in different states may be used to show engine oil evolution. Differences in the fingerprints may correspond to changes to chemical and / or physical properties of the engine oil. In the present application, a nuanced analysis is provided by looking at a larger fingerprint (rather than, for example, changes to one or two peak heights or changes to one or two variables) to identify and analyse changes across the engine oil chemistry and physical properties. This allows, for example, certain chemical evolution changes to be linked to certain changes in contamination. The Applicants have determined that there are numerous factors that influence engine oil evolution in a combustion engine. These may include, but are not limited to, (a) a local temperature to which the engine oil is exposed within a component, sub-system or system of the combustion engine; (b) contamination of many forms to which the engine oil is exposed; (c) surface area between the engine oil and any contaminants; and (d) contact area between the engine and the engine oil in varying friction regimes. As noted, contaminants may be produced by the combustion engine and may include gaseous or solid combustion byproducts or component wear material. Since engine factors such as these directly influence the way in which the engine oil evolves, the engine oil evolution determined herein may be used to analyse engine performance and / or engine health. Changes to the engine oil may be evident in an analysis of chemistry of the engine oil, such as a spectral analysis of the engine oil. As an example, a spectral analysis of engine oil might provide information relating to the chemical species within the engine oil or to the chemical bonds that are present or that have formed or have broken within the components of the engine oil. The changes that are shown in a chemical analysis of the engine oil may include one or more of physical changes, chemical changes and contamination. An evolution pathway of engine oil including one or more of physical changes, chemical changes and contamination may therefore be interrogated via chemical analysis. A vector or vectors in which engine oil chemically and / or physically evolves may, therefore, be used to determine engine operation. Engine operation may include how the engine has been used and / or engine health. Separately, evolution of engine oil may be used to determine engine oil performance. The characterisation of the evolution of the engine oil may include “normal” and “abnormal” evolutions of the engine oil. The normal and abnormal evolution of engine oil may be specific to the specific engine components, subsystems or systems in which the oil resides. The normal and abnormal evolution of engine oil may differ for engine oil residing in different engine architectures. The normal and abnormal evolution of engine oil may be indicative of normal and abnormal health, respectively, of engine components or subsystems. In other words, particular changes to the engine oil may correlate to particular changes to the engine. Abnormal evolution of engine oil may indicate damage or issues with engine components or subsystems, before the damage is observed at an engine level. The normal and abnormal evolution of engine oil may be indicative of normal and abnormal operation, respectively, of engine components or subsystems. Normal engine operation may comprise expected levels of one or more of overall engine performance; oil consumption; oil physical properties; chemistry of the engine oil; contamination of engine oil; and magnitude and rate of component wear. In this context, “expected” may mean within limits, between which the performance of an engine, component, system or sub- system may be considered acceptable. For example, the engine, component, system or sub-system may be considered to meet customer requirements of engine, machine or application performance or lifespan. Abnormal engine operation may comprise performance outside the expected limits. Changes to the engine oil are caused by conditions or states to which the engine oil has been exposed and that have caused chemical reactions to occur, contaminants to form, or other changes to the engine oil to occur. The conditions or states to which the engine oil is exposed are defined by the engine operation. Changes to the engine oil may be indicative of changes to the engine itself, including but not limited to changes to the physical state of the engine components or systems (such as wear) or changes to the engine operating conditions (such as speed, load, etc.). The conditions to which engine oil is exposed may vary between engine subsystems and components, so engine oil in or near to a particular engine subsystem or component is exposed to a local condition. This causes local engine oil evolution. However, larger quantities of the engine oil circulate through the engine than is found at a given moment in a local area. The engine oil may return to an oil pan or sump and may be considered as a substantially homogeneous mixture within the oil pan, wherein the engine oil in the oil pan is a mixture of the engine oil that has passed through the various engine components, systems and sub-systems. The engine oil in the oil pan or sump may be referred to as bulk engine oil. The bulk oil may be approximately homogeneous, but is not static while in the engine. The engine oil is transported from the sump via the oil pump and distributed into the lubrication system continuously and repeatedly while the engine is in operation. The engine oil in the lubrication system is distributed to specific locations within the engine. After passing through a local component, sub-system or system in the engine, the engine oil flows into the sump. In the sump, the engine oil flowing from the local area is mixed to re-establish a new bulk oil. Bulk engine oil may further describe engine oil that is added to the engine lubrication system at fill, and the engine oil within the engine lubrication system before the engine is operated. Once the engine begins to be operated, the bulk engine oil is made up of engine oil that has passed through local components, sub-systems or systems of the engine. At a given moment, engine oil in different local components, sub-systems or systems of the engine may be exposed to different local conditions, such that the engine oil in different local components, sub-systems or systems of the engine undergoes different local engine oil evolutions. The bulk engine oil is a mixture of engine oil that has passed through local components, sub-systems or systems of the engine, and therefore the bulk engine oil has a bulk chemical evolution that is a consequence of various local engine oil evolutions. Since the engine oil may follow different routes through the engine, and may have different mass flow rates through different engine subsystems, the bulk engine oil evolution is a complex combination of the various local engine oil evolutions. Bulk oil evolution describes the changes over time of the bulk engine oil. These changes may include one or more of physical changes, chemical changes and contamination. As the bulk engine oil is made up of engine oil received from local areas, the bulk oil evolution is affected by the evolution of engine oil within local areas of the engine. Local oil evolution may comprise changes over time to the local engine oil, while the engine oil is within a component interface or within a system or sub-system. The changes may include one or more of physical changes, chemical changes and contamination. The local oil evolution within a component interface or within a system or sub-system may be affected by the local residence time for that component interface or system or sub-system. The local residence time is an average length of time that a particular quantity of oil remains in a particular local area, such as a component interface or system or sub-system. The local area may, for example, be within lubrication surfaces of a system or sub-system or within a system or sub-system. The particular quantity of oil may be in motion within that local area during the local residence time. In certain embodiments, the local residence time may be an average length of time that a droplet of oil remains in a particular local area, or an average length of time that a cubic millimetre of oil remains in a particular local area. The local residence time may be defined for any system, sub-system or component interface. For example, a local residence time may apply to an oil pan or sump, an oil passage of a cylinder block, a bearing, or other system or subsystem. In a certain example, a local residence time for a bearing may be the average length of time that a particular quantity of oil remains in a clearance volume of a bearing. A clearance volume of a bearing comprises a volume between the bearing and a component, wherein the volume is configured to be filled with engine oil. The component may, for example, be a journal (shaft) or other component. The engine oil between the bearing and the component may be a film of engine oil, such that the bearing and component are separated by the film of engine oil. In a particular example, the bearing may have a hollow cylindrical shape such that an inner diameter of the bearing may contact engine oil or a component. Said component may be cylindrical and pass through the bearing such that an outer diameter of the component may contact engine oil or the inner diameter of the bearing, wherein the clearance volume has a shape of a thin- walled cylinder between the bearing inner diameter and the component outer diameter. The local residence time for said bearing may be the average time taken for the particular quantity of oil to enter and exit the clearance volume of a bearing. The local residence time may differ between different components, systems or subsystems. The local residence times may differ between directly and indirectly fed components, systems or subsystems. For example, the local residence time of engine oil on the cylinder wall may be significantly longer than the local residence time in the piston cooling gallery. The local residence time on the piston cooling gallery may be longer than the local residence time of main or rod bearings. In certain examples, the local residence time of engine oil on the cylinder wall may be in the order of minutes. In certain examples, the local residence time of engine oil on the piston cooling gallery may be in the order of tenths of a second. In certain examples, the local residence time of engine oil in the main or rod bearings may be in the order of hundredths of a second. However, these values are examples only. The local residence times may vary from these examples. The local residence time of engine oil in an engine component, subsystem or system may depend on a mass flow rate of engine oil through the engine component, subsystem or system. As discussed above, specific evolution pathways of the engine oil can be associated with proximity to specific engine components, subsystems or systems, or to specific operating conditions within those engine components, subsystems or systems. The rate of evolution of engine oil as a result of proximity to or operating conditions within specific engine components, subsystems or systems may be affected by mass flow rate of engine oil though the engine components, subsystems or systems. For example, engine components, subsystems or systems that have a higher mass flow rate of engine oil may cause a higher rate of evolution of engine oil than engine components, subsystems or systems that have a lower mass flow rate of engine oil. A higher mass flow rate through a particular engine component, subsystem or system may result in a higher rate of evolution of engine oil than a lower mass flow rate through that engine component, subsystem or system. However, other examples are possible. For example, an engine component, subsystem or system that has a relatively low mass flow rate of engine oil may cause a relatively high rate of evolution of engine oil. According to certain embodiments of the present disclosure, there is provided a method of characterising evolution of engine oil during operation of a combustion engine, wherein the engine oil is configured to lubricate the combustion engine. The method comprises obtaining more than one sample of engine oil at step 110, wherein each sample of engine oil has been exposed to one of a plurality of operational conditions of the combustion engine. At step 120, the method further comprises using spectrometry to analyse elemental composition and / or chemical bonds of the more than one sample of engine oil to provide a spectral analysis 121, 122, 123 of each sample. At step 130, the method further comprises using multi-dimensional analysis to interrogate variation between the spectral analyses of the more than one sample, wherein the variation characterises an evolution pathway of the engine oil. It is noted that three spectral analyses 121, 122 and 123 are shown, but this is for illustrative purposes only and any number of samples may be obtained and analysed to provide any number of spectral analyses. The variation between the spectral analyses may be expressed as vectors of changes. A particular evolution pathway of engine oil may be expressed by at least one vector of change. An evolution pathway may be expressed by a set of vectors of change. The set of vectors of change may provide a reference frame for the evolution pathway, wherein engine oil at a certain point along the evolution pathway is a certain distance along each vector of change. The characterisation of an evolution pathway obtained by the method may provide the set of vectors of change. In use of the characterisation, it may be determined which evolution pathway engine oil in an engine is currently following. Magnitudes or values of the vectors of change may be identified for used engine oil to determine how far along the evolution pathway the engine oil has progressed. The samples may be taken from a volume or volumes of engine oil that have been exposed to operational conditions. Or, the samples may remain part of a volume of volumes of engine oil that have been exposed to operational conditions. Each operational condition may be identified by one or more than one engine variable. The one or more engine variable may comprise a temperature to which engine oil is exposed. The one or more engine variable may comprise chemistry of at least one contaminant to which the engine oil is exposed. A contaminant might comprise a gas (such as NOx), particles, soot, a metal (including a metal surface and / or metal particles), or other contaminant. The one or more engine variable may comprise concentration of at least one contaminant to which the engine oil is exposed. The one or more engine variable may comprise a surface area of an interface between the engine oil and one or more contaminants (such as a bubble size, area of metal, particle size, and so on). An operational condition may be identified by the one or more than one engine variable, or the operational condition may be identified by at least one simplified metric obtained from more than one engine variable. The simplified metric may identify a region of interest of the operational condition. For example, the simplified metric may identify a variable or type of variable that is particularly relevant to engine oil evolution. In an example, for a particular engine operational condition, it might be the temperature that has the most effect on engine oil evolution. In another example, it might be the concentration of a particular contaminant that has the most effect on engine oil evolution. For each evolution pathway of the engine oil, the samples may have been exposed to operational conditions having at least one similar engine variable. For example, a plurality of engine oil samples may be exposed to operational conditions having particular magnitudes of certain engine variables for different lengths of time. Or, a plurality of engine oil samples may be exposed to operational conditions described by certain engine variables, wherein each sample has been exposed to an operational condition with a different magnitude of those engine variables. In other words, each sample may be taken from engine oil that has been exposed to operational conditions that are at a different point along an engine vector, where the engine vector may comprise any engine variable or time. Variation between spectral analyses of samples on the same evolution pathway may be analysed. Variation between spectral analyses of samples on different evolution pathways may be analysed. More than one evolution pathway may be characterised by exposing samples to different operational conditions. The method may comprise obtaining more than one sample of engine oil for each of more than one set of samples. Each sample of engine