Engine oil service schedule

Abstract

There is provided a characterisation of an engine oil service schedule after operation of a combustion engine, and a method of determining an engine oil service schedule after operation of a combust

Description

Field of the disclosure The disclosure relates to the field of engine oil. 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, neutralizing 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 characterisation of an engine oil service schedule after operation of a combustion engine, wherein the engine oil is configured to lubricate the combustion engine, wherein the characterisation comprises an engine operation function configured to: a. receive as an input data indicative of engine operation during an operation period; and b. provide an output indicative of a predicted property of the engine oil; and wherein the characterisation further comprises a comparison of the output to a condemnation limit. In this way, a service schedule of engine oil may be determined without taking samples of the engine oil during operation of the engine. The service schedule may instead be based on an engine operation function that received data from the engine, and uses that data to predict one or more properties of the engine oil. The data indicative of engine operation may be data that an operator or controller is likely to obtain in normal use. The predicted property or properties of the engine oil may be indicative of performance, health or age of the engine oil, and may therefore be used to determine a predicted remaining life of the engine oil and, therefore, a service schedule for when the engine oil should be changed. The condemnation limit may be a threshold or trend to which the predicted property may be compared. For example, the condemnation limit may be indicative of an acceptable value of the property, wherein in an event that a value of the property passes the condemnation limit it is indicative that the engine oil should be changed within a certain time period. There is also provided: a method of determining an engine oil service schedule after operation of a combustion engine, wherein the engine oil is configured to lubricate the combustion engine, the method comprising: a. operating the engine during an operation period; b. obtaining data indicative of engine operation during the operation period; c. using the data as an input to an engine operation function to provide an output indicative of a predicted property of the engine oil; and d. determining the engine oil service schedule by comparing the output to a condemnation limit. 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 determining an engine oil service schedule according to an embodiment of the present disclosure. Figure 2 shows a flowchart illustrating a method of creating an engine operation function, according to an embodiment of the present disclosure. Figure 3 shows examples of changes to peaks of a spectrum. Figure 4 shows examples of changes to troughs of a spectrum. Detailed description During operation of a 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, neutralizing 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 the 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), sub-system and / or 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 engine oil performance evolves over time. 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. 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 the base oil (or base stock) and any additives, if present. The changes to the engine oil may comprise changes to the base oil itself and / or changes to the additives, if additives are present. As a result of the changes to the engine oil, 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. 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 neutralizing 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, subsystem 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. 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 an accelerated rate of 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 and / or excessive wear or abnormal wear mechanisms. A system or component of the engine may be undergoing initial or intermediate phases of a component or system failure or performance degradation. As described above, 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 form, 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. 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, including one or more of chemical changes, physical 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. 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 any physical analysis techniques. 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, sub-system or system may be considered acceptable. For example, the engine, component, sub-system or 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, sub-systems and 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 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 millimeter of oil remains in a particular local area. The local residence time may be defined for any component interface, sub-system or system. 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 in 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. With reference to Figure 1, a method of determining an engine oil service schedule is provided. At step 110, a user of a vehicle or equipment comprising an engine operates the engine to perform work during an operation period. At step 120, data indicative of engine operation is obtained during the operation period. The data may be obtained at any time during the operation period. The data may be obtained once, or more than once. During operation of the engine, the engine oil evolves. The data is used as an input to an engine operation function at step 130. At step 150, an output 140 of the engine operation function is compared to one or more condemnation limit 160. In this context, operating an engine to perform work may mean any operation of an engine that drives equipment, a machine or a vehicle. 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. Operating the engine at step 110 may comprise operating the engine to perform work according to the definition provided above. For example, operating the engine may comprise any of operating the engine to provide power to a vehicle, machine, equipment or tool; operating the engine as a traction engine; operating the engine as a generator; or operating the engine for some other purpose. The operation period of the combustion engine may comprise any time period during which the combustion engine is operated, either continuously or over discrete sub-intervals. During the operation period, the engine may be operated continuously over the operation period or the engine may be operated over discrete sub-intervals, wherein the engine is not operated between the sub-intervals. The operation period of the combustion engine may comprise any time period during which the combustion engine is operated, either continuously or over discrete sub-intervals. The operation period may comprise one or more sub-intervals during which the combustion engine is operated. The operation period may comprise one or more sub-intervals during which the combustion engine is in standby, turned off, or in storage. The data may be obtained from an engine control module (or engine control unit), or from any sensor or controller associated with the engine. The data may be obtained at intervals, or continuously. In certain embodiments, the data may be obtained at constant intervals. In other embodiments, the data may be obtained at varying intervals. For example, the intervals may reduce in length over time so that data is obtained more frequently. Data may be obtained more frequently in circumstances where the risk of engine, or engine oil, health or performance degradation is considered to be higher. For