Two-stroke engines with lubrication deficiency detection

Abstract

A crankcase scavenged two-stroke engine (200, 900) comprising a cylinder wall (260) with an intake port (250) and an exhaust port (251 ), where the engine (200, 900) is associated with an idle mode of operation and an active mode of operation, where a speed (w) of the engine (200, 900) is higher in the active mode of operation compared to the idle mode of operation, the engine (200, 900) comprising at least one engine vibration sensor (270) arranged to sense vibration by the engine (200, 900) and to output an engine vibration signal (275), and a control unit (280) configured to detect a lubrication deficiency in the engine (200, 900) based on the engine vibration signal (275), where the lubrication deficiency detection by the control unit (280) is conditioned on that the engine (200, 900) is operating in the active mode of operation.

Description

[0001]TITLE TWO-STROKE ENGINES WITH LUBRICATION DEFICIENCY DETECTION TECHNICAL FIELD The present disclosure relates to hand-held power tools and construction equipment such as wall saws, power cutters, and chain saws. There are disclosed two-stroke crankcase scavenged engines, vibration sensor arrangements, control units and methods for detecting when a combustion engine suffers from insufficient lubrication and for mitigating consequences of insufficient engine lubrication. Some aspects of the disclosure relate to detection of other faults, such as damaged air filters. BACKGROUND Most combustion engines require some form of lubrication system in order to operate successfully. More advanced combustion engines, such as four-stroke engines, often comprise separate oil pumps that provide the required lubrication. Lubrication deficiency in a combustion engine is undesired since it may lead to overheating, increased fuel consumption, and eventually also piston seizure. US 10,900,393 discloses a combustion engine with a control unit configured to control an oil pump based on a sensed vibration of the combustion engine. CH613495 relates to an automatic oil feed arrangement that is controlled based on a sensed temperature or vibration by the combustion engine. US 5,062,399 describes a piston protection system for a two-stroke engine that is based on vibration data obtained from a vibration sensor. There is a need for less complex lubrication deficiency detection methods suitable for smaller crankcase scavenged two-stroke combustion engines. An increase in detection reliability is also desired. There is furthermore a need for cost-efficient manufacturing methods for producing the components that are desirable for execution of the above- mentioned less complex lubrication deficiency detection methods. SUMMARY It is an objective of the present disclosure to provide engines, sensors and sensor systems for detecting insufficient lubrication in smaller two-stroke crankcase scavenged combustion engines of the type used in hand-held power tools and in smaller construction equipment such as floor saws. At least some of the sensors and sensor systems disclosed herein are of limited complexity, and do not require processing circuitry with significant computational power. The objective is at least in part obtained by a crankcase scavenged two-stroke engine comprising a cylinder wall with at least one intake port and at least one exhaust port. The engine is associated with an idle mode of operation and an active mode of operation, where the speed of the engine is higher in the active mode of operation compared to the idle mode of operation. The engine is normally in its active mode of operation when the power tool driven by the engine is actively used to perform a work task, and in the idle mode of operation when to power tool is not being actively used. The engine comprises at least one engine vibration sensor arranged to sense vibration by the engine and to output an engine vibration signal. The engine also comprises or is associated with a control unit that is configured to detect a lubrication deficiency in the engine based on the engine vibration signal in at least one delimited frequency band, such as a predetermined frequency band. A delimited frequency band may be a low-pass band that is delimited by a highest frequency, or a high-pass band which is delimited by a lowest frequency, or a frequency band that is delimited by low and high end frequencies. It has been found that lubrication deficiency gives different vibration signatures in different frequency bands. Vibration magnitude at low frequencies may for instance result in a decrease in vibration magnitude while the same lubrication deficiency results in an increased vibration magnitude at higher frequencies. Thus, by monitoring the engine vibration signal in at least one delimited frequency band, such as a low frequency band and / or a high frequency band, a more robust detection of engine lubrication deficiency is obtained compared to lubrication deficiency detection systems which do not make any frequency band limitations. The lubrication deficiency detection by the control unit is preferably also conditioned on that the engine is in the active mode of operation, i.e., that the engine is not operating in the idle mode of operation. This manner of