oil may have been exposed to one of a plurality of operational conditions of the combustion engine, where each operational condition is identified by one or more than one engine variable. Each sample from a set of samples may be on the same evolution pathway, at different points along the evolution pathway. Each set of samples may represent a different evolution pathway. Spectrometry is used to analyse elemental composition and / or chemical bonds of the more than one sample of engine oil to provide a spectral analysis of each sample. For each set of samples, multi-dimensional analysis may be used to interrogate variation between the spectral analyses of the more than one sample of the set of samples with respect to one or more than one engine variable. The variation is associated with the one or more engine variable of the operational condition to which the sample was exposed, and with the evolution pathway of the set of samples. For each set of samples, an evolution pathway of the engine oil is characterised. A set of samples may each be exposed to one of a certain set of operational conditions wherein the set of operational conditions are indicative of using a combustion engine in a certain manner over a certain period of time. In this way, an evolution pathway of engine oil used in a combustion engine that is operated in that manner may be characterised. Evolution pathways may be characterised for different operational conditions. Normal evolution pathways may be characterised for operational conditions that are indicative of a combustion engine operating normally. Abnormal evolution pathways may be characterised for operational conditions that are indicative of a combustion engine operating abnormally. The method may comprise obtaining a plurality of samples of engine oil, each sample having been exposed to one of a plurality of operational conditions of the combustion engine. The plurality of samples of engine oil may be on a variety of engine oil evolution pathways. Spectrometry may be used to analyse elemental composition and / or chemical bonds of each sample of engine oil to provide a spectral analysis of each sample. Multi- dimensional analysis may be used to interrogate variation between the spectral analyses of plurality of samples. The variations may be clustered to identify any outliers. One or more vectors of normal change may be identified that describe the variations excluding the outliers. These vectors of normal change may describe normal engine oil evolution pathways. One or more vectors of abnormal change may be identified that describe the outliers. The vector(s) of abnormal change may be interrogated to determine the reason for the abnormal engine oil evolution, for example by looking at the operational conditions to which the samples that are represented by the outliers were exposed. The outliers may be removed from the data, and the data may be clustered once more to identify any other outliers. In this way, abnormal engine oil evolutions may be iteratively removed from the variations. The remaining variations may represent normal engine oil evolution. The spectral analysis may comprise a plurality of engine oil variables extracted from spectral data. The engine oil variables may represent physical properties of the engine oil (such as elemental composition, information regarding the chemical bonds in the molecules of the engine oil, oxidation, nitration, sulfation, and so on), or the engine oil variables may be simplified metrics that are indicative of physical properties of the engine oil but do not necessarily map one-to-one onto physical properties. In certain embodiments, the multi- dimensional analysis interrogates variation between the plurality of engine oil variables to provide at least one component that is indicative of a difference in elemental composition and / or chemical bonds between the more than one sample. The more than one sample may be on the same evolution pathway. Or, the more than one sample may be on different evolution pathways. The spectral analysis may comprise spectral data. The number of vectors of change output from the multi-dimensional analysis may be fewer than the number of engine oil variables of the spectral analyses. The multi-dimensional analysis may be achieved via dimension reduction. Each vector of change may be indicative of a proportion of a total variation in spectral analyses. In this way, a sum of the vectors of change provides the total variation in spectral analyses. Each vector of change may be indicative of a variation of more than one engine oil variable. Certain proportions of each vector of change may be indicative of a variation of certain different engine oil variables, such that variation in more than one engine oil variable is represented by a given vector of change. Different vectors of change may represent the different engine oil variables in different proportions. For example, a first proportion of a first vector of change may be indicative of a variation of a first engine oil variable, a second proportion of the first vector of change may be indicative of a variation of a second engine oil variable, and so on. A third proportion (different from the first proportion) of a second vector of change may be indicative of a variation of the first engine oil variable and a fourth proportion (different from the second proportion) of the second vector of change may be indicative of a variation of the second engine oil variable, and so on. In certain examples, a given vector of change may comprise a proportion that is indicative of each of the plurality of engine oil variables such that variation in all the engine oil variables is represented by a given vector of change. Spectrometry is used to analyse elemental composition and / or chemical bonds of the engine oil samples. Elemental composition may describe a concentration of any vector of change of the engine oil, including concentrations of elements, molecules or compounds that would be found in the initial engine oil formulation and also any contaminant found in the engine oil after use of the engine oil. The initial engine oil formulation is the formulation of “new” engine oil that has not been used in an engine or otherwise exposed to operational conditions. Analysis of chemical bonds of the sample may include any spectral analysis that provides information as to the chemical bonds present in the molecules present in the samples. For example, FTIR or Raman spectroscopy each provide information relating to the chemical bonds present. analysis of an FTIR or Raman spectrum may permit analysis of the same chemical bond (such as carbon-oxygen) in different molecules, and provide information as to the evolution of the engine oil. Analysis of the chemical bonds can provide information both relating to composition of the sample, and to processes such as oxidation, sulfation and nitration that vectors of change of the sample may have undergone. Using spectral analysis to analyse elemental composition and / or chemical bonds of the sample may comprise using more than one spectrometry type to provide a spectral analysis of the sample. In other words, a spectral analysis of a sample may comprise outputs from more than one spectrometry type. A spectrometry type may include, for example, FTIR, Raman or ICP-AES spectrometry, or another type of spectrometry. Each sample may be further analysed to provide oil viscometric information, or another physical property of the engine oil. The oil viscometric information may be provided as a function of engine oil temperature, or engine oil shear rate, or other parameter. Multi- dimensional analysis may be used to interrogate variation between the spectral analyses and the viscometric information of the more than one sample. The spectral analysis of each sample may be processed based on viscometric information, wherein multi-dimensional analysis is then used to interrogate variation between the processed spectral analyses. The processed spectral analysis may comprise a spectrum or spectral features as a function of viscosity. The spectrum or spectral features as a function of viscosity may be provided for a specific engine oil temperature. For example, a processed spectral analysis may comprise a maximum of a peak of a spectrum as a function of viscosity at a specific temperature of the engine oil. The processed spectral analysis may comprise a spectrum or spectral features normalised to a change in viscosity at a specific condition. The processed spectral analysis may comprise a moment of a spectrum normalised to a change in viscosity at a specific condition. The processed spectral analysis may comprise a ratio of two or more peaks of a spectrum, normalised to a viscosity of the engine oil at a certain condition (such as a certain shear rate). A moment of a spectrum or distribution may be a quantitative measure of the spectrum that is indicative of a shape and / or a spread and / or an average of the spectrum. For example, a moment of a spectrum may be a mean, a variance or standard deviation, a skewness, or kurtosis. The following description describes in more detail how the samples may be exposed to operational conditions, how the samples may be analysed, and how the variation between samples may be interrogated. Other examples are also provided for certain steps of the method, such as how variation may be determined. According to certain embodiments of the present disclosure, a method of characterising evolution of engine oil during operation of a combustion engine is provided, wherein the engine oil is configured to lubricate the combustion engine. With reference to Figure 2, at step 210 the method comprises using a first spectrometer to analyse a primary sample of a first volume of the engine oil and provide a primary spectrum 211 of the first volume of engine oil. The primary sample may be a sample of bulk engine oil. The method further comprises exposing the first volume of engine oil to a first operational condition (or conditions) of the combustion engine at step 220. At step 230 the first spectrometer is then used to analyse a secondary sample of the first volume of engine oil after the volume of engine oil has been exposed to the first operational condition, and provide a secondary spectrum 231 of the first volume of engine oil. The primary spectrum 211 and the secondary spectrum 231 of the first volume of engine oil are compared at step 240, to determine one or more differences between the primary spectrum and the secondary spectrum wherein the one or more differences are indicative of a change in the first volume of engine oil. As discussed above, a change in the first volume of engine oil may include one or more of a physical change, chemical change or contamination. Physical changes might include changes to the engine oil viscosity, such as those due to permanent shear thinning, or other physical changes to the engine oil. Chemical changes might include chemical reactions in the engine oil, and / or chemical species being formed or consumed. Contamination increasing or decreasing, or deposits forming in the engine might result in changes to the engine oil. The method further comprises associating the one or more differences with the first operational condition at step 250. In other words, spectral analysis may be used to analyse samples of engine oil that have been exposed to operational conditions. In this way, boundary conditions to which engine oil is exposed by engine operation may be determined. Spectral analysis may be carried for an engine oil sample after the engine oil sample has been exposed to engine boundary conditions or engine operational conditions. The engine oil sample may be exposed to engine operational conditions either on engine or off engine. The spectral analysis is used to generate a fingerprint of how the engine oil evolves when exposed to particular engine operational conditions. The spectral analysis may be used to generate the fingerprint by itself, or in combination with other measured chemical or physical properties of the engine oil sample. The operational condition to which the difference or differences in the spectra are associated may include any parameter of the engine or use of the engine. For example, an operational condition may comprise one or more engine operating parameter such as rotational speed, load or temperature. An operational condition may comprise engine oil mass flow and / or residence time of engine oil. A local operational condition may comprise any parameter associated with an engine component, subsystem or system, such as temperature, engine oil mass flow through the engine component, subsystem or system, or local residence time of the engine oil within the engine component, subsystem or system. The method may further comprise exposing samples of engine oil to various operational conditions. In this way, a reference characterisation may be built up that associates different engine oil evolution pathways with different operational conditions. The engine oil evolution pathways may be associated with different local operational conditions. Bulk engine oil may comprise engine oil that has been exposed to different local operational conditions (and that has, therefore, evolved at local interfaces). Isolating the engine oil evolution of a particular local operational condition therefore allows a spectrum of bulk engine oil to be analysed to identify the different local operational conditions to which that engine oil has been exposed. As discussed, chemical and physical properties of engine oil are a function of engine operating conditions. Engine operating conditions may, in turn, be a function of engine health. Associating an operational condition to an evolution of the engine oil means that analysis of a sample of engine oil may be used to identify health of the engine, a system of the engine, a subsystem of the engine, a component of the engine, or the engine oil itself. The method described herein allows an expected engine oil evolution to be obtained for bulk engine oil that has evolved at local interfaces, by associating engine oil evolution to local environment variables such as combustion gas composition, local temperature, or local pressure and / or other local engine parameters and / or bulk engine parameters. The resulting engine oil evolution may comprise multivariable vectors, wherein the way in which the engine oil changes provides information as to the engine operation. By obtaining an expected engine oil evolution for particular engine operational conditions by the method outlined herein, any deviation of actual engine oil evolution from expected engine oil evolution may be identified. A deviation of actual engine oil evolution may be linked to a particular local engine state or local engine operational condition, using the results of the method described herein. The following description provides more detail as to how the engine oil may be exposed to operational conditions, and so on. The description focusses on a first volume of engine oil exposed to a first operational condition, from which primary and secondary samples may be obtained. However, the description may apply to any volume or sample of engine oil that is exposed to operational conditions. For example, the following may be read with any volume or sample of engine oil in place of “the first volume” and any operational condition in place of “the first operational condition”. A volume (such as the first volume) of engine oil may comprise any quantity of engine oil that is exposed to an operational condition (such as the first operational condition). The volume may be of constant or varying quantity. The volume of engine oil may be exposed to the operational condition for any length of time. For example, the operational condition may include an oil volume of engine oil residing in or passing through an engine, or a component, subsystem or system of an engine for a particular length of time. The volume of engine oil may be exposed to the operational condition in a combustion engine, or the volume of engine oil may be exposed to an experimental condition corresponding to the operational condition or conditions of the combustion engine. Where primary and secondary samples are taken, the first volume of engine oil may be exposed to the first operational condition or conditions of the engine operation between the sample times at which the primary and secondary samples are taken from the first volume of the engine oil. For example, a first volume of engine oil may comprise the engine oil that is in an engine. A primary sample of engine oil taken from an engine at a first time, and analysed to provide a primary spectrum. The engine oil remaining in the engine may then be exposed to operational conditions while the engine is in use. After a period of use of the engine, a secondary sample of engine oil may be taken from