example, the interval might be longest immediately after the engine oil has been changed, and may then reduce over time so that as the engine oil becomes older data is obtained more frequently. The engine operation function may comprise any function or model configured to receive data 120 indicative of engine operation as an input and provide an output 140. For example, the engine operation function may comprise any of a function, an analytical model, a numerical model, a computer program or model, an algorithm, or other type of function or model. An analytical model may comprise a mathematical model, physics-based model, quantitative model or computational model. The output or outputs 140 of the engine operation function may be indicative of one or more predicted properties of the engine oil. The output or outputs 140 of the engine operation function may be indicative of how properties of the engine oil are expected to change over time based on the engine operation data 120. Changes to the properties of the engine oil may be indicative of one or more of physical changes to the engine oil, chemical changes to the engine oil and contamination of the engine oil. In an example, an output 140 of the engine operation function may comprise a generated spectrum of the engine oil, wherein a generated spectrum of the engine oil may provide predicted properties of the engine oil at a particular time point. A generated spectrum of the engine may be a predicted spectrum corresponding to a spectrum that would be obtained if the engine oil were analysed using a spectrometer. The generated spectrum may, for example, be a generated Fourier-transform infrared (FTIR) spectrum. In another example, the generated spectrum may be a generated inductively coupled plasma atomic emission (ICP-AES) spectrum (also referred to as an inductively coupled plasma optical emission (ICP-OES) spectrum). An ICP-AES spectrum may provide elemental composition of the engine oil (or other elemental analysis), including the presence of engine oil additive elements such as phosphorous, zinc or boron, and contaminant elements such as iron or aluminium. In another example, an output 140 of the engine operation function may comprise a predicted viscosity of the engine oil, or an indicator of a predicted proportion of fuel in the engine oil, or an indicator of a predicted proportion of water in the engine oil, or some other output. In certain embodiments, the output of the engine operation function may comprise one or more generated spectra of the engine oil. Generated spectra may provide predicted properties of the engine oil at a particular time point. The generated spectra may, for example, indicate elemental composition of the engine oil, or provide information regarding the bonds of the molecules within the engine oil. The engine operation function may be used to obtain more than one generated spectrum, wherein each generated spectrum is a predicted spectrum of the engine oil at a particular time point. The generated spectra may be in graphical form or may be provided as generated spectral data. The output of the engine operation function may comprise predicted variances between spectra of the engine oil at different time points, wherein the predicted variances are obtained by comparing two or more generated spectra. The output of the engine operation function may comprise a combination of generated spectra as a function of time and predicted variances between generated spectra of the engine oil at different time points. In certain embodiments, the output of the engine operation function may comprise predicted variances between spectra of the engine oil at different time points, without outputting the spectra themselves. The output of the engine operation function may be characterised by the predicted variances between spectra of the engine oil at different time points. An output 140 of the engine operation function is compared to one or more condemnation limit 150 at 160. The condemnation limit may be any threshold or trend to which the output or outputs of the engine operation function may be compared. The condemnation limit may be indicative of a remaining life of the engine oil. For example, in an event that the output 140 of the engine operation function meets the condemnation limit, it may be indicative that the engine oil has reached a certain percentage remaining life. The output 140 may be compared to more than one condemnation limit, wherein each condemnation limit is indicative of a different remaining life of the engine oil. There may be one or more outputs 140 of the engine operation function. In an event that there is more than one output of the engine operation function, the condemnation limit may comprise a limit for each output, or a limit that is indicative of a relationship between the more than one output. Examples of a condemnation limit may include, but are not limited to, a maximum value of a property of the engine oil; a minimum value of a property of the engine oil; minimum or maximum elemental concentration of a particular element in the engine oil when normalised against an elemental concentration of another element; an integral of a region of a spectrum; a maximum peak of a region of a spectrum; a relative peak height of a region of a spectrum; a ratio of heights of two peaks of a spectrum; a skew across a spectrum; or a combination of the preceding examples. Comparing the output 140 to the condemnation limit 150 may be used to determine when to replace the engine oil. This may allow the interval between replacing the engine oil to be extended safely, by more accurately predicting when the engine oil might need to be replaced. When the engine oil needs to be replaced may depend on multiple factors. In general, the engine oil does not need to be replaced as long as the engine oil performance is such that the engine oil safely protects the engine in all conditions. In an event that the engine oil performance diminishes such that the engine oil cannot safely protect the engine in all conditions, the engine oil may need to be replaced. However, the extent to which the performance can diminish before the engine oil needs to be replaced may depend on several factors, including the specifics of the engine and the operation of the engine. For example, a particular engine may be able to withstand a lower performing engine oil than another engine, or a mode of operation of an engine may affect the importance of engine oil performance. In another example, the local engine oil performance may be used to determine whether any local issues resulting from this oil evolution and performance are likely to occur, such that the engine oil can be replaced before the issues arise. Determining when to replace the engine oil may comprise determining a predicted remaining life of the engine oil as a proportion or percentage of the total life of the engine oil, or as a time to the end of life of the engine oil. The total life of the engine oil may comprise a total useful life of the engine oil over which the engine oil performance is such that the engine oil safely protects the engine in all conditions. The end of the life of the engine oil may be when the engine oil performance diminishes such that the engine oil cannot safely protect the engine in all conditions, or when the engine oil performance drops below a threshold. The predicted remaining life of the engine oil may assume that the engine continues to be used in the same way that it has been since the previous engine oil change. Changing the way in which the engine is operated may alter the remaining life of the engine oil. Repeating the method of Figure 1 during a period of operation of the engine allows the predicted life of the engine oil to be updated based on various factors, including how the engine is operated. The engine operation function may be developed by any appropriate