detecting lubrication deficiency is advantageous since the vibration signature of the engine that indicates lubrication insufficiency becomes much more pronounced at higher engine speeds, at least in crankcase scavenged two- stroke engines, which allows for less complex detection mechanisms compared to, e.g., continuously adapting the lubrication deficiency detection algorithm to the current engine speed as done in US 5,062,399. Attempting to detect lubrication deficiency at too low engine speeds, such as during idling, may result in a significant number of false detections even if detection thresholds or the like are adapted to current engine speed. In other words, lubrication deficiency is, according to some aspects of the teachings herein, only detected based on vibration data collected during high-speed operation of the engine, and not based on vibration data collected during low-speed operation such as idling. This way the number of false detections, i.e., detecting lubrication deficiency when in fact there is no deficiency, can be reduced significantly. The active mode of operation may, e.g., be defined as an engine operation when the engine speed is above a threshold, or some other engine speed dependent acceptance criterion, such as when a trigger or throttle leaves a standby or default position. The engine vibration signal can also be weighted by the engine speed, such that engine vibration data collected during high speed engine operation is given more weight compared to engine vibration data collected during low speed operation. The control unit may be arranged to be powered by a Peltier element attached to the engine, and does not need a dedicated power source, which is an advantage. Some aspects of the present disclosure also relate to vibration sensors that are powered by the ignition system of the engine, e.g., by an ignition module attached to the engine. According to some aspects the vibration sensor is integrated in an ignition module of the engine, allowing cost- effective manufacturing of power tools comprising the engine and the engine vibration sensor. The engine vibration sensor is preferably arranged in connection to a port in the cylinder wall, such as the intake port or the exhaust port of the engine. The engine vibration sensor may for instance be arranged in direct connection to a scavenging channel of the engine that connects the crankcase of the engine to the cylinder, or an exhaust channel of the engine through which exhaust is ventilated from the cylinder, where particularly relevant vibration signals are generated during lubrication deficiency. Some engines comprise scavenging channels that are partly delimited by lids that cover the scavenging channel. It may be advantageous to attach the vibration sensor to such a lid. The at least one engine vibration sensor may comprise a piezoelectric element, such as a piezoelectric ceramic element. This type of vibration sensor is both low cost and robust. Two or more piezoelectric elements can be arranged facing in different directions, in order to provide two- or three- dimensional vibration data. In other words, the vibration sensor may be a one- dimensional sensor arranged to sense vibration by the engine in a single dimension. However, improvements to the system can be obtained if a two- dimensional or even a three-dimensional sensor is used to sense engine vibration. Higher-dimensional vibration data may be particularly suitable for machine learning based algorithms for lubrication deficiency detection, i.e., detection methods that are at least partly based on artificial intelligence (AI). According to preferred aspects, the control unit is configured to detect lubrication deficiency in the engine based on a decrease in vibration magnitude in a first frequency band below 20 kHz and preferably below 15 kHz. This behavior where the vibration magnitude decreases as a result of lubrication deficiency is a particular characteristic of crankcase scavenged two-stroke engine that allows efficient and reliable detection of lubrication insufficiency at an early stage before piston seizure, or other more serious effects result from the lubrication insufficiency. Vibration magnitude may, e.g., be measured as a root-mean-squared (RMS) value of vibration amplitude over some time window, or as a squared value after some type of low-pass filtering. The first frequency band may comprise frequencies above 5 Hz and preferably above 10 Hz or so, and up to about 15 kHz or so. The control unit may also be configured to detect lubrication deficiency in the engine based on an increase in engine vibration magnitude in a second frequency band above 50 kHz and preferably above 100 kHz and up to about 450 kHz or so. This increase in vibration magnitude is more commonly seen also in other engine types, as indicated in some of the related literature mentioned above. It is an advantage that two different mechanisms can be used to detect lubrication deficiency. Some occurrences of lubrication deficiency may show up better as a decrease in magnitude for lower frequencies of vibration, while other occurrences of lubrication deficiency may show up as a more pronounced increase in vibration magnitude at higher frequencies. The second frequency band may comprise