an engine at a second time, and analysed to provide a secondary spectrum. Otherwise, a sample of engine oil may simply be taken from a volume of engine oil that has been exposed to an operational condition. A sample may or may not have been taken from that volume of engine oil prior to the volume of engine oil being exposed to the operational condition. The primary and secondary samples may be taken from the first volume of engine oil. The primary sample may be taken from the first volume of engine oil before the first volume of engine oil has been exposed to the first operational condition, and the secondary sample may be taken from the first volume of engine oil after the first volume of engine oil has been exposed to the first operational condition. The primary sample may be analysed prior to exposing the first volume of the engine oil to the first operational condition and the secondary sample may be analysed after exposing the first volume of the engine oil to the first operational condition, or both the primary and secondary samples may be analysed after the exposing the first volume of the engine oil to the first operational condition. Otherwise, the primary and secondary samples may remain in the first volume of engine oil, and may be analysed by the first spectrometer while remaining in the first volume of engine oil. The primary sample may be analysed before the first volume of engine oil has been exposed to the first operational condition, and the secondary sample may be analysed after the first volume of engine oil has been exposed to the first operational condition. An operational condition (such as the first operational condition of step 220) may be achieved by any method that can be used to implement the operational condition(s), or to replicate the operational condition(s), or to achieve an experimental condition(s) corresponding to the operational condition(s). In an example, an experimental condition may be achieved using an experimental rig configured to replicate conditions found within one or more subsystems, systems or components of an engine. As used herein, an experimental rig may be any experimental equipment or laboratory equipment, such as bench-top test equipment. In another example, the operational condition may be achieved using an engine. The volume of engine oil may be distributed within the lubrication system of the engine. A sample taken from the volume of engine oil may comprise a sample of bulk engine oil. The samples of bulk engine oil may be removed from the engine after the engine oil has passed through an oil pump but before the engine oil has passed through the oil filter. Alternatively, the samples of bulk engine oil may be removed from the engine after of bulk engine oil may be removed from the engine. At this point the engine oil comprises bulk oil and is under pressure, so will flow into a collection container. A sample of bulk engine oil may also be removed from the sump or oil pan, which may require use of a vacuum or other extraction device. Where the operational condition is achieved using an engine, the engine may or may not be used to drive equipment. In an example where the operational condition is achieved using an engine or cell that is not used to drive equipment, the operational condition may be achieved using a test engine or test cell to implement experimental conditions(s) corresponding to the operational condition(s), for example using a dynamometer. The test engine or cell does not drive equipment. In this context, driving equipment refers to using the engine to drive any equipment, machine or vehicle in an end use application or in a customer application configuration. Driving equipment does not, in this example, refer to: a dynamometer; any test device used to either measure torque or speed of the engine; or any test device used to provide simulated loading of the engine. In an example where the operational condition is achieved using an engine that is used to drive equipment, the operational condition may be achieved by using an engine to drive equipment such that the operational condition(s) is / are implemented. Alternatively, the operational condition may be identified by using an engine to drive equipment and using data relating to engine use to identify the operational condition(s). A sample taken from the volume of engine oil used in the engine comprises a bulk engine oil sample. In this context, equipment may refer to any equipment, machine or vehicle driven by the engine. The engine may be used for propulsion or in a stationary configuration. Using an engine to drive equipment may mean using an engine in an end use or in a customer application configuration. The end use may be any application or function for which an engine is designed or used. A customer may be a customer of the engine manufacturer. A customer application configuration may be used at a site or location of said customer. An application configuration may refer to any equipment, machine or vehicle driven by the engine. The end use of the engine or a customer application configuration of the engine may include any of an engine in a vehicle, an engine in a work machine or work equipment, a marine propulsion engine, a locomotive engine, an engine used in a stationary or mobile or towable configuration for electrical power generation, an engine used in a stationary or mobile or towable configuration to support petroleum industry functions, an engine used on a vehicle or marine vessel for auxiliary power, an industrial machine, or other engine applications. In other words, equipment driven by the engine may include any of a vehicle, a work machine or work equipment, a marine vessel, a locomotive vehicle, an electrical power generator, an auxiliary power provider on a vehicle or marine vessel, an industrial machine, or any other engine application. These examples of methods for obtaining an operational condition will be discussed in more detail below. At step 240, the primary spectrum 211 and the secondary spectrum 231 of the first volume of engine oil are compared. The one or more differences determined between the primary spectrum 211 and the secondary spectrum 231 of the first volume of engine oil may comprise one or more of peak (and / or trough) height, peak (and / or trough) location and peak (and / or trough) shape differences. The one or more differences may comprise differences to one peak (or trough) or to multiple peaks (and / or troughs). In certain embodiments, the first volume of engine oil may be homogenous such that the composition, chemical properties and physical properties of the primary and secondary samples are substantially the same as those of the first volume of engine oil. In other embodiments, the first volume of engine oil may be inhomogeneous. The primary and secondary samples may be indicative of properties of the first volume of engine oil. Similarly, where multi-dimensional analysis is used to interrogate variation in spectral analyses for more than one sample, the variation may comprise one or more of peak (and / or trough) height, peak (and / or trough) location and peak (and / or trough) shape differences. The one or more differences may comprise differences to one peak (or trough) or to multiple peaks (and / or troughs). As an example of a homogeneous volume, a volume of engine oil may be a bulk oil volume, and so the spectra of any samples of the volume are indicative of bulk oil evolution. As described above, the bulk engine oil is made up of engine oil that has passed through local components, sub-systems or systems of the engine. The bulk evolution of the bulk engine oil is, therefore, a result of a combination of a plurality of local oil evolutions. Analysis of a volume of bulk oil may be used to identify local evolution of the engine oil in a particular component, sub-system or system. The features of the primary and secondary spectra may be used to isolate local oil evolution and local conditions to which the oil has been exposed. The engine may change over time, in which case the conditions to which the engine oil is exposed will change and the local oil evolution for a particular subsystem may change. The local engine oil evolution may be used to identify changes to the conditions to which the engine oil has been exposed, and therefore to identify changes to the engine. Changes to the engine may include changes to a component, sub-system or system. Changes to a component, sub-system or system may include physical changes, such as wear, and / or changes to performance of a component, sub-system, system or engine. The difference(s) between the primary spectrum and the secondary spectrum (or variations between spectral analysis of any samples of engine oil) are indicative of the conditions to which the engine oil has been exposed. The difference(s) between the spectra are associated with the first operational condition (step 250). This may entail associating the secondary spectrum with the first operational condition, and / or associating variances in spectra features (based on difference(s) between the primary spectrum and the secondary spectrum) with the first operational condition. This may entail associating an output of multi- dimensional analysis of variation between a spectral analysis of a sample of engine oil and at least one spectral analysis of at least one other sample of engine oil with the operational condition to which the sample was exposed. Associating a spectrum or variance with an operational condition may comprise recording the operational condition against the spectrum or variance, or labelling the spectrum and / or variance such that it is linked to the operational condition, or other method of associating a spectrum or variance with an operational condition. The operational condition may be identified by one or more than one engine variable. Other information may be associated with the operational condition (or with the secondary spectrum or variances in spectra features, or output of multi-dimensional analysis of variation between spectra analyses). Examples include but are not limited to: a mass flow rate of the engine oil; a length of time for which the engine oil has been exposed to the first operational conditions; a length of time for which the engine oil has been exposed to any engine conditions; data relating to engine operation (such as a percentage duty cycle, or other data); information relating to engine hardware; engine age; or other information. Metadata from the engine may be associated with an operational condition. An operational condition(s) (such as the first operational condition) may correspond to conditions that would be experienced by the engine oil within components or systems of the engine or in proximity to components or systems of the engine, while those engine components or systems are undergoing certain engine conditions. In an event that a volume of engine oil is exposed to an experimental condition corresponding to the operational condition(s), the experimental condition may aim to isolate operational condition(s) for particular components or systems of the engine, or may aim to correspond to operational condition(s) for multiple components or systems of the engine. Similarly, the experimental condition may aim to isolate a particular engine condition or may aim to correspond to more complex engine conditions. For example, the operational condition(s) may correspond to engine oil within a particular component or system of the engine, or to engine oil in proximity to or within multiple components or systems of the engine, or to engine oil within the engine as a whole. The operational condition(s) may correspond to the engine undergoing one particular engine condition, such as a steady state engine operation. The operational condition(s) may correspond to the engine undergoing multiple engine conditions. The operational condition may isolate one or more of the multiple engine conditions. For any given engine condition, the engine oil may experience multiple operational conditions. The operational condition(s) may correspond to conditions experienced under “normal” operation of a combustion engine. Alternatively, the operational condition(s) may correspond to conditions experienced under “abnormal” combustion operation of an engine, such as in an event that an engine component, sub-system or system is experiencing a failure mode or has suffered a failure mode. A failure mode may be an error, a defect, or any operational condition in which an engine component, subsystem or system is not performing as expected. A particular failure mode may be indicated by a unique signature of changes of the engine oil, which can be identified in the secondary spectrum (or spectral analysis of a sample that has been exposed to the failure mode). However, a change of the engine oil does not necessarily represent a change in performance of a component, subsystem or system of the engine. Examples of parameters that are influenced by engine operational conditions and that affect the evolution of engine oil may include temperature, pressure, engine oil volume, engine oil flow rate, final combustion products, interim combustion products, local engine oil chemical state, air or other gas concentrations, shear, friction, or other property of the engine. Combustion products may include nitrogen oxides, blowby gases, intermediary gases, soot, or other products. For example, the contact area between a gas and the engine oil may affect the engine oil, for example if the gas is in bubbles or a sheet. Changes in these parameters may be indicative of changes to certain engine components or to the operation of certain engine components. As described above, exposing a volume (such as the first volume) of engine oil to an operational condition (such as the first operational condition) or to an experimental condition (such as the first experimental condition) corresponding to the operational condition may comprise using experimental equipment, a test engine or test cell, or use of an engine to drive equipment. The volume of engine oil may comprise all of or a portion of the engine oil within the experimental equipment, test engine, or engine. A sample of engine oil may comprise portion of the volume of engine oil extracted from the volume of engine oil. In an event that primary and secondary samples are taken, the primary sample may comprise a portion of the first volume of engine oil that is extracted from the first volume of engine oil prior to exposing the first volume of engine oil to the first operational condition. The secondary sample may comprise a portion of the first volume of engine oil that is extracted from the first volume of engine oil after exposing the first volume of engine oil to the first operational condition. In other words, the analysis of samples may occur separately to the volume of engine oil, such that the analysis occurs separately to the equipment or engine used to expose the volume to the operational condition. Or, the samples may remain as part of the volume of engine oil during analysis. The analysis of the samples may occur in situ in the equipment or engine used to expose the volume to the operational condition. The analysis of a sample taken from a volume of engine oil prior to the volume being exposed to an operational condition may occur prior to exposing the volume to the operational condition. Alternatively, in an event that analysis of the sample occurs separately to the volume of engine oil, the analysis of the sample may occur after exposing the volume to the operational condition. The analysis of the primary and secondary samples may occur at any time and in any order. In an event that the analysis of sample(s) occurs separately to the equipment or engine used to expose the volume to the operational condition, the analysis may be performed on- site. Here, “on-site” refers to a location of the equipment or engine used to expose the volume to the operational condition, such as a laboratory, test site, work site or other location. Alternatively, the analysis may be performed off-site. The sample(s) may be extracted by any means from the equipment or engine used to expose the volume to the operational condition. In an event that the analysis of the sample(s) occurs in situ in the equipment or engine used to expose the volume to the operational condition, the sample(s) may remain part of the volume and the analysis of the sample(s) may be performed while the sample(s) remain in the equipment or engine used to expose the volume to the operational condition. An analysis device may be permanently fitted to the equipment or engine used to expose the volume to the operational condition, or may be a removable device that may be removably fitted to the equipment or engine used to expose the volume to the operational condition. In an example, an analysis device may be fitted to an engine oil circuit