method. Several methods will be described in more detail by way of example, but it will be understood that other methods may be used. The examples of methods of obtaining the engine operation function described below may be used alone or in combination. With reference to Figure 2, the engine operation function may be based on measured engine operating condition data 210 from an engine that is operated over a test period, and measured engine oil properties 220 from engine oil samples taken from the same engine during and / or after the test period. The measured engine operating data may be used at step 230 to characterise engine usage patterns. The measured engine oil properties may be used at step 240 to characterise evolution of the engine oil. The relationship between the engine usage patterns and the evolution of the engine oil may then be characterised at step 250. Operating the engine over the test period may be as described above in relation to operating the engine over the operation period. The engine operation function may be based on measured engine operating data 210 from a plurality of engines that are each operated over a test period, and measured engine oil properties 220 from engine oil samples taken from each engine during and / or after each test period. The measured engine operating data 210 may be any data relating to operating conditions of the engine. For example, the measured engine operating data 210 may comprise one or more of engine speed, engine oil temperature, engine load, engine fuelling amount, air temperature, jacket water temperature, temperature of another fluid, pressure of engine oil, ambient temperature or other data. The measured engine operating data 210 may be as a function of time. The measured engine operating data 210 may be in the form of an array, spreadsheet, database, real data, or other format. The measured engine operating data 210 may be recorded as a function of time, such as by recording a data point at certain time intervals. Examples of suitable time intervals may be every second, every 5 seconds, every 30 seconds, or other interval. The measured engine operating data 210 may be recorded as a histogram, for example by recording a percentage of time over which a particular variable is in each of a plurality of bins. For example, a histogram may indicate a percentage of time over which the engine speed is in each of a plurality of bins, wherein each bin is a range of engine speeds. The measured engine oil properties 230 may be obtained by analysing engine oil samples taken from the same engine during and / or after the test period. The time point(s) of the sample(s) may be known, such that the engine oil sample(s) may be associated with the measured engine operating data prior to and / or at the same time point. The measured engine oil properties 230 may comprise one or more spectra obtained by analysing the engine oil sample using a spectrometer (such as and FTIR spectrometer or an ICP-AES spectrometer), and / or viscosity of the engine oil, and / or an indicator of a proportion of fuel in the engine oil, and / or indicator of a predicted proportion of water in the engine oil, and / or some other output. The engine operation function may be further based on meta-data for the engine, such as location, customer, application, and so on. The measured engine operating data 210 may be used at step 220 to characterise engine usage patterns. The engine usage patterns may be characterised by visual inspection of histograms (or minima or maxima of each variable of engine operating data), dimension reduction techniques, or other techniques. The measured engine oil properties 230 may be used at step 240 to characterise evolution of the engine oil. The evolution of the engine oil may be characterised by visual inspection of minima or maxima of each engine oil property, by dimension reduction techniques, or other techniques. Examples of characterising engine usage patterns by analysis of measured engine operating data 210 will now be described, and may also apply to characterising evolution of the engine oil by analysis of measured engine oil properties 230. For example, engine usage patterns may be characterised using simple mathematical comparisons of the measured engine operating data 210, such as finding numerical differences between the raw data. Engine usage patterns may be characterised using other techniques. These may include data science techniques, such as dimension reduction techniques or binary additive operations, or simplifying the data by other methods. Data science is an interdisciplinary method, that 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 the measured engine operation data. Trends of changes to a particular variable, either in isolation or in combination with trends of changes of other variables, allow a better description of the engine operation. Similarly, evolution of the engine oil may be characterised using simple mathematical comparisons of the measured engine oil properties 230, or by other techniques such as data science techniques. Techniques for identifying changes to the measured engine operation data 210 (or measured engine oil properties 230) may be applied to a matrix or matrices of measured engine operation data 210 (or measured engine oil properties 230). The matrix or matrices may comprise data for two or more time points, such that analysis of the data can be used to identify changes in the measured engine operation data 210 (or measured engine oil properties 230). A simple mathematical assessment or assessments may be applied to a matrix of the data, or more advanced techniques may be used to identify changes between data. Such advanced techniques may include data science techniques. Data science techniques may be used to identify how variations are occurring and to isolate variations to specific variables. In this case, data science may be used to analyse the measured engine operation data 210 in order to establish how the operation of the engine changes over time, or to analyse the measured engine oil properties 230 in order to establish how the engine oil properties change over time. There are many data science techniques that can be used to identify variations in the measured engine operation data 210 (or measured engine oil properties 230). As a non-limiting example, a matrix containing the data can be simplified via dimension reduction. It may then be identified where in the measured engine operation data 210 (or measured engine oil properties 230) a change is seen. 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. In a specific example, a set of data can be presented as a matrix (rows and columns). The set of data may comprise data for more than one time point and, optionally, more than one variable. 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 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 measured engine operation data 210 (or measured engine oil properties 230), or to processed measured engine operation data 210 (or processed measured engine oil properties 230). In certain examples, each set of data in a matrix of measured engine operation data 210 (or measured engine oil properties 230) 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 measured engine operation data 210 (or measured engine oil properties 230) may comprise a plurality of sets of data. Each set of data may provide measured engine operation data 210 (or measured engine oil properties 230) at a particular time point. 