frequencies below 500 kHz and preferably below 450 kHz or so. This dual band detection principle provides a more robust detection of lubrication deficiency in crankcase scavenged two-stroke engines compared to systems which only use one frequency band for the detection. It is an advantage that the control unit is configured to detect lubrication deficiency in the engine based on the engine vibration signal in at least one delimited frequency band, since the lubrication deficiency affects vibration characteristics differently depending on the frequency band that is being monitored. The control unit can be configured to detect lubrication deficiency in the engine based on various principles, such as classic thresholding techniques, different statistical tests, and also based on machine learning functions, as will be discussed in more detail below. The engine advantageously comprises a plurality of engine vibration sensors arranged at respective (different) locations in connection to the cylinder wall to sense respective vibrations by the engine. This allows the control unit to get a better idea of the operating condition of the engine and the control unit may therefore be able to detect lubrication insufficiency more reliably. The control unit may also be able to infer and / or predict other types of malfunctions in the engine using the data from the plurality of vibration sensors mounted at different locations around the engine, which is an advantage. These other types of malfunctions may comprise, e.g., damaged or faulty air intake filters, damaged crankshafts, and damage on the power tool driven by the engine, such as damaged saw blades, damaged drill bits, and the like. According to some aspects, the control unit is arranged to receive data indicative of an external vibration generated by an external source. In this case the control unit can be configured to detect lubrication deficiency in the engine based on a combination of the sensed engine vibration and the external vibration. The control unit can use the data indicative of the external vibration to suppress unwanted distortion in the engine vibration signal generated, e.g., by vibration in a tool driven by the engine, such as a saw blade. According to some aspects, the control unit is arranged to limit the maximum engine speed or prevent engine operation in response to detecting lubrication deficiency. This way the control unit can prevent more serious damage to the engine as a result of the detected lubrication deficiency. Control of engine speed can be efficiently implemented via control of the spark plug action in case the lubrication deficiency system is integrated in an ignition module of the engine. The control unit may also be arranged to control a variable oil dispensing system in response to detecting lubrication deficiency, to increase the amount of dispensed oil in response to detecting lubrication deficiency. An oil reservoir for lubricating the engine may, e.g., be provided in connection with the engine and used on demand to provide lubrication as needed, which reduces the amount of oil required to operate the engine. The oil reservoir can be a sealed oil reservoir that lacks an oil refill opening. This sealed oil reservoir is then dimensioned to contain an amount of oil that lasts for the expected lifetime of the power tool driven by the engine. This means that an operator does not have to worry about refilling or monitoring the oil level in the oil reservoir, which is an advantage. There is also disclosed a crankcase scavenged two-stroke engine comprising a cylinder wall with an intake port and an exhaust port, a spark plug, and an ignition module fixedly attached to a motor block of the engine and electrically connected to the spark plug. The engine comprises an engine vibration sensor that is arranged to sense vibration by the engine and to output an engine vibration signal. The engine also comprises a control unit configured to detect a lubrication deficiency in the engine based on the engine vibration signal. The engine vibration sensor is integrated with the ignition module of the crankcase scavenged two-stroke engine. The control unit may also be integrated with the ignition module. This particular placement of the vibration sensor has at least two benefits. The ignition module is normally directly attached to the motor block, and therefore receives the relatively weak vibrations generated by the piston and cylinder interaction during lubrication deficiency with sufficient magnitude. The ignition module also generates electrical power for driving the spark plug. This electrical power can be used to power the vibration sensor, and optionally also the control unit. An integration of the vibration sensor system and the ignition module also allows the lubrication deficiency detection system to be assembled with various power tools in a cost-efficient manner. An ignition module comprising an engine vibration sensor and a control unit according to the teachings herein can also control the engine speed directly by adjusting the spark plug action in dependence of the vibration signal. The engine speed can, for instance, be reduced by reducing the amount of sparks generated over a time window. The present disclosure also relates to a crankcase scavenged two-stroke engine comprising a cylinder wall with an intake port and an exhaust port. The engine comprises at least one engine vibration sensor arranged to sense vibration by the engine and to output an engine vibration signal, and a control unit configured to detect a lubrication deficiency in the engine based on the engine vibration signal, where the control unit is configured to detect lubrication deficiency in the engine based on a decrease in vibration magnitude in a first frequency band and on an increase in engine vibration magnitude in a second frequency band above the first frequency band. There are also disclosed herein various methods and technical features that can be used in a stand-alone manner, as will be described in the following. Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a / an / the element, apparatus, component, means, step, etc." are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated. Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the following description. The skilled person realizes that different features of the present invention may be combined to create embodiments other than those described in the following, without departing from the scope of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS The present disclosure will now be described in more detail with reference to the appended drawings, where Figure 1 illustrates an example hand-held power-tool; Figure 2 schematically illustrates a two-stroke engine, Figures 3A-B illustrate vibration by a piston ring passing a cylinder wall port, Figures 4A-B are graphs illustrating vibration magnitude vs time, Figure 5 is a block diagram illustrating a signal processing function, Figure 6 is a flow chart that illustrates an example method, Figure 7 shows an example engine vibration sensor, Figure 8 is a flow chart illustrating a method for producing an engine vibration sensor, Figures 9A-B show an example two-stroke engine with an ignition module, Figure 10 schematically illustrates a piezoelectric vibration sensor arrangement, Figure 11 illustrates a three-dimensional vibration sensor, and Figure 12 shows an example control unit comprising processing circuitry. DETAILED DESCRIPTION The invention will now be described more fully hereinafter with reference to the accompanying drawings, in which certain aspects of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments and aspects set forth herein; rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout the description. It is to be understood that the present invention is not limited to the embodiments described herein and illustrated in the drawings; rather, the skilled person will recognize that many changes and modifications may be made within the scope of the appended claims. Figure 1 illustrates an example hand-held power tool 100, in this case a power cutter that can be used to cut hard material work objects such as concrete and stone. This type of hand-held power tool is often driven by a two-stroke crankcase scavenged combustion engine 110 that normally offers a high power-to-weight ratio. The present disclosure relates generally to lubrication in two-stroke crankcase scavenged combustion engines of the kind used in power cutters, chain saws, leaf blowers, clearing saws, and other hand-held power tools. Some of the techniques discussed herein can also be used in light construction equipment such as floor saws and the like. Figure 2 schematically illustrates an example crankcase scavenged two-stroke engine 200. The engine 200 comprises a piston 210 that reciprocates in a cylinder 220 that also comprises a spark plug 230 or other ignition means in a known manner. A crankshaft 245 comprised in a crankcase 240 is brought into rotation by the reciprocating action of the piston 210, to drive the crankshaft at an engine speed ^. The engine speed ^ is controlled by the trigger 120 which is connected to the throttle arrangement of the engine 110, 200. The engine speed can be determined directly using an engine speed sensor, such as a Hall effect element or rotary encoder arranged on or in connection to the engine drive shaft. The engine speed can also be indirectly determined from a position of the trigger 120. Hand-held power tools such as the tool 100 are associated with an idle mode of operation where no work is performed, and an active mode of operation where the trigger is depressed, and the engine speed is increased compared to the idle mode of operation. The engine normally has a maximum speed of operation, obtained at wide open throttle (WOT). The crankcase scavenged two-stroke engine 200 comprises a cylinder wall 260 that partly defines the interior volume of the cylinder 220 in which the piston 210 reciprocates. This cylinder wall comprises at least an intake port 250 and an exhaust port 251 and may also comprise other ports such as a transfer port 252 and / or one or more airhead ports for stratified scavenging of the combustion engine 200. A transfer channel 253 extends between the crankcase 240 and the combustion chamber in the cylinder 220. The transfer channel has a transfer port 252 that opens up into the cylinder 220. The intake port 250 is used to transfer air into the engine to enable combustion in the combustion