at a point at which the engine oil is pressurised, such that the engine oil flows through the analysis device. The engine oil may return to the engine oil circuit from the analysis device. The engine oil flowing through the analysis device may be analysed, for example via spectroscopy. The engine oil may be diluted in the analysis device, for example using a solvent. In a particular, non-limiting, example, the analysis device may comprise a microfluidic channel into which the engine oil flows, wherein the engine oil is analysed while in the microfluidic channel. Exposing a volume of engine oil to an experimental condition corresponding to the operational condition(s) may comprise using experimental equipment to replicate conditions found within one or more specific component, sub-system or system of the combustion engine. The experimental equipment may therefore be used to age or react the first volume of the engine oil as though it was being used only in that specific component(s), sub-system(s) or system(s) of the combustion engine under the first operational condition. Effectively, the experimental condition may isolate one or more component, sub-system or system of the combustion engine. Alternatively, the experimental equipment may be used to replicate conditions found within multiple components, sub-systems or systems, or to replicate conditions found within the engine as a whole. The experimental equipment may be configured to control one or more of the temperature, pressure, engine oil volume, engine oil flow rate, intermediate and final products of combustion, engine oil evolution, air or other gas concentrations, shear, friction, or other property of the engine. Gas flows may be controlled, and reaction rates may be measured. In certain embodiments, the experimental equipment may comprise a pressure burette, which may be used to measure reaction rates. The experimental equipment may comprise one or more of a stirrer, a condenser and a high pressure, high temperature beaker. The high pressure, high temperature beaker may be used to apply temperatures and pressures at and above room temperature and atmospheric pressure. Different gases may flow through the system, and the flows of those gases can be controlled. The experimental equipment may comprise components configured to shear the engine oil thereby introducing frictional forces into the engine oil, such as a high frequency reciprocating rig or a ball on disc tribometer. In certain embodiments, the experimental equipment may comprise more than one experimental rig, wherein the engine oil is put into the more than one experimental rig sequentially. For example, the engine oil may be first exposed to experimental conditions of high temperature and limited shear using a first experimental setup, and second exposed to lower temperature and high shear using second experimental setup. This approach may be used to replicate operational conditions for engine oil having proximity to more than one component or system of the engine. In certain embodiments, use of an experimental rig to expose the engine oil to experimental conditions may comprise holding the oil under a number of conditions. The experimental rig may be used to individually change a plurality variables. For example, a volume of engine oil may be held under a plurality of values of a first variable. A sample of the volume of engine oil may be analysed after exposure to each of the plurality of values of the first variable. The sample is a bulk oil sample, indicative of the volume of engine oil within the experimental rig. The volume of engine oil may be exposed to the plurality of values of first variable sequentially, to analyse the cumulative effect of changing the first variable. Otherwise, a new volume of engine oil may be exposed to each of the plurality of values of first variable. This may be repeated for one or more other variables. Analysis of a sample of engine oil from the volume of engine oil may allow trends to be identified and linked to changes in a particular variable. The particular variable that controls a certain reaction in or change to the volume of engine oil may be isolated. Using knowledge of parameters and variables within an engine, the analysis may be interpreted to identify engine conditions that result in certain reactions of the engine oil or change to the engine oil. For example, certain systems or locations within the engine may be highly sensitive to combustion conditions. To replicate this, the engine oil may be exposed to high temperatures and multiple combustion products. In other systems or locations within the engine, the engine oil may be exposed to lower temperatures and higher shear rates. The engine oil may undergo reactions while under experimental or operational conditions. These reactions may be driven by different energies, depending on the reaction. For some reactions, the activation energy may be provided by temperature and / or pressure. For other reactions, shear may reduce the activation energy required. As described above, an operational condition may be achieved using a test engine or using an engine to drive equipment. The operational condition may include the time interval over which the engine oil is within the engine may be set. For example, samples may be taken of the engine oil at specific time intervals. The operational condition(s) may correspond to engine oil within the engine as a whole, wherein the engine undergoes one or more particular engine conditions. The operational condition may depend on the time period over which the engine oil is in the engine under the particular engine conditions. Data from the equipment driven by the engine or from the test engine may provide information as to the operational conditions to which the engine oil was exposed over the time period. The engine may be run in a particular way to aim to achieve particular experimental conditions corresponding to operational conditions. Or, the engine may be run without aiming for a particular experimental condition, wherein data from the engine or equipment provides information as to the actual operational conditions. Samples of engine oil are analysed using spectrometry. A spectrum may comprise a plurality of features. In certain embodiments, those spectra (in either numerical or graphical format) may be compared to identify one or more than one difference between the spectra. The primary and secondary spectra may each comprise a plurality of features. There may be one difference between the primary and the secondary spectra that may comprise a change to one feature. For example, one peak might change in height. There may be multiple differences between the primary and secondary spectra, comprising changes to multiple features. The combination of differences may indicate a particular change to the engine oil and its performance. For example, a change to a first feature may indicate one of two changes to the engine oil, depending on a change to a second feature. In an example, a first peak increasing in height may indicate a first change to the oil in an event that a second peak decreases in height, but the first peak increasing in height may indicate a second change to the oil in an event that the second peak increases in height. In another example, there may be one difference between the primary and secondary spectrum, comprising a change to a first feature, wherein the change to the engine oil indicated by the difference depends on another unchanged feature of the spectrum. For example, a first peak increasing in height may indicate a first change to the oil in an event that a second peak is lower than the first peak, but may indicate a second change to the oil in an event that the second peak is not lower than the first peak. The features of the primary and secondary spectra may comprise peaks and troughs. The difference(s) between the primary spectrum and the secondary spectrum may include one or more of the following: one or more peaks shifting with respect to the y axis; one or more peaks shifting with respect to the x axis; one or more peaks scaling with respect to the y axis; one or more peaks scaling with respect to the x axis; one or more troughs shifting with respect to the y axis; one or more troughs shifting with respect to the x axis; one or more troughs scaling with respect to the y axis; one or more troughs scaling with respect to the x axis. The differences may comprise a combination of more than one of these differences. The difference(s) between the primary spectrum and the secondary spectrum may include one or more of a change to peak height ratios; a change to the number of peaks in a specific wavenumber range; a change to integrals of the spectrum within a specific wavenumber range; and roughness over a specific wavenumber range. A change in peak height may comprise a change in the absolute value of absorbency of the peak (i.e. measured from the origin), or a change in height as measured from the base of the peak. Several differences may be associated with an operational condition. In certain embodiments, spectral analysis of a sample may comprise processing the spectrum or spectra to identify a plurality of engine oil variables, such as variables indicative of elemental composition and / or chemical bonds of the sample. The variables may be extracted from spectrum or spectra. For example, a spectral analysis of a sample may comprise values of concentrations of certain elements or molecules in the sample, levels of oxidation, levels of sulfation, levels of nitration, and so on. The output of the methods described herein may comprise a characterisation of engine oil, comprising a plurality of associations of differences between spectra with a plurality of operational conditions. A given operational condition may be associated with one difference between spectra, or with a particular combination of differences between spectra. A given operational condition may be associated with a variation between spectral analyses of more than one sample, wherein the variation comprises a value of at least one vector of change indicative of the variation. An operational condition may be identified by at least one engine variable. In use, it may be possible to identify one or more changes in an engine oil volume (by analysing a sample of the volume of engine oil using spectrometry), and use the characterisation to look up the associated operational condition. Figures 3 and 4 to indicates some simple examples of possible differences between the primary spectrum and the secondary spectrum. These examples are merely illustrative, and show simplified peaks (Figure 3) and troughs (Figure 4). For each graph, the solid line indicates a part of the primary spectrum, and the dashed and dotted lines indicate parts of possible secondary spectra. Figure 3A shows a peak shifting with respect to the y-axis. The dashed line 312 shows a positive translation of the peak 311 of the primary spectrum with respect to the y-axis (i.e. translated to higher y values), and the dotted line 313 shows a negative translation of the peak 311 of the primary spectrum with respect to the y-axis (i.e. translated to lower y values). Figure 3B shows a peak shifting with respect to the x-axis. The dashed line 322 shows a positive translation of the peak 321 of the primary spectrum with respect to the x- axis (i.e. translated to higher x values), and the dotted line 323 shows a negative translation of the peak 321 of the primary spectrum with respect to the x-axis (i.e. translated to lower x values). Figure 3C shows a peak scaling with respect to the y-axis. The dashed line 332 shows the peak 331 of the primary spectrum stretched parallel to the y-axis. The dotted line 333 shows the peak 331 of the primary spectrum compressed parallel to the y-axis. Figure 3D shows a peak scaling with respect to the x-axis. The dashed line 342 shows the peak 341 of the primary spectrum stretched parallel to the x- axis. The dotted line 343 shows the peak 341 of the primary spectrum compressed parallel to the x-axis. Figure 4A shows a trough shifting with respect to the y-axis. The dashed line 412 shows a positive translation of the trough 411 of the primary spectrum with respect to the y-axis (i.e. translated to higher y values), and the dotted line 413 shows a negative translation of the trough 411 of the primary spectrum with respect to the y-axis (i.e. translated to lower y values). Figure 4B shows a trough shifting with respect to the x-axis. The dashed line 422 shows a positive translation of the trough 421 of the primary spectrum with respect to the x- axis (i.e. translated to higher x values), and the dotted line 423 shows a negative translation of the trough 421 of the primary spectrum with respect to the x-axis (i.e. translated to lower x values). Figure 4C shows a trough scaling with respect to the y-axis. The dashed line 432 shows the trough 431 of the primary spectrum stretched parallel to the y-axis. The dotted line 433 shows the trough 431 of the primary spectrum compressed parallel to the y-axis. Figure 4D shows a trough scaling with respect to the x-axis. The dashed line 442 shows the trough 441 of the primary spectrum stretched parallel to the x- axis. The dotted line 443 shows the trough 441 of the primary spectrum compressed parallel to the x-axis. The primary and secondary spectra may comprise multiple peaks and troughs. As discussed above, the difference between the secondary and primary spectra may comprise one change to one peak or trough, such as one of those changes described in relation to Figures 3 and 4. The difference between the secondary and primary spectra may comprise multiple changes to multiple peaks or troughs, such as a combination of those changes described in relation to Figures 3 and 4. A given change to a given feature of the primary spectrum may be associated with different evolution pathways of the engine oil, depending on the change to another feature(s) of the spectrum, or the lack of change to another feature(s) of the spectrum, or some other characteristic of other feature(s) in the spectrum. For example, a given change to a given feature of the primary spectrum may be associated with different evolution pathways of the engine oil depending on whether or not another feature has changed. In another example, a given change to a given feature of the primary spectrum may be associated with different evolution pathways of the engine oil depending on how another feature has changed. In another example, a given change to a given feature of the primary spectrum may be associated with different evolution pathways of the engine oil depending on characteristics of another feature relative to a changed feature (such as relative peak / trough height, or relative peak / trough position, etc.). The first spectrometer may comprise a Fourier-transform infrared (FTIR) spectrometer, allowing analysis of the chemical bonds of the molecules within the oil. Analysis of individual peaks allows analysis of the same chemical bond within different molecules. For example, a double chemical bond between carbon and oxygen may be found in two different molecules, and the changes of that chemical bond in the two molecules may have different implications for the evolution of the oil. FTIR spectrometry is a well-known technique. In brief, an FTIR spectrum indicates the absorption of light at various wavelengths. Light at different frequencies is incident on the sample, and absorption or transmittance is measured for each frequency. This is achieved by using a Michelson interferometer to block or transmit certain wavelengths of light from a light source by altering a mirror position, such that the frequency of light incident on the sample varies as a function of time. The absorbance or transmittance may be measured against each frequency as a function of time. A Fourier transform may be used to convert the displacement of the mirror into the wavenumber of the light, providing a spectrum of absorption against wavenumber. As described, an interpretation of a change to a particular peak or trough of a spectrum may change depending on other features of the spectrum. With reference to Figure 5, an example of a section of FTIR spectra is illustrated. Absorbance is plotted against wavenumber, and each line represents a sample taken at a different time. This is anexample only. Alternatively, transmittance (wherein = 2log( )) may be plotted against wavenumber, or the absorbance ortransmittance may be plotted against wavelength or frequency. In this example, each sample was taken approximately 50 hours apart, but this is merely illustrative. The spectrum of Figure 5 is cropped with respect to the y-axis, and shows only a selected range of wavenumbers. The FTIR spectra used may have a different range of wavenumbers, and may show an absorbance axis from 0 to 1. To provide examples of changes that may occur