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. Irrespective of the technique used to obtain the characterisation of engine usage patterns, the characterisation may provide an easily recognisable visual representation of differences between variables of the measured engine operation data 210 at different time points, which may be more easily interpreted than a visual assessment of the raw data. Similarly, or to obtain the characterisation of engine oil evolution, irrespective of the technique used to obtain the characterisation of engine oil evolution, the characterisation may provide an easily recognisable visual representation of differences between measured engine oil properties 210 at different time points, which may be more easily interpreted than a visual assessment of the raw data. Patterns between measured engine operation data 210 or measured engine oil properties 230 can be established by comparing principle components of the characterisation. The relationship between the engine usage patterns and the evolution of the engine oil may be characterised at step 250. This may be achieved by any suitable method, such as using regression modelling or machine learning. The engine usage patterns and the evolution of the engine oil may be provided as numerical inputs, or as an image or graph. For example, a heat map of time spent at different engine conditions may be used, identified by channels such as engine speed. A heat map for two inputs may be a histogram of time, with buckets for the first input on the x axis and buckets for the second input on the y axis. The first and second inputs may, for example, be engine speed and engine load. Machine learning may be used to correlate the engine usage patterns and the evolution of the engine oil. The inputs for characterising the relationship between the engine usage patterns and the evolution of the engine oil may comprise any quantity that may be measured from a sensor on the engine, such as inlet manifold temperature and / or pressure, exhaust manifold temperature and / or pressure, engine oil temperature, turbo speed, and so on. The inputs for characterising the relationship between the engine usage patterns and the evolution of the engine oil may be calculated. For example, cylinder pressure may be determined from a model that uses engine speed and fuelling to determine where on a speed-load map the operating condition is. The inputs for characterising the relationship between the engine usage patterns and the evolution of the engine oil may vary dependent on the use of the method. Different uses of the method may require different levels of accuracy in the outputs of the engine operation function, requiring different levels of complexity in the creation of the engine operation function. For example, to identify when the engine oil should be changed, a relatively simple engine operation function may be achieved using inputs of speed and fuelling. The relationship between these inputs (speed and fuel) and the output (properties of the engine oil) may be simple such as a linear relationship, or more complex such as quadratics or square root relationships. Additional mathematical relationships may be required such as creating a histogram of the time spent at engine speed and / or fuel points, and / or the skew or kurtosis of the histogram calculated, and / or mathematical functions combining these variables such as the ratio of fuel and speeds may be used. In certain examples, certain ranges of inputs may result in different outputs. For example, below a certain engine load the engine oil may be determined to have negligible evolution for certain engine operation patterns. However, if the engine load is above the certain engine load, the engine oil evolution function may not be negligible and the engine operation function may be more complex. A more complicated model may be required in other scenarios, such as validating new engine platforms and identifying whether the new engine platform stresses oil in different ways or in a harsher manner. In this case, additional inputs may be required when creating the engine operation function. For example, a combination of engine speed, fuelling, inlet manifold temperature, exhaust manifold temperature, cylinder pressure and variables calculated from cylinder pressure, oil temperature in local areas of an engine such as turbo, turbo speed, and other inputs may be used to describe the engine operation. These inputs may have mathematical functions calculated to describe these inputs in similar manners as above, which are then correlated to the characterisation of engine oil evolution. In another instance, these inputs may also be characterised using dimension reduction techniques, and these lower dimension outputs then correlated to calculate oil chemical state in similar methodologies as above. As described, the engine operation function may be based on engine operating data and measured spectra of engine oil samples. In more detail, an engine may be operated, either in a test scenario or to drive equipment (wherein 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). As a result of operating the engine, the engine oil evolves. Engine operating data may be recorded using sensors on the engine, or obtained from the Engine Control Module (ECM). Data obtained from the ECM may comprise calculated parameters or data based on sensor data. Examples of engine operating data may include, but not be limited to, engine speed, engine fuelling amount, engine load, fluid temperatures (such as air temperature, jacket water temperature, engine oil temperature), engine oil pressure, ambient temperature, engine oil type, engine type, and so on. Engine operating data may be recorded as a function of time, or as a histogram with respect to a parameter. Samples of the engine oil may be obtained at sample time points during the operation of the engine (where a sample taken at time = zero is new engine oil that has not been used in engine operation, and a sample taken at time thas been in an engine that has been operated for time f). The samples of the engine oil may be taken when the combustion engine is in operation or when the combustion engine is not in operation. In an event that the samples are taken when the combustion engine is in operation, the sample may be taken downstream of the oil pump such that the oil is under pressure and can flow out of the engine into a sampling container when a valve is opened. The samples may be taken automatically on the engine during operation of the combustion engine. The samples may be taken during a service of the combustion engine, for example by a service engineer. The samples may be taken by an operator, for example before or after the operator operates the combustion engine. Each sample may be analysed to obtain one or more measurements associated with the sample. For example, each sample may be analysed using a spectrometer to obtain a measured spectrum for each sample time point. The measured spectra for each sample time may be associated with the engine operating data for the sample time point. The engine operating data for a sample time point may comprise engine operating data at the sample time point or during a time period preceding the sample time point. At time zero (new engine oil in the engine), certain types of engine operating data that requires operation of the engine may not be available, but certain engine operating data such as engine oil type and engine type may still be associated with the measured spectra at time zero. Meta-data may also be associated with the measured spectra at the sample time points. In another example, a viscosity or other property of the engine oil sample may be measured. An engine operation function may be obtained from the engine operating data and the measured spectra using analytics, algorithms, or other mathematical techniques. For example, a regression model may be used to fit the data, or other data science techniques may be used. The resulting engine oil evolution function may take engine operating data as inputs, and output generated spectra (or generated variances) or other another output for a particular time point. In an event that the output comprises generated spectra, the generated spectra comprise fitted