chamber. The intake flow via the intake port often also comprises fuel in an air / fuel mixture, although fuel injection systems that inject fuel directly into the crankcase 240 or into the cylinder 220 are also known. The exhaust port 251 is used to allow exhaust from the combustion to escape the cylinder after combustion has occurred. The scavenging system on a two- stroke engine can be rather complex, with features such as stratified scavenging where clean air is let into the cylinder via airhead channels before the air / fuel mixture is allowed to enter the cylinder. Stratified scavenging can be achieved by intricate channels formed in the piston skirt, or by forming separate ports in the cylinder wall. The present disclosure is applicable to a wide range of crankcase scavenged two-stroke engines 200, with or without stratified scavenging. The present disclosure is applicable to both carburettor- based engines and to engines with fuel injection systems. The combustion engine 200 comprises a lubrication system which reduces friction between the different surfaces of the engine. Some two-stroke combustion engines are driven by fuel where motor oil has been added, and other combustion engines have separate oil systems that are arranged to dispense oil in a controlled amount into the combustion engine during operation. It is important that the engine is not operated with lubrication deficiency, i.e., without a sufficient amount of oil present in the engine during operation. Lubrication deficiency can occur for many reasons, such as malfunction in an oil dispensing system. A common reason for lubrication deficiency in a two-stroke crankcase scavenged combustion engine is that the operator is using fuel without an appropriate amount of motor oil additive, i.e., that the operator is using the wrong type of fuel. The engine 200 is associated with an idle mode of operation and an active mode of operation. The idle mode of operation is a stand-by mode of operation that is obtained when the engine is started without the trigger 120 being depressed. The active mode of operation is obtained by activating the tool, such as by depressing the trigger 120, which opens up the engine throttle and accelerates the engine. The speed ^ of the engine 200 is higher in the active mode of operation compared to the idle mode of operation. The engine 200 comprises at least one engine vibration sensor 270 arranged in connection to the cylinder wall 260 to sense vibration by the engine 200 and to output an engine vibration signal 275. The engine also comprises or is at least associated with a control unit 280 that is configured to detect a lubrication deficiency in the engine based on the engine vibration signal in at least one delimited frequency band. This means that the control unit monitors engine vibration in a frequency selective manner. The control unit may, e.g., monitor vibration in a high frequency band above some start frequency and / or in a low frequency band below some end frequency. The control unit may also process the vibration signal in one or more different frequency bands delimited by respective start and stop frequencies. A delimited frequency band is, generally, a subset of frequencies that the control unit processes in some way. The control unit 280 may perform the frequency selective processing of the engine vibration signal 275 by applying analog or digital frequency filters, such as low-pass filters, high-pass filters, or band-pass filters. The control unit 280 may also perform the frequency selective processing of the engine vibration signal 275 by signal processing operations such as Fourier transforming the engine vibration signal. The control unit 280 generally applies different detection rules to detect lubrication deficiency in dependence of the particular frequency band that is being monitored, as will be discussed in more detail below. The engine 200 preferably also comprises a control unit 280 configured to detect lubrication deficiency in the engine 200 based on the engine vibration signal 275 and also based on the mode of operation of the engine 200. In other words, the detection of lubrication deficiency is based on both the engine vibration and on the speed of the engine. It has been found through practical experimentation that it is particularly advantageous to combine frequency selective processing of the engine vibration signal with conditioning on engine speed. I.e., to only attempt to detect lubrication deficiency if the engine is operating above idle speed, and to perform the lubrication deficiency detection in a frequency selective manner by monitoring different frequency bands of the engine vibration signal and applying different lubrication deficiency detection principles for the different frequency bands of the engine vibration signal. A look-up-table (LUT), analytical function, or the like can be used to translate roughly between trigger position or throttle state and engine speed and / or operating mode of the engine 200. Thus, data indicative of trigger position and / or throttle state is considered equivalent to engine speed data herein and indicates an operating state of the engine. Thus, according to some aspects the control unit 280 is configured to detect lubrication deficiency