to the spectrum, four peaks are indicated (510, 520, 530, 540). In an event that peak 510 grows (i.e. absorbance increases) over time, it may be an indication that the engine oil is degrading. However, in an event that peak 520 has low absorbance before peak 510 grows, the growth of peak 510 may not indicate that the engine oil is degrading; the performance of the engine oil may improve or remain the same. The growth of peak 510 may also indicate other changes to an engine component. Changes to peak 530 may indicate changes to a characteristic that is improving, but the interpretation of changes to peak 530 may depend on the height of peak 520. Peak 540 may relate to another characteristic of the engine oil. If peak 540 decreases, this may indicate either an improvement to or a worsening of that characteristic of the engine oil, depending on other peaks of the spectrum. Examples of changes to engine characteristics that may be inferred by scrutiny of the spectra include changes to anti-wear performance of an engine component, growth of deposits on an engine component, changes to the turbo bearing, rheological changes, soot induced wear and other changes. With reference to Figures 6 and 7, further examples of spectra indicative of engine oil evolution are illustrated. Figures 6 and 7 show schematics that are representative of sections of FTIR spectra, but that do not show real data. Figure 6 illustrates a section of an FTIR spectra for engine oil undergoing a “normal” evolution pathway. Spectra are illustrated for the engine oil at 0 hours, 100 hours, 200 hours, 300 hours and 400 hours. A peak at a particular wavenumber is indicated by arrow 610 and a trough at a particular wavenumber is indicated by arrow 620. The peak 610 increases in height over time. The trough 620 increases in depth over time. Figure 7 shows a section of an FTIR spectra for engine oil undergoing an “abnormal” evolution pathway, for the same wavenumber range as shown in Figure 6. The spectra shown in Figure 7 are for engine oil undergoing a particular failure mode. Spectra are illustrated for the engine oil at 0 hours, 100 hours, 200 hours, 300 hours and 400 hours. Arrow 710 indicates a peak at the same wavenumber as peak 610 of Figure 6. The peak 710 has a greater increase in height over time than peak 610. Arrow 720 indicates a peak at the same wavenumber as trough 620 of Figure 6. Rather than the peak 610 of normal evolution, the abnormal evolution shows a small peak 710. The combination of these two changes may be a signature of a particular failure mode. One of those changes without the other may be a signature of a different failure mode. With reference to Figures 8 and 9, further examples of spectra indicative of engine oil evolution are illustrated. Figures 8 and 9 show schematics that are representative of sections of FTIR spectra, but that do not show real data. Figure 8 illustrates a section of an FTIR spectra for engine oil undergoing a “normal” evolution pathway. Spectra are illustrated for the engine oil at 0 hours, 100 hours, 200 hours, 300 hours and 400 hours. A peak at a particular wavenumber is indicated by arrow 810 and a trough at a particular wavenumber is indicated by arrow 820. The peak 810 increases in height over time. The trough 820 increases in depth over time. Figure 9 shows a section of an FTIR spectra for engine oil undergoing an “abnormal” evolution pathway, for the same wavenumber range as shown in Figure 8. The spectra shown in Figure 9 are for engine oil undergoing a particular failure mode. Spectra are illustrated for the engine oil at 0 hours, 100 hours, 200 hours, 300 hours and 400 hours. Arrow 910 indicates a peak at the same wavenumber as peak 810 of Figure 8, but that decreases in height over time and eventually becomes a trough. Arrow 920 indicates a feature at the same wavenumber as trough 820 of Figure 8. The trough 920 decreases in depth with time, and becomes a peak increasing in height over time. The combination of these two changes may be a signature of a particular failure mode. One of those changes without the other may be a signature of a different failure mode. Figures 6 to 9 each show spectra for engine oil at intervals of 100 hours. However, other sampling intervals may be used. The sampling interval may be shorter or longer than 100 hours, and may be any length. The sampling intervals may be consistent, or may vary in length. The sampling interval may be defined by a time period or by reference to an engine oil change interval. For example, samples of engine oil may be obtained at certain points in an engine oil change interval. In specific, non-limiting examples, an engine oil change interval may be used as a sampling interval. In certain examples, the engine oil change interval may be between 250 and 1,000 hours, or an engine oil change interval may be less than or equal to 4,000 hours, or an engine oil change interval may be greater than or equal to 10 hours. Other engine oil change intervals may be used. Samples of engine oil may be taken at certain points in an engine oil change interval other than (instead of or in addition to) at the end of an engine oil change interval. In another specific example, one sample of engine oil may be obtained in the middle of an engine oil change interval, and one sample of engine oil may be obtained at the end of an engine oil change interval (when the engine oil is changed). Or, the samples of engine oil may be obtained more frequently during the engine oil change interval. The sampling interval may depend on one or more of: the type of engine an operator is able to or wishes to pause operation of the engine; whether the operator suspects an issue; and so on. For example, a shorter sampling interval may be required for a diesel engine than for a spark ignited industrial natural gas engine. In another example, if an operator suspects an issue with the engine or engine oil, the sampling interval may be in the region of 10 hours. Other sampling intervals may be used. The spectra illustrated in Figures 6 to 9 show each feature (peak or trough, in this case) having a linear change with respect to time. In other words, the change in height of a peak or change in depth of a trough is shown as being linear with time. In fact, the changes may be non-linear with respect to time, for normal or abnormal evolutions. A charge may be linear with respect to time for a certain length of time, and then may develop to be non- linear with respect to time. For example, a change may be linear with respect to time in a normal evolution, but if a failure mode develops the change may become non-linear with respect to time. In other examples, a change may be non-linear with respect to time in a normal evolution. Furthermore, the spectra illustrated in Figures 6 to 9 show peaks and troughs changing in height or depth over time. As discussed above, other changes to features of the spectra may occur. The changes to the features of the spectra can indicate both how the engine oil evolves, and how the engine components are performing. For example, a rate of consumption (i.e. rate of decrease in peak height, where peak height is measured from the base to the top of the peak) of a particular peak may be affected by NOx gas exposure. Inspection of the rate of consumption of that peak may indicate a change to the combustion within the engine. In another example, the rate of change of the whole spectrum may be affected by temperature. Inspection of this effect may indicate how the turbo is performing. In another example, a change in a ratio of certain peaks may indicate a local high temperature and may be indicative of piston top ring conditions. In another example, the quantity of soot may affect the skew of the trace. The changes to the features of the spectra can be indicative of performance of any component, subsystem or system that interacts with the engine oil. The method may further comprise analysing further samples of further volumes of engine oil exposed to different operational conditions, with a view to associating a plurality of different operational conditions with differences between spectra of the engine oil. The different operational conditions may correspond to conditions experienced in an engine operating under “normal” and / or “abnormal” conditions. The method may further comprise analysing further samples of further volumes of the engine oil. The volumes of engine oil may be exposed to further operational conditions. In an event that an experimental rig is used, further experimental conditions may correspond to different operational conditions of the same part of the combustion engine, and / or operational conditions of a different part of the engine. In an event that a test engine is used or an engine is used to drive equipment, the further experimental conditions may correspond to a different time interval with the same engine conditions, and / or different engine conditions. The method may further comprise using the first spectrometer to analyse a second volume of the engine oil. To do so, a primary sample of the second volume of engine oil may be analysed, providing the primary spectrum of the second volume of engine oil. The second volume of engine oil may be exposed to a second operational condition, wherein the second operational condition corresponds to a second operational condition of a second part of the combustion engine. The first spectrometer may then be used to analyse a secondary sample of the second volume of engine oil and provide a secondary spectrum of the second volume of engine oil. The method may further comprise comparing the primary spectrum and the secondary spectrum of the second volume of engine oil to determine one or more differences between the primary spectrum and the secondary spectrum, wherein the one or more differences are indicative of a change in the engine oil. The one or more differences may be associated with the second operational condition. The first volume and the second volume may be the same engine oil, wherein the engine oil is sequentially exposed to the first operational condition and then to the second operational condition. The first operational condition may differ simply in time periods, such that the first volume is exposed to a particular condition or conditions for a first time period, and the second volume is exposed to the same condition or conditions for a second time period. The first and second time periods may be adjacent time periods, or may have a time period between them, or may overlap. In certain embodiments, the second volume may be exposed to the first operational condition prior to being exposed to the second operational condition, such that the primary sample of the second volume has been exposed to the first operational condition but not to the second operational condition. For example, the primary sample of the first volume may be taken at time t0. The secondary sample of the first volume may be taken at time t1, later than time t0. The primary sample of the second volume may be taken at time t2, which may be the earlier than, the same as or later than t1 (the primary sample of the second volume may the same as or different from the secondary sample of the first volume). The secondary sample of the second volume may be taken at time t3, which is later than time t2. Between t0 and t3, the conditions to which the engine oil is exposed may remain the same or may be varied. For example, a test engine may be run with a certain quantity of engine oil. The first volume of engine oil may refer to that quantity of engine oil exposed to a first operational condition within the test engine over a first time period. The second volume of engine oil may refer to that same quantity of engine oil exposed to a second operational condition within the same test engine over a second time period. The second time period may immediately follow the first time period, may occur some time after the first time period ends, or may overlap with the first time period. The first volume of engine oil and second volume of engine oil may be exposed to the first and second operational conditions, respectively, using the same equipment or engine. However, the engine oil may be changed between using the equipment or engine to expose engine oil to the first and second operational conditions, such that the first and second volumes of engine oil are different engine oil. The second volume of engine oil has not, therefore, been exposed to the first operational condition prior to being exposed to the second operational condition. The first volume of engine oil and the second volume of engine oil may be exposed to the first and second operational conditions, respectively, using different equipment or engines. The method may further comprise exposing further volumes of engine oil to further operational conditions of further components, sub-systems or systems of the engine. For each volume of engine oil exposed to an operational condition, a primary spectrum may be obtained prior to exposing the volume of engine oil to the operational condition and a secondary spectrum may be obtained after exposing the volume of engine oil to the operational condition. The further volumes be the same as or different to each other and / or to the first and second volumes. The further operational conditions may be applied using the same as or different equipment or engines to each other and / or the first and second operational conditions. In addition to or instead of the second volume of engine oil, the method may comprise using the first spectrometer to analyse a third volume of the engine oil and provide a primary spectrum of the third volume of engine oil. To do so, a primary sample of the third volume of engine oil may be analysed, providing the primary spectrum of the third volume of engine oil. The method may comprise exposing the third volume of engine oil to a third operational condition of the same component, sub-system or system of the combustion engine as the first operational condition. The first spectrometer may then be used to analyse a secondary sample of the third volume of engine oil and provide a secondary spectrum of the third volume of engine oil. The primary spectrum and the secondary spectrum of the third volume of engine oil may be compared to determine one or more differences between the primary spectrum and the secondary spectrum, wherein the one or more differences are indicative of a change in the engine oil. The one or more differences may be associated with the third operational condition. In addition to or instead of the second and / or third volumes of engine oil, the method may comprise using the first spectrometer to analyse a primary sample of a fourth volume of the engine oil and provide a primary spectrum of the fourth volume of engine oil. The method may comprise exposing the fourth volume of engine oil to a fourth operational condition. The fourth operational condition may correspond to different engine condition(s) from the second and / or third operational conditions. The first spectrometer may then be used to analyse a secondary sample of the fourth volume of engine oil and provide a secondary spectrum of the fourth volume of engine oil. The primary spectrum and the secondary spectrum of the fourth volume of engine oil may be compared to determine one or more differences between the primary spectrum and the secondary spectrum, wherein the one or more differences are indicative of a change in the engine oil. The one or more differences may be associated with the fourth operational condition. In addition to or instead of the second and / or third and / or fourth volumes of engine oil, the method may comprise using the first spectrometer to analyse a primary sample of a fifth volume of the engine oil and provide a primary spectrum of the fifth volume of engine oil. The method may comprise exposing the fifth volume of engine oil to a fifth operational condition. The fifth operational condition may correspond to the same engine condition(s) as another operational condition, over a different time interval. The first spectrometer may then be used to analyse a secondary sample of the fifth volume of engine oil and provide a secondary spectrum of the fifth volume of engine oil. The primary spectrum and the secondary spectrum of the fifth volume of engine oil may be compared to determine one or more differences between the primary spectrum and the secondary spectrum, wherein the one or more differences are indicative of a change in the engine oil. The one or more differences may be associated with the fifth operational condition. The method may comprise exposing any number of volumes of engine oil of engine oil to any number of operational conditions, and analysing the volumes of engine oil before and after exposing the volumes of engine oil to each operational condition. Any of the volumes of engine oil may be analysed