data, and are not the measured spectra used to create the engine operation function. The techniques used to create the engine oil evolution function may further reduce the number of input parameters needed, from the initial number of parameters (data types or channels) used to create the engine oil evolution function. The engine operation function may be obtained from engine operating data and measured spectra obtained from a plurality of engines. The engine operation function may be verified by further operating engines and obtaining further engine operating data and further measured spectra of engine oil samples. The further engine operating data may be used as inputs to the engine operation function, and the outputs of the engine operation function compared with the further measured spectra of engine oil samples. In another example, the engine operation function may be based on one or more models that describe engine operation and / or engine oil evolution. The one or more model may comprise any function or model. For example, the one or more model may comprise any of a function, an analytical model, a numerical model, a computer program or model, an algorithm, or other type of function or model. An analytical model may comprise a mathematical model, physics-based model, quantitative model or computational model. In an example, the engine operation function for an engine may be based on two separate models: an engine operation model and a model of predicted engine oil evolution. The engine operation model may provide a modelled engine operation. Local conditions in the engine may be monitored using sensors. Data obtained from the sensors on the engine may be used as inputs to the model of engine operation to model the local conditions within the engine. The data inputs may include engine speed and fuelling information, and / or other inputs such as manifold pressures, temperatures, or other parameters. The model of predicted engine oil evolution may provide an expected evolution path of the engine oil over its life cycle for particular engine conditions. The modelled engine operation may be used in combination with the expected evolution path to determine at what point in the expected evolution path the engine oil in the engine currently is, providing the output of the engine operation function. For example, a numerical fit may be carried out between the modelled engine operation and the modelled evolution path to determine the extent to which the engine oil has evolved along the expected evolution path. This may provide a generated spectrum of the engine oil at a particular point in time for particular engine conditions, and / or a generated variance between spectra of the engine oil at particular points in time for particular engine conditions. The generated spectrum and / or generated variance may depend on the length of time that the engine oil has been in the engine and on the engine conditions to which the engine oil was exposed during that time. The output of the engine operation function may be compared to the condemnation limit. Furthermore, the future engine oil evolution may be predicted, since the extent to which the engine oil has evolved so far (i.e. the point at which the engine oil has reached along the predicted evolution path) has been determined. In certain embodiments, the engine operation model may provide information regarding the current state of the engine oil. The current state of the engine oil may be a local engine oil state. The engine operation model may use data from one or more engine sensors as inputs, in order to provide the current state of the engine oil. The engine operation model may be weighted to emphasise engine parameters of particular significance, such as fuel or engine speed. The weighting may be carried out by any method. For example, dimension reduction may be used. In certain embodiments, the outputs of the engine operation model may include a data set that is a time series. The time series data set may be arranged into a histogram, with data in a certain time range put into a certain histogram bin. The size (time range) of the histogram bins may vary. Smaller histogram bins can be used to provide more data where the risk to degradation of engine oil performance is greater. For example, the size of the histogram bins might reduce at later times in the time series, since the evolution of the engine oil may be at a later stage and therefore the risk of a decrease in performance may increase. Alternatively, the size of the histogram bin might depend on the engine operating conditions, such that the histogram bin is smaller for riskier engine operating conditions. In certain embodiments, the model of predicted engine oil evolution may be based on a database of engine oil data. For example, the data may be based on spectra of engine oil. In a specific example, the spectra may be Fourier-transform infrared (FTIR) spectra. The predicted evolution path may be obtained from the database, for example using data science techniques. In certain embodiments, dimension reduction techniques may be used to obtain the predicted evolution path. The accuracy of the predicted evolution path may be weighted to variables or areas related directly to engine oil performance. For example, in certain implementations the concentration of phosphorus and calcium might be important. In other implementations, the weighted variables might differ. The predicted evolution path may identify how the engine oil is expected to evolve, depending on the current or previous parameters of the engine oil. For example, the engine oil may be expected to evolve in different ways from the same set condition, depending on the composition of the engine oil. A relationship between the output of the engine operation model and the model of predicted engine oil evolution may be obtained using prior data. In certain embodiments, the relationship may be obtained using a neural network or other modelling techniques. The relationship may be used to determine the extent to which the engine oil has evolved (i.e. the point reached on the predicted evolution path). The relationship may be used to predict the future evolution of the engine oil. Determining the extent to which the engine oil has evolved allows the output of the engine operation function to be compared to the condemnation limit. This comparison allows a remaining life of the engine oil to be determined. In another example, the engine operation model may be developed from the measured engine operating condition data and measured engine oil properties using machine learning algorithms. In another example, the engine operation function may be generated using experimental equipment. The experimental equipment may comprise bench-top laboratory equipment such as a bench-top engine oil reactor. The experimental equipment may be used to replicate the conditions of an individual subsystem. For example, conditions in a top ring turn around zone may be replicated. Changes to the engine oil, including physical changes and chemical changes, may be evaluated. The changes to the engine oil may, for example, be evaluated using spectral analysis of the engine oil. This may be repeated, using the experimental equipment to replicate conditions of a plurality of different individual subsystems. The changes to the engine oil may be associated with each set of conditions, such that the results may be used to generate the engine operation function, wherein engine conditions are associated with changes to the engine oil such that data from an engine may be used as an input to the engine operation function. In certain examples, the engine operation function may then be validated and / or expanded using data from an engine. Data from the engine may include spectral analysis of engine oil samples from an engine, and / or data that is indicative of properties of the engine oil