in the engine 200 based on the engine vibration signal 275 and also based on the state of the throttle and / or based on the trigger position. A LUT, analytical function, or the like, can also be used to translate roughly between engine vibration magnitude and engine speed and / or operating mode of the engine 200. Thus, data indicative of vibration magnitude is considered equivalent to engine speed data herein and indicates an operating state of the engine. Thus, according to some aspects the control unit 280 is configured to detect lubrication deficiency in the engine 200 based on the engine vibration signal 275, conditioned on that the engine vibration magnitude is strong enough (e.g., above a threshold). It is also possible to weight the engine vibration data by its magnitude, such that strong engine vibration (during the active mode of operation) is given more consideration when detecting engine lubrication deficiency compared to weak engine vibration (such as during idling). The engine vibration sensor 270 is connected to the control unit 280 via interface wires that carry the engine vibration signal 275. The interface between the engine vibration sensor 270 and the control unit 280 can be single-ended or differential (one or two wires). The control unit 280 may implement a lubrication deficiency detection function that is activated as soon as the engine leaves the idle mode of operation and inactivated otherwise. The control unit, when in the active mode of operation, monitors the vibrations generated by the combustion engine, looking for signs of lubrication deficiency. It has been realized that the detection efficiency in detecting lubrication deficiency by the control unit is improved if said detection routine focuses on time periods where the combustion engine is operated at engine speeds above the idle engine speed. This is because the vibration patterns indicative of lubrication deficiency become more pronounced at higher engine speeds. A two-stroke combustion engine is also normally capable of operating at idle speeds for an extended period of time despite suffering from lubrication deficiency. Hence it is not as critical to detect lubrication deficiency when the engine is in the idle mode of operation, compared to when the two- stroke engine is in the active mode of operation. The active mode of operation can be defined as engine operating modes where the engine is operating at speeds above some percentage of its maximum speed, such as above 80% of the maximum engine speed. The detection can be conditioned on that the engine is operating in the active mode of operation. In this case the detection routine is simply suspended when the engine is not in the active mode of operation. The detection can also be performed on weighted engine vibration signal data collected during engine operation, where the weight is configured as an increasing function of engine speed. This way engine vibration data collected during engine operation at high speeds is given more weight compared to engine vibration data collected during engine operation at low speed, such as during idling. Suppose for instance that the control unit 280 collects engine vibration data samples ^[^], where ^ is a time index. A test variable can then be formed by weighting the data samples by the engine speed ^, as ^^[^] = ^ ^[^] The weighting can also be non-linear, e.g., ^^[^] = ^^^[^] The weights can also be binary, such that only vibration data samples ^[^] collected by the control unit 280 during engine operation above some engine speed threshold is considered. Engine vibration data samples ^[^] collected during engine operation below the engine speed threshold are then discarded by the control unit 280. The detection mechanism can be based on a simple thresholding of the vibration magnitude or other test statistic after some low-pass filtering or averaging, and other more advanced detection principles can of course also be applied here. A test statistic ^[^]can for instance be formed over time ^ as ^[^]= ^[^ − 1]+ ^^^^^^[^]^− ^[^ − 1]^where ^ ≪ 1 is a filter weight and where ^^^^[^]^is a magnitude function such as an absolute value ^^^^[^]^ = |^^[^]| or a squaring operation ^^^^[^]^ = ^^^[^]^^. It is appreciated that there are several different ways in which the control unit 280 can be configured to detect lubrication deficiency in the engine 200 based on the engine vibration signal 275 and on the speed ^ of the engine 200. One example is a straight-forward threshold operation applied on the test statistic ^[^]. Several detection methods are known in the art. Detection methods will therefore not be discussed in more detail herein. It has been realized that at least two different types of vibration occur in a two- stroke combustion engine. A first type of vibration occurs as the moving parts of the engine interact with each other during operation of the engine. The piston 210, for instance, will not slide perfectly evenly against the cylinder wall, even if an oil film of sufficient thickness is present between the piston and the cylinder wall. The relative motion of the crankshaft and connecting rod, and the relative motion of the piston and the connecting rod will also not be perfectly even but will give rise to vibration during