by one or more of the methods described herein. In certain embodiments, the same volume of engine oil may be exposed to multiple operational conditions in succession, with the volume of engine oil being analysed between each operational condition. The operational conditions may have a cumulative effect on the engine oil. For example, the second volume of engine oil may comprise the first volume of engine oil after the first volume of engine oil has been exposed to the first operational condition. In an event that a given volume of engine oil is exposed to successive operational conditions, the secondary sample taken after exposing the volume of engine oil to a particular operational condition may also be used as the primary sample of the volume of engine oil taken prior to exposing the volume of engine oil to the next operational condition. Or, in an event that a given volume of engine oil is exposed to successive operational conditions, the primary sample of the volume of engine oil taken prior to exposing the volume of engine oil to a particular operational condition may be different from the secondary sample taken after exposing the volume of engine oil to the previous operational condition. In other embodiments, different volumes of engine oil may each be exposed to one or more different operational conditions. For example, a one volume of engine oil may be exposed to a first operational condition followed by a second operational condition, while another volume of engine oil may be exposed to only a third operational condition. In certain embodiments, one or more volumes of engine oil may each be exposed to multiple operational conditions in succession, and one or more volumes of engine oil may be exposed to different operational conditions. Any number of volumes of engine oil may be exposed to any number of operational conditions, either separately or in succession. The method may further comprise using a second spectrometer to analyse a different property of the engine oil to the first spectrometer. For example, the first spectrometer may analyse chemical bonds, and the second spectrometer may carry out elemental analysis. The method may further comprise, before exposing the first volume of engine oil to the first operational condition, using a second spectrometer to analyse a tertiary sample of the first volume of the engine oil and provide a tertiary spectrum of the first volume of engine oil. After exposing the first volume of engine oil to the first operational condition, the method may comprise using the second spectrometer to analyse a quaternary sample of the first volume of engine oil and provide a quaternary spectrum of the first volume of engine oil. The tertiary spectrum and the quaternary spectrum of the first volume of engine oil may then be compared to determine one or more differences between the tertiary spectrum and the quaternary spectrum, wherein the one or more differences are indicative of a change in the engine oil. The one or more differences may be associated with the first operational condition. The tertiary sample may be the same as or separate from the primary sample of the first volume of engine oil. The quaternary sample may be the same as or separate from the secondary sample of the first volume of engine oil. In any of the examples provided above, a second spectrometer may be used to analyse a tertiary sample of the volume of the engine oil and provide a tertiary spectrum of the volume of engine oil. After exposing the volume of engine oil to the particular operational condition, the method may comprise using the second spectrometer to analyse a quaternary sample of the volume of engine oil and provide a quaternary spectrum of the volume of engine oil. The tertiary spectrum and the quaternary spectrum of the volume of engine oil may then be compared to determine one or more differences between the tertiary spectrum and the quaternary spectrum, wherein the one or more differences are indicative of a change in the engine oil. The one or more differences may be associated with the operational condition. Any of the methods or parts of the methods or features described above with relation to the first volume and the first operational condition may be applied to other volumes and other operational conditions. In certain embodiments, the second spectrometer may comprise an inductively coupled plasma atomic emission spectrometer (ICP-AES, also referred to as an inductively coupled plasma optical emission spectrometer ICP-OES). Whereas FTIR spectrometry analyses chemical bonds, an ICP-AES provides elemental analysis. The ICP-AES may be used to determine the reduction (consumption rate) or increase in concentration of different elements of the engine oil to determine how the engine oil changes over time. For example, the consumption of different elements may be compared in order to understand how the engine uses certain additives compared to other additives. The consumption of additives can provide information as to the engine conditions. Furthermore, ICP-AES may be used to detect contaminations in the engine oil. For example, the iron contamination rate may be determined. As an example, additive elements in the engine oil, such as calcium or phosphorous, may either decrease or increase in concentration in the engine oil over time. Other compounds that may arise due to wear of the engine, such as iron, may increase in concentration in the engine oil over time. In certain embodiments, Raman spectrometry may be used to analyse samples of engine oil. Other spectrometry techniques may be used. The spectra obtained in any of the methods above are compared to identify changes between them. This may be carried out by any suitable method. More than one method of analysis may be combined. Analysis of the spectra aims to identify the degree of variation between the spectra associated with different oil volumes of engine oil. Changes to the features of the spectra can indicate both how the engine oil evolves, and how the engine components are performing. The changes to the features of the spectra can be indicative of performance of any component, subsystem or system that interacts with the engine oil. In certain embodiments, trends may be identified in the changes to the spectra. As described above, the spectra may be compared (either in graphical form, or as data), or engine oil variables may be compared that have been extracted from the spectra. The spectra may be inspected visually to identify changes. The spectra may be visually inspected in graphical form, or the raw data may be inspected. The raw data may be compared using simple mathematical comparisons, such as finding numerical differences between the raw data. Given the complex nature of the spectra, other techniques may be used to identify the changes to the spectra. These techniques may include binary additive operations, simplifying the spectra through a combination of experiments and mathematical functions, or data science tools such as dimension reduction techniques. Data science is an interdisciplinary subject, which may use one or more of statistics, algorithms, scientific methods, numerical methods of analysis, domain knowledge, and other methods to analyse complex data sets. The application of data science techniques to chemical information may also be referred to as chemometrics. Data science may be used to identify trends in the changes in features of the spectra. Trends of changes to a particular feature of the spectra, either in isolation or in combination with trends of changes of other features of the spectra, allow a better description of the evolution of the engine oil and the associated operational conditions. The degree of variation between the spectra can be analysed, with the variation then being characterised as being due to a condition or conditions of an individual engine component, subsystem or system. This information may be recorded, and may be used as a reference characterisation for engine oil, for example when analysing engine oil of an engine in use. Techniques for identifying changes to the spectra may be applied to a matrix or matrices of spectrum data. The matrix or matrices may comprise data for two or more spectra, such that analysis of the data can be used to identify changes in the spectra. A simple mathematical assessment or assessments may be applied to a matrix of the spectra data, or more advanced techniques may be used to identify changes between spectra. Data science may be used to analyse spectra of volumes of engine oil in order to establish how the engine oil evolved. There are many data science techniques that can be used to identify variations in the spectra. Multi-dimensional analysis may be used to interrogate variations between spectra that are indicative of a plurality of engine oil variables, or to interrogate variations between a plurality of engine oil variables extracted from the spectra. As a non-limiting example, a matrix containing the data can be simplified via dimension reduction. It may then be identified where in the spectrum a change is seen, and that variation may be linked to a property of the engine, for example. Once these variations have been identified, they may be interlinked to identify changes in features that are affected by other features or by changes to other features. Knowledge of the engine and its components may also be used to assist in interpreting the changes, such as to link particular engine conditions to particular reactions of the engine oil or changes to the engine oil. In an example, spectral analysis of more than one sample (wherein each sample has been exposed to a respective operational condition) may provide values of more than one engine oil variable for each sample. Dimension reduction may be used to describe the variation between the more than one engine oil variables in fewer dimensions than the number of engine oil variables. The output of dimension reduction may comprise at least one vector of change, wherein each vector of change is indicative of variation between the spectral analysis of each sample. Each vector of change may be indicative of variation of more than one engine oil variable, wherein each vector of change is indicative of variations of different engine oil variables in different proportions. The vectors of change that are output from the dimension reduction may be orthogonal, allowing vectors of change to be extracted for more detailed analysis or for the effects of certain vectors of change to be removed. For each sample, the values of the vectors of change may be associated with the operational condition to which the sample was exposed, thereby characterising the evolution of the sample of engine oil. In another specific example, a set of spectroscopy data can be presented as a matrix (rows and columns). The set of data may comprise data for more than one spectrum. Either the rows or the columns may be described as features or dimensions. The rows (or columns) may be individual ‘dimensions” such that a data set with numerous rows (or columns) is known as multidimensional data. Examples of a data science technique that can be applied to multidimensional spectra data sets are dimensional reduction techniques, used to reduce the number of rows (or columns) of the data set and thereby increase interpretability of data while maintaining the trends and patterns within the data. In other words, the data set is simplified without losing the patterns in the data that need to be extracted and analysed, and particular variations in the data may be isolated. The dimension reduction techniques may be applied directly to the spectral data, or to processed spectral data. In certain examples, each set of data in a matrix of spectroscopy data may be compared, and a new matrix may be determined containing numerical indications of differences between each data set and / or variations within each data set. Dimension reduction techniques or other techniques may be used to isolate specific variations. In a specific example, a matrix of spectrometer data may comprise a plurality of sets of data. Each set of data may provide spectrometry data for an engine oil sample, in the form of signal magnitudes and wavenumbers. A new matrix containing numerical indications of differences between and / or within each data set may be obtained, and analysed to determine the degree of variation between and / or within the data sets. Specific variations may be isolated, for example using dimension reduction techniques or otherwise. An example of a common data science dimensional reduction technique is Principal Component Analysis (PCA). PCA linearly transforms a data set into a new coordinate system. In the new coordinate system, variations in the data can be described with fewer dimensions than in the original data. In use, PCA is carried out using a matrix of spectrometer data comprising a plurality of sets of data, wherein each set of data provides spectrometry data for an engine oil sample in the form of signal magnitudes and wavenumbers. For each wavenumber, the mean of signal magnitudes across the sets of data may be obtained. The data sets may be normalised with respect to that mean value. A covariance matrix may then be obtained. Further mathematical functions may be applied to the covariance matrix to determine the degree of variation between and / or within the data sets. PCA can be used to characterise primary variation between the spectra. For example, changes in the shapes of the curves may be characterised, including slopes or gradients, and magnitude and position of peaks and valleys. Alternatively, or additionally, the variations between the spectra may be characterised by other techniques. Irrespective of the technique used to obtain the characterisation, the characterisation may provide an easily recognisable visual representation of differences between spectra, which may be more easily interpreted than a visual assessment of the raw data. Patterns between samples of oils can be established by comparing principle components of the characterisation or differences between the spectra data sets. This may yield an understanding of how the engine oil evolved within the spectra data sets. The primary variation between the engine oil samples due to the evolution can be isolated using this technique. Any variation can then be characterised as being due to a condition or conditions of an individual engine component, subsystem or system. This can be used to determine changes in engine component, subsystem or system health, and to predict subsequent engine oil performance in that engine component, subsystem or system. Associating the one or more differences between spectra with an operational condition may be carried out by any suitable method. More than one method may be combined. For normal engine oil evolution, one or more volumes of engine oil may be exposed to one or more operational conditions of the combustion engine, and the one or more volumes of engine oil may be analysed as described above. This may be repeated for a plurality of operational conditions to build up a reference characterisation of normal engine oil evolution comprising a plurality of reference spectra or reference variations between spectra that are indicative of normal engine oil evolution. More than one volume of engine oil may be exposed to the same operational condition to validate the normal engine oil evolution. The reference characterisation for a normal engine oil evolution may be associated with certain engine types, engine meta data, or other information. For example, a normal engine oil evolution may differ between different engines or between different operation modes of a particular engine. As an example, consistently operating an engine at a low engine load may result in a different normal engine oil evolution than consistently operating the same engine at a high engine load. For abnormal engine oil evolution, one or more volumes of engine oil may be exposed to one or more operational conditions of the combustion engine, wherein the one or more operational conditions correspond to a failure mode or other condition resulting in abnormal engine oil evolution. The one or more volumes of engine oil may be analysed as described above. This may be repeated for a plurality of operational conditions to build up a reference characterisation of abnormal engine oil evolution comprising a plurality of reference spectra or reference variations between spectra that are indicative of abnormal engine oil evolution. More than one volume of engine oil may be exposed to the same operational condition to validate the abnormal engine oil evolution. The reference characterisation for a normal engine oil evolution may be associated with certain engine types, engine meta data, or other information. The resulting spectra for volumes of engine oil exposed to operational conditions resulting