or conditions to which the engine oil was exposed in the engine (such as the number of hours for which the engine oil has been used, or a mass flow rate of the engine oil, or meta data associated with the engine). The data may be associated with changes to the engine oil and / or to engine conditions replicated by the experimental equipment. As described above, the engine operation function may be based on measured engine operating data 210 from a plurality of engines that are each operated over a test period, and measured engine oil properties 220 from engine oil samples taken from each engine during and / or after each test period. The engine operation function may be based on measured engine operating data 210 from a first group of engines that are each operated over a test period, and measured engine oil properties 220 from engine oil samples taken from each engine of the first group of engines during and / or after each test period. The resulting engine operation function may be validated against data from a second group of engines. The second group of engines may each be operated over a validation period, and measured engine operation data may be obtained from each engine of the second group of engines. Measured engine oil properties may also be obtained from engine oil samples taken from each engine of the second group of engines during and / or after each validation period. The measured engine oil properties of the second group of engines may be used as in input to the engine operation function. The output of the engine operation function for each engine of the second group of engines may then be compared to the measured engine oil properties for the same engine. In an event that the output of the engine operation function corresponds closely to the measured engine oil properties for each engine, the engine operation function may be deemed as being ready to use. In an event that the output of the engine operation function corresponds less closely to the measured engine oil properties for each engine, creation of the engine operation function may continue using more data from more engines. Comparing the output of the engine operation function for each engine of the second group of engines to the measured engine oil properties for the same engine may be carried out by any suitable method. More than one method of analysis may be combined. For example, in an event that the output of the engine operation function comprises generated spectra and the measured engine oil properties comprise measured spectra, the generated spectra may be compared to the measured spectra by visual inspection, mathematical comparisons, data science techniques or other methods. In an event that differences between features of the generated spectra and the measured spectra are smaller than a threshold, the output of the engine operation function may be deemed to correspond closely to the measured engine oil properties. Differences between the spectra may comprise one or more of peak (and / or trough) height, peak (and / or trough) location and peak (and / or trough) shape differences. Differences may comprise differences to one peak (or trough) or to multiple peaks (and / or trough). The generated spectra and measured spectra may each comprise a plurality of features. There may be one difference between the generated spectra and measured spectra that may comprise a change to one feature. For example, one peak might change in height. There may be multiple differences between the spectra, comprising changes to multiple features. Similarly, characterising the engine oil evolution may comprise generated spectra, wherein there may be one or more differences between generated spectra at different time points. 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 spectra, 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 generated spectra and measured spectra may comprise peaks and troughs. The difference(s) between the generated spectra at different time points or between generated spectra and measured spectra 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 differences 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. Figures 3 and 4 to indicates some simple examples of possible differences between spectra. 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 a primary spectrum, and the dashed and dotted lines indicate parts of possible secondary spectra, wherein the primary spectrum and secondary spectrum would be spectra of samples of engine oil taken at different times from an engine or generated spectra at different time points. 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 342 shows the trough 441 of the primary spectrum stretched parallel to the x-axis. The dotted line 344 shows the trough 341 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 3and 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 output of the engine operation function is compared to the condemnation limit. As discussed above, the output of the engine operation function may, for example, comprise one or more spectra, such as an FTIR spectrum and / or an ICP-AES spectrum. In an event that the output of the engine operation function comprises an FTIR spectrum, the condemnation limit may comprise one or more of: an integral of certain regions of the spectrum; a ratio of certain peak heights; a moment of certain regions of the FTIR; a number of peaks in a certain region of the spectrum; underlying trends over a full spectrum or a region of the spectrum; and maximum values of specific peaks of the spectrum. 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. In an event that the output of the engine operation function comprises an ICP-AES spectrum, the condemnation limit may comprise one or more of: a minimum concentration of certain elements; a maximum concentration of certain elements; a ratio of new and used concentrations of certain elements; a normalised ratio of new and used concentration of certain elements when compared to another element; a rate of change of element concentration in the engine oil; a rate of change of element concentration in engine oil, normalised to account for engine oil consumption; and a ratio of the rates of change of two element concentrations in oil, normalised to account for oil consumption. In an event that the output of the engine operation function comprises both an FTIR spectrum and a ICP-AES spectrum, the condemnation limit may comprise one or more of: a moment of certain regions of the FTIR at a certain normalised ratio of new and used concentration of certain elements, compared to another element; a number of peaks in a certain region of the FTIR as a function of the concentration of an element, normalised to oil consumption; a ratio of certain peaks in a controlled region of the FTIR, normalised to the ratio of new and used concentration of certain elements. In an event that the output of the engine operation function comprises both an FTIR spectrum and oil viscometries information the condemnation limit may comprise one or more of: the maximum of a peak in a certain region of the FTIR as a function of the viscosity at a specific temperature of the oil, a moment of certain regions of the FTIR normalised to the change in viscosity at a specific condition, a ratio of certain peaks in a controlled region of the FTIR, normalised to the viscosity of the oil at a certain shear rate. There is also provided a characterisation of an engine oil service schedule after operation of a combustion engine, which may be obtained using any of the methods described 5 herein. The characterisation comprises an engine operation function configured to receive as an input data indicative of engine operation during an operation period, and provide an output indicative of a predicted property of the engine oil. The characterisation further comprises a 10 comparison of the output to a condemnation limit.