operation of the engine. This uneven motion between engine components gives rise to high frequency vibration, i.e., vibrations in a frequency band above 50-100 kHz or so. A lubrication deficiency in the engine will make this vibration more pronounced, such that the magnitude of the high frequency vibrations increase. A second type of vibration occurs as the piston 210, and in particular the piston ring or rings arranged on the piston, interacts with the cylinder wall ports 250, 251, 252. Figure 3A schematically illustrates an example of a piston ring 310 that moves across a port 320. As the piston ring traverses the port it expands a bit into the port 320, and then hits the port edge on its way past the port. This “collision” 330 generates an impulse in the piston ring and in the surrounding material which triggers an oscillation in the material, as illustrated schematically in Figure 3B, where an amplitude A of the oscillation is schematically plotted vs time t. This generates vibration in a lower frequency range compared to the high frequency range vibrations caused by uneven sliding motion of the parts of the engine as they interact with each other in use. Interestingly, it has been found that the magnitude of these lower frequency vibrations decrease during lubrication deficiency, at least in two-stroke crankcase scavenged combustion engines, since the oscillations are damped stronger due to the increase in friction between moving parts of the engine. In Figure 3B, this is illustrated by the solid line 340 and by the dashed line 350, where the dashed line 350 illustrates the stronger damping seen during lubrication deficiency in the combustion engine 200. Important vibration data is generated as the piston passes the one or more ports 320 of the cylinder wall. Thus, it makes sense to emphasize vibration data collected as the piston moves across a port when detecting lubrication deficiency. Thus, according to some aspects, the control unit 280 is arranged to obtain crankshaft angle data indicative of a crankshaft angle, and to selectively obtain and / or process the engine vibration signal 275 as function of the crankshaft angle. According to one example the control unit 280 only uses vibration data associated with a certain range of crankshaft angle. According to another example, the control unit 280 emphasizes vibration data associated with a certain range of crankshaft angle, e.g., by applying a larger weight to this data in the detection process. Figures 4A-B illustrate measurement results from an example two-stroke crankcase scavenged combustion engine operated alternately at an idle mode of operation 410 and at an active mode of operation 420, i.e., the trigger 120 on the hand-held power tool is first depressed, then released, then depressed again, and so on. Figure 4A shows an example 400 of root-mean-square (RMS) value of magnitude for vibrations in a high frequency range from about 100kHz to about 450kHZ in an oil deficient operating scenario. It is noted that the magnitude of the high frequency vibrations shows an increasing trend 430 over time. Lubrication deficiency can for instance be detected based on the sensed vibration by comparing the magnitude or power of the vibration in the high frequency range to a threshold 440, or by using some other suitable detection criteria. Figure 4B shows an example 450 of root-mean-square (RMS) value of magnitude for vibrations in a low frequency range from about 10Hz to about 15kHZ in an oil deficient operating scenario. In this case the trend 460 is instead decreasing, which supports the discussion above. A threshold 470 or some other suitable detection criterion can be used also here to detect lubrication deficiency in the two-stroke crankcase scavenged combustion engine 200. Based on this understanding of the vibrations generated in a crankcase scavenged two-stroke engine, the control unit 280 may advantageously be configured to detect lubrication deficiency in the engine 200 based on a decrease in vibration magnitude in a first frequency band below 20 kHz and preferably below 15 kHz, i.e., in a low frequency band. The first frequency band may be delimited on the low side at 5-10 Hz or so in order to avoid too low frequencies that have a tendency to appear as biases over shorter time periods. The vibration magnitude may, e.g., be measured as a root-mean- squared value of the vibration picked up by the vibration sensor 270, or by some other suitable metric of vibration magnitude, as discussed above. The control unit 280 may also be configured to detect lubrication deficiency in the engine 200 based on an increase in engine vibration magnitude in a second frequency band above 50 kHz and preferably above 100 kHz, i.e., in a high frequency band. The second frequency band may be delimited on the upper side at 450-500 kHz or so to avoid unnecessarily high requirements on sampling frequency by the vibration sensor. The detection criteria used by the control unit 280 to detect lubrication deficiency in the engine 200 may be based on straight forwards thresholds 440, 470 as exemplified in Figures 4A-B. This is a detection mechanism that is associated with a small computational burden and therefore advantageous in control units 280 with limited processing