in abnormal engine oil evolution may, in certain scenarios, be associated with operational conditions by analysis of the spectra to determine which chemical bonds have changed, and using knowledge of certain operating conditions that would result in these changes. For example, it may be known that a certain failure mode of the combustion engine causes a certain reaction in the engine oil that results in a particular change to the chemical bonds of a component of the engine oil. In other scenarios, a reference characterisation for abnormal engine oil may be produced by observing or replicating failure modes (or other conditions that lead to abnormal engine oil evolution) and establishing correlation and causality between a particular failure mode or condition and particular variances between spectra. For example, a volume of engine oil may be exposed to one or more operational conditions corresponding to a particular failure mode or other condition that leads to abnormal engine oil evolution, either by deliberately implementing or replicating the operational conditions or by observing the operational conditions. The volume of engine oil may be analysed as above. The resulting spectra may be compared to a reference characterisation for normal engine oil evolution, and / or to any other reference characterisation for abnormal engine oil evolution. Differences may be identified between the spectra and the reference characterisation for normal engine oil evolution (and / or any other reference characterisation for abnormal engine oil evolution) such that particular variances in the spectra may be associated with the particular failure mode or other condition that leads to abnormal engine oil evolution. This may be repeated, exposing other volumes of engine oil to the same operational condition, to validate an association between particular variances and the particular failure mode or other condition that leads to abnormal engine oil evolution. By way of example, several specific conditions or failure modes of a combustion engine that may result in an abnormal engine oil evolution will now be described. These are examples only, and other conditions or failure modes may result in abnormal engine oil evolution. As a first example, abnormal wear of a cylinder bore top ring turn around (TRTA) zone may lead to an abnormal engine oil evolution. A cylinder bore wall may comprise grooves on an inner surface. The grooves may, for example, be arranged in a cross-hatching pattern. The grooves may be configured to accommodate a flow of engine oil to lubricate an interface between the cylinder bore and a piston and / or between the cylinder bore and piston rings. For example, the interface between the cylinder bore and the top ring of the piston. In an event that abnormal wear of the cylinder bore occurs, the grooves may be worn away such that there is a loss of lubrication to the piston ring(s). In an event that lubrication to the top ring of the piston is lost, the top ring may rub against the cylinder bore, causing damage to the top ring and affecting a sealing ability of the top ring. This change in performance may, in turn, affect the evolution of the engine oil. As a second example, carbon deposits at a piston top land may result in an abnormal engine oil evolution. There may typically be a clearance between a surface of the piston top land and a cylinder bore. This clearance may allow for thermal expansion of the piston. Certain conditions (such as engine oil entering the combustion chamber, certain combustion conditions, or piston temperatures exceeding a threshold) may lead to carbon deposits building up on the top land of the piston. As the piston changes direction, the piston may rock around a piston pin, which may bring the top land of the piston closer to the cylinder bore. Carbon deposits on the top land may reduce the clearance between the top land and the cylinder bore, such that in an event that the top land of the piston is brought closer to the cylinder bore due to the piston changing direction, there may be contact between the carbon deposits on the top land and the cylinder bore. The carbon deposits may then abrade the cylinder bore along the bore. The cylinder bore may have grooves configured to accommodate a flow of engine oil to lubricate an interface between the cylinder bore and the piston. Abrasion of the cylinder bore along the bore may wear away the grooves such that there is a loss in lubrication between the cylinder bore and the top land of the piston. This change in performance may, in turn, affect the evolution of the engine oil. Other examples of failure modes of a combustion engine may include abnormal wear of a camshaft lobe to follower interface; piston ring groove deposits; piston ring sticking, leading to scuff; abnormal wear of a piston ring face; abnormal wear or scuff of a piston second land; abnormal wear or scuff of a piston skirt; scuff or bore marking of a piston ring liner; abnormal wear or scuff or seizure of a piston pin joint; connecting rod small end bushing chemical leaching; abnormal wear, scuff or corrosion of a connecting rod crank end bearing; abnormal wear, scuff or corrosion of a main bearing; abnormal wear, scuff or corrosion of a cam bushing; abnormal wear of valve to seat; valve sticking in guide; abnormal wear, scuff or corrosion of a rocker arm bushing; scuff of a pushrod to rocker arm spherical joint; or other failure modes. A characterisation of evolution of engine oil during operation of a combustion engine, wherein the engine oil is configured to lubricate the combustion engine, may be obtained by any of the methods described herein. A characterisation of evolution of engine oil during operation of a combustion engine may comprise variation between spectral analyses of more than one sample of engine oil with respect to one or more than one engine variable. Each sample has been exposed to one of a plurality of operational conditions of the combustion engine. Each operational condition is identified by one or more than one engine variable. The variation between the spectral analyses of the more than one sample of engine oil are obtained by using multi- dimensional analysis to interrogate variation between the spectral analyses of the more than one sample with respect to one or more than one engine variable. The characterisation may further comprise an association of each variation with the one or more engine variable of the operational condition to which the sample was exposed, to characterise one or more than one evolution pathway of the engine oil. Spectral analyses of each sample are obtained by using spectrometry to analyse elemental composition and / or chemical bonds of the more than one sample of engine oil. As described above in relation to the methods, for each evolution pathway of the engine oil, the samples may have been exposed to operational conditions having at least one similar engine variable. For example, a plurality of engine oil samples may be exposed to operational conditions having particular magnitudes of certain engine variables for different lengths of time. Or, a plurality of engine oil samples may be exposed to operational conditions described by certain engine variables, wherein each sample has been exposed to an operational condition with a different magnitude of those engine variables. In other words, each sample may be taken from engine oil that has been exposed to operational conditions that are at a different point along an engine vector, where the engine vector may comprise any engine variable or time. Variation between spectral analyses of samples on the same evolution pathway may be analysed. More than one evolution pathway may be characterised by exposing samples to different operational conditions. The method may comprise obtaining more than one sample of engine oil for each of more than one set of samples. Each sample of engine oil may have been exposed to one of a plurality of operational conditions of the combustion engine, where each operational condition is identified by one or more than one engine variable. Each sample from a set of samples may be on the same evolution pathway, at different points along the evolution pathway. Each set of samples may represent a different evolution pathway. Spectrometry is used to analyse elemental composition and / or chemical bonds of the more than one sample of engine oil to provide a spectral analysis of each sample. For each set of samples, multi-dimensional analysis may be used to interrogate variation between the spectral analyses of the more than one sample of the set of samples with respect to one or more than one engine variable. The variation is associated with the one or more engine variable of the operational condition to which the sample was exposed, and with the evolution pathway of the set of samples. For each set of samples, an evolution pathway of the engine oil is characterised. A set of samples may each be exposed to one of a certain set of operational conditions wherein the set of operational conditions are indicative of using a combustion engine in a certain manner over a certain period of time. In this way, an evolution pathway of engine oil used in a combustion engine that is operated in that manner may be characterised. Evolution pathways may be characterised for different operational conditions. Normal evolution pathways may be characterised for operational conditions that are indicative of a combustion engine operating normally. Abnormal evolution pathways may be characterised for operational conditions that are indicative of a combustion engine operating abnormally. The spectral analysis may comprise a plurality of engine oil variables. In certain embodiments, the multi-dimensional analysis interrogates variation between the plurality of engine oil variables to provide at least one vector of change that is indicative of a difference in elemental composition and / or chemical bonds between the more than one sample. The more than one sample may be on the same evolution pathway. Or, the more than one sample may be on different evolution pathways. The number of vectors of change output from the multi-dimensional analysis may be fewer than the number of engine oil variables of the spectral analyses. The multi-dimensional analysis may be achieved via dimension reduction. Each vector of change may be indicative of a proportion of a total variation in spectral analyses. In this way, a sum of the vectors of change provides the total variation in spectral analyses. Each vector of change may be indicative of a variation of more than one engine oil variable. Certain proportions of each vector of change may be indicative of a variation of certain different engine oil variables, such that variation in more than one engine oil variable is represented by a given vector of change. Different vectors of change may represent the different engine oil variables in different proportions. For example, a first proportion of a first vector of change may be indicative of a variation of a first engine oil variable, a second proportion of the first vector of change may be indicative of a variation of a second engine oil variable, and so on. A third proportion (different from the first proportion) of a second vector of change may be indicative of a variation of the first engine oil variable and a fourth proportion (different from the second proportion) of the second vector of change may be indicative of a variation of the second engine oil variable, and so on. In certain examples, a given vector of change may comprise a proportion that is indicative of each of the plurality of engine oil variables such that variation in all the engine oil variables is represented by a given vector of change. Spectrometry is used to analyse elemental composition and / or chemical bonds of the engine oil samples. Elemental composition may describe a concentration of any vector of change of the engine oil, including concentrations of elements, molecules or compounds that would be found in the initial engine oil formulation and also any contaminant found in the engine oil after use of the engine oil. The initial engine oil formulation is the formulation of “new” engine oil that has not been used in an engine or otherwise exposed to operational conditions. Analysis of chemical bonds of the sample may include any spectral analysis that provides information as to the chemical bonds present in the molecules present in the samples. For example, FTIR or Raman spectroscopy each provide information relating to the chemical bonds present. analysis of an FTIR or Raman spectrum may permit analysis of the same chemical bond (such as carbon-oxygen) in different molecules, and provide information as to the evolution of the engine oil. Analysis of the chemical bonds can provide information both relating to composition of the sample, and to processes such as oxidation, sulfation and nitration that the sample may have undergone. The characterisation comprises one or more differences between a primary spectrum and a secondary spectrum of a first volume of the engine oil, wherein the one or more differences are indicative of a change in the engine oil. The characterisation further comprises an association of the one or more differences to a first operational condition of the combustion engine. The primary spectrum is obtained by using a first spectrometer to analyse a primary sample of the first volume of engine oil and the secondary spectrum is obtained by using the first spectrometer to analyse a secondary sample of the first volume of engine oil. The primary sample is indicative of the first volume of engine oil prior to being exposed to the first operational condition, and the secondary sample is indicative of the first volume of engine oil after being exposed to the first operational condition. The characterisation may further comprise one or more differences between a primary spectrum and a secondary spectrum of a second volume of the engine oil, wherein the one or more differences are indicative of a change in the engine oil. The characterisation may comprise an association of the one or more differences to a second operational condition of the combustion engine. The primary spectrum may be obtained by using a first spectrometer to analyse a primary sample of the second volume of engine oil and the secondary spectrum may be obtained by using the first spectrometer to analyse a secondary sample of the second volume of engine oil. Between obtaining the primary spectrum of the second volume of engine oil and the second spectrum of the second volume of engine oil, the first volume of engine oil may be exposed to a second operational condition. In other words, the primary sample may be indicative of the second volume of engine oil prior to being exposed to the second operational condition, and the secondary sample may be indicative of the second volume of engine oil after being exposed to the second operational condition. The second volume of engine oil may be the first volume of engine oil after the first volume of engine oil has been exposed to the first operational condition. Otherwise, the second volume of engine oil may be separate to the first volume of engine oil. The second operational condition may correspond to any of those detailed above. The characterisation may comprise differences between a primary spectrum and a secondary spectrum of multiple volumes of engine oil exposed to multiple operational conditions, as described in any of the methods above. The characterisation may further comprise one or more differences between a tertiary spectrum and a quaternary spectrum of the first volume of the engine oil, wherein the one or more differences are indicative of a change in the engine oil. The characterisation may further comprise an association of the one or more differences to a first operational condition of a first part of the combustion engine. The tertiary spectrum and the quaternary spectrum may be obtained by using a second spectrometer to analyse a tertiary sample and a quaternary sample of the first volume of engine oil, respectively. Between obtaining the tertiary spectrum and the quaternary spectrum, the first volume of engine oil may be exposed to a first operational condition. The characterisation may comprise differences between primary and secondary spectra of primary and secondary samples, respectively, of multiple volumes of engine oil, wherein each volume of engine oil was exposed to an operational condition between taking the primary sample and the secondary sample. The characterisation may further comprise any processed data provided by any of the methods herein, and / or any trends in the data identified by any of the methods herein, and / or any association of differences in spectra of engine oil samples to operational conditions of a combustion engine provided by any of the methods herein.

Claims

CLAIMS 1. A method of characterising evolution of engine oil during operation of a combustion engine, wherein the engine oil is configured to lubricate the combustion engine, the method comprising: obtaining more than one sample of engine oil, wherein each sample of engine oil has been exposed to one of a plurality of operational conditions of the combustion engine; using spectrometry to analyse elemental composition and / or chemical bonds of the more than one sample of engine oil to provide a spectral analysis of each sample; and using multi-dimensional analysis to interrogate variation between the spectral analyses of the more than one sample, wherein the variation characterises an evolution pathway of the engine oil.

2. The method of claim 1, wherein the variation between the spectral analyses of the more than one sample is expressed as at least one vector of change.

3. The method of claim 2, wherein one or more than one vector of change defines an engine oil evolution pathway.

4. The method of claim 2 or 3, wherein a vector of change is indicative of a change elemental composition and / or chemical bonds in the first volume of engine oil.

5. The method of any of claims 2 to 4, wherein each vector of change is indicative of variation of more than one engine oil variable, wherein variation of each engine oil variable is represented as a certain proportion of the vector of change.

6. The method of any preceding claim, wherein the method is repeated for more than one volume of engine oil exposed to more than one operational condition.

7. The method of any preceding claim, wherein the method characterises more than one evolution pathway of engine oil evolution, wherein each evolution pathway is described by at least one vector of change obtained by carrying out the method of any preceding claim for at least one sample of engine oil exposed to at least one operational condition.

8. The method of any of claims 2 to 7, wherein a particular vector of change is associated with either normal engine oil evolution or abnormal engine oil evolution.

9. The method of any of claims 2 to 8 wherein a set of more than one vector of change comprises more than one orthogonal vector of change.

10. The method of any preceding claim, wherein in an event that a volume of engine oil is exposed to an operational condition that results in abnormal engine oil evolution, the at least one vector of change associated with the volume of engine oil are associated with at least one metric indicative of the operational condition to which the volume of engine oil was exposed.

11. The method of any preceding claim wherein the spectral analysis of the sample comprises a plurality of engine oil variables and wherein optionally the multi- dimensional analysis is carried out via dimension reduction techniques such that the number of vectors of change is fewer than the number of engine oil variables.

12. The method of any preceding claim wherein each operational condition is identified by one or more than one engine variable.

13. The method of any preceding claim wherein exposing a sample of engine oil to an operational condition is achieved by: using the engine oil in a combustion test engine that is not used to drive equipment, wherein the combustion test engine is configured to replicate the operational condition; and / or. using the engine oil in a combustion engine that is used to drive equipment such that the operational condition is implemented; and / or by exposing the engine oil to an experimental condition, wherein the experimental condition corresponds to the operational condition of at least one component, sub-system or system of the combustion engine.

14. The method of claim 13 wherein the sample is exposed to an operational condition using an engine and wherein the variation between the spectral analyses is associated with metadata from the combustion engine.

15. The method of any preceding claim wherein at least a portion of the plurality of operational conditions may comprise local operational condition of a component or sub- system or system of the combustion engine.

16. The method of any preceding claim wherein the spectral analysis of each sample of engine oil comprises at least one of: spectral data; and a plurality of engine oil variables extracted from spectral data.

17. The method of any preceding claim wherein the multi-dimensional analysis is carried out via dimension reduction.

18. The method of any preceding claim wherein the variation(s) between the spectral analyses of the more than one sample comprises at least one of: a change to a peak height of one or more peaks; a change to a location of one or more peaks; a change to a shape of one or more peaks; a change to a trough height of one or more troughs; a change to a location of one or more troughs; and a change to a shape of one or more troughs.

19. The method of any preceding claim further comprising associating data relating to the engine oil or to the use of the engine oil with the operational condition to which a sample was exposed, wherein optionally the data comprises one or more of: a temperature to which the sample was exposed; chemistry of any contaminants to which the sample was exposed; concentration of any contaminants to which the sample was exposed; a surface area of an interface between the sample and any contaminants; a length of time for which the engine oil has been exposed to the operational conditions; a type or model of combustion engine; in an event that exposing the engine oil to the operational condition is achieved by using the engine oil in a combustion engine, data relating to engine operation and / or engine hardware and / or engine age.

20. The method of any preceding claim, wherein more than one sample is taken from the same volume of engine oil and wherein in between taking each sample, the volume of engine oil is exposed to one of a plurality of operational conditions.

21. The method of any preceding claim wherein more than one sample is exposed to operational conditions identified by the same engine variable, such that the more than one sample of engine oil are indicative of engine oil that has been used in a combustion engine operating: over different periods of time; and / or with different values of the engine variable.

22. The method of any preceding claim, wherein the plurality of operational conditions comprises: more than one operational condition of the same component, sub-system or system of the combustion engine; and / or operational conditions of different component, sub-system or system of the combustion engine.

23. The method of any preceding claim wherein using spectrometry to analyse each sample comprises using: Fourier Transform Infrared (FTIR) spectrometry; and / or inductively coupled plasma atomic emission spectroscopy (ICP-AES); and / or Raman spectroscopy; to analyse each sample.

24. The method of any preceding claim wherein using spectrometry to analyse each sample comprises using more than one spectrometer to analyse each sample.

25. The method of any preceding claim wherein the multi-dimensional analysis may be carried out using Principle Component Analysis.

26. The method of any preceding claim wherein the more than one sample is further analysed to provide oil viscometrics information, and wherein multi-dimensional analysis is used to interrogate variation between the spectral analyses and the viscometric information of the more than one sample.

27. The method of claim 26 wherein the viscometric information is obtained as a function of engine oil temperature.

28. The method of claim 26 or 27 wherein the spectral analysis of a sample is processed based on viscometric information and wherein multi-dimensional analysis is used to interrogate variation between the processed spectral analyses, wherein the processed spectral analyses comprise one or more of: a maximum of a peak of a spectrum as a function of viscosity at a specific temperature of the engine oil; a moment of a spectrum normalised to a change in viscosity at a specific condition; and a ratio of two or more peaks of a spectrum, normalised to a viscosity of the engine oil at a certain shear rate.

29. A characterisation of evolution of engine oil during operation of a combustion engine, wherein the engine oil is configured to lubricate the combustion engine and wherein the characterisation comprises variation between spectral analyses of more than one sample of engine oil, wherein: each sample has been exposed to one of a plurality of operational conditions of the combustion engine; and the variation between the spectral analyses of the more than one sample of engine oil are obtained by using multi-dimensional analysis to interrogate variation between the spectral analyses of the more than one sample; wherein spectral analyses of each sample are obtained by using spectrometry to analyse elemental composition and / or chemical bonds of the more than one sample of engine oil.

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