Claims

1. A characterisation of an engine oil service schedule after operation of a combustion engine, wherein the engine oil is configured to lubricate the combustion engine, wherein the characterisation comprises an engine operation function configured to: a. receive as an input data indicative of engine operation during an operation period; andb. provide an output indicative of a predicted property of the engine oil; and wherein the characterisation further comprises a comparison of the output to a condemnation limit.

2. The characterisation of claim 1, wherein the engine operation function comprises any of:a function;an analytical model;a numerical model;a computer program;a computer model; or an algorithm.

3. The characterisation of claim 1 or 2, wherein the output of the engine operation function comprises one or more generated spectrum of the engine oil.

4. The characterisation of claim 3, wherein the output of the engine operation function comprises:more than one generated spectrum of the engine oil, wherein each generated spectrum is a predicted spectrum of the engine oil at a particular time point; and / orpredicted variances between spectra of the engine oil at different time points.

5. The characterisation of any preceding claim, wherein the output of the engine operation function comprises one or more of:a predicted viscosity of the engine oil;an indicator of a predicted proportion of fuel in the engine oil; and an indicator of a predicted proportion of water in the engine oil.

6. The characterisation of any preceding claim, wherein the engine operation function is obtained using engine operating data of a combustion engine and measured engine oil properties of engine oil samples from the combustion engine.