resources. However, more advanced detection mechanisms may also be contemplated, such as statistical detection tests. The detection may be based solely on monitored vibration in the first frequency band (the low frequency band), or on a combination of monitored vibrations in the first and the second frequency bands. A detection of lubrication deficiency may according to some aspects be declared if one out of the two detection criteria is fulfilled (at least one out of the high frequency band and the low frequency band vibration indicates lubrication deficiency). A detection of lubrication deficiency may according to some other aspects be declared only if both of the two detection criteria are fulfilled simultaneously. According to some aspects, the control unit 280 is configured to detect lubrication deficiency in the engine 200 based on a machine learning function. A machine learning function, often also referred to as an artificial intelligence structure or function, is a mathematical model that is first trained using example vibration data from engines suffering from lubrication deficiencies of varying degree, and also vibration data from engines that are sufficiently lubricated. The different data sets are preferably also collected from different use cases and may also be collected solely from a certain type of tool, such as a power cutter if the control unit is to be used with engines in power cutters, and from chain saws if the control unit is to be used together with engines in chain saws. The machine learning function may, e.g., comprise a random forest structure, a convolutional network, or some other machine learning structure, and is trained using known techniques from machine learning. After convergence, i.e., after the machine learning structure has been trained to a point where it fulfils some predetermined detection performance criteria, such as a given percentage of false alarms and a given percentage of missed detections, it may be deployed in the control unit and used to detect lubrication deficiency in the engine. The inputs to the machine learning function are the engine vibration signal 275 and the engine speed or a signal indicative of engine operating mode, such as the trigger position. Several actions may be taken in response to detecting lubrication deficiency in the engine 200. For instance, the control unit 280 can be arranged to limit the maximum engine speed or prevent engine operation in response to detecting lubrication deficiency. This prevents the lubrication deficiency from causing irreparable damage to the engine 200, which is an advantage. The control unit 280 can also be arranged to control a variable oil dispensing system in response to detecting lubrication deficiency. This way a smaller amount of oil can be used overall, and oil only has to be added to the engine internals when actually needed. The oil dispensing system may be configured to dispense oil according to a timer function or the like and dispense extra oil in case lubrication deficiency is detected. The oil dispensing system may also be configured to only dispense oil when lubrication deficiency is detected by the control unit 280. According to some aspects, the variable oil dispensing system comprises a sealed oil reservoir, i.e., an oil reservoir which lacks a refill opening that is easily accessible by an operator. The sealed oil reservoir may be a plastic container without opening that is directly attached to the oil pump or attached to the oil pump via an oil conduit. The hand-held power tool is delivered with the sealed oil reservoir filled with oil, and the amount of oil in the reservoir is dimensioned so as to last for the lifetime of the tool. The control unit 280 can be powered by some sort of energy harvesting mechanism, such as a Peltier element attached to the engine 200. This is an advantage sine the mechanism then does not require an external power source to operate, such as a battery or capacitor. The at least one engine vibration sensor 270 preferably comprises a piezoelectric element. Piezoelectricity is the creation of electric potential in certain materials when they are under mechanical stress, such as bending, stretching, or compressing. The materials that exhibit these characteristics are called piezoelectric materials, and there are many such materials. A sub-class of the piezoelectric materials are the piezoelectric ceramic materials, which include, e.g., Barium Titanate, Potassium Niobate, Sodium Tungstate and Lead Zirconate Titanate (PZT). The latter is widely used in practical applications and is a mix of lead zirconate and lead titanate. PZT has higher piezoelectric sensitivity and greater stability at high temperatures than many other materials. In addition, the piezoelectric properties of PZT can be formulated to be hard or soft. Vibration sensors based on piezoelectric elements are cost effective and have been found to yield reliable results in detecting lubrication deficiency in two-stroke crankcase scavenged combustion engines of the type used in hand-held power tools. The engine vibration sensor 270 is advantageously arranged in connection to a port 250, 251, 252 in the cylinder wall 260. This way the vibration induced by interaction between the piston ring and the port