7. The characterisation of claim 6, wherein the engine operating data comprises one or more of:engine speed;engine oil temperature;engine load;engine fuelling amount;air temperature;jacket water temperature;temperature of another fluid;pressure of engine oil; and ambient temperature.

8. The characterisation of claim 6 or 7, wherein the engine operating data is obtained as a function of time over test period.

9. The characterisation of claim 8, wherein the measured engine oil properties of engine oil samples are obtained by analysing engine oil samples taken from the same engine during and / or after the test period.

10. The characterisation of any of claims 6 to 9, wherein the measured engine oil properties comprise one or more spectra obtained by analysing each engine oil sample using a spectrometer.

11. The characterisation of claim 10 wherein the one or more spectra are obtained by analysing the engine oil sample using:an FTIR spectrometer; and / oran ICP-AES spectrometer.

12. The characterisation of any of claims 6 to 11 wherein the measured engine oil properties comprise one or more of:viscosity of the engine oil;an indicator of a proportion of fuel in the engine oil; and an indicator of a predicted proportion of water in the engine oil.

13. The characterisation of any of claims 6 to 12 wherein the measured engine operating data is used to characterise engine usage patterns and wherein the measured engine oil properties are used to characterise evolution of the engine oil.

14. The characterisation of claim 12, wherein characterising engine usage patterns and / or characterising evolution of the engine oil is achieved using one or more of: visual inspection;mathematical comparison;binary additive operations;simplification of spectra; and data science tools.

15. The characterisation of claim 13 or 14, wherein the relationship between the engine usage patterns and the evolution of the engine oil is characterised to obtain the engine operation function.

16. The characterisation of claim 15, wherein characterising the relationship between the engine usage patterns and the evolution of the engine oil is achieved using one or more of:visual inspection; mathematical comparison; binary additive operations; simplification of spectra; regression modelling;machine learning; and data science tools.

17. The characterisation of any preceding claim, wherein the condemnation limit comprises any threshold or trend to which the output or outputs of the engine operation function may be compared, wherein the condemnation limit is indicative of a remaining life of the engine oil.

18. The characterisation of any preceding claim, wherein the condemnation limit comprises one or more of:a maximum value of a property of the engine oil;a minimum value of a property of the engine oil;minimum or maximum elemental concentration of a particular element in the engine oil when normalised against an elemental concentration of another element;an integral of a region of a spectrum;a maximum peak of a region of a spectrum;a relative peak height of a region of a spectrum;a ratio of heights of two peaks of a spectrum; and a skew across a spectrum.

19. The characterisation of any preceding claim, wherein the output of the engine operation function comprises an FTIR spectrum and wherein the condemnation limit comprises one or more of:an integral of certain regions of the spectrum;a ratio of certain peak heights;a moment of certain regions of the FTIR spectrum;a number of peaks in a certain region of the spectrum;underlying trends over a full spectrum or a region of the spectrum; and maximum values of specific peaks of the spectrum.

20. The characterisation of any preceding claim, wherein in an event that the output of the engine operation function comprises an ICP-AES spectrum, the condemnation limit comprises one or more of:a minimum concentration of certain elements;a maximum concentration of certain elements;a ratio of new and used concentrations of certain elements;a normalised ratio of new and used concentration of certain elements when compared to another element;a rate of change of element concentration in the engine oil;a rate of change of element concentration in engine oil, normalised to account for engine oil consumption; and a ratio of the rates of change of two element concentrations in oil, normalised to account for oil consumption.

21. The characterisation of any preceding claim, wherein in an event that the output of the engine operation function comprises both an FTIR spectrum and a ICP-AES spectrum, the condemnation limit comprises one or more of:a moment of certain regions of the FTIR at a certain normalised ratio of new and used concentration of certain elements,compared to another element; a number of peaks in a certain region of the FTIR as a function of the concentration of an element, normalised to oil consumption; anda ratio of certain peaks in a controlled region of the FTIR, normalised to the ratio of new and used concentration of certain elements.

22. The characterisation of any preceding claim, wherein in an event that the output of the engine operation function comprises both an FTIR spectrum and oil viscometries information the condemnation limit comprises one or more of:the maximum of a peak in a certain region of the FTIR as a function of the viscosity at a specific temperature of the oil,a moment of certain regions of the FTIR normalised to the change in viscosity at a specific condition, anda ratio of certain peaks in a controlled region of the FTIR, normalised to the viscosity of the oil at a certain shear rate.

23. The characterisation of any preceding claim wherein the data is obtained from an engine control module.

24. A method of determining an engine oil service schedule after operation of a combustion engine, wherein the engine oil is configured to lubricate the combustion engine, the method comprising:a. operating the engine during an operation period;5 b. obtaining data indicative of engine operation during the operation period;c. using the data as an input to an engine operation function to provide an output indicative of a predicted property of the engine oil; andd. determining the engine oil service schedule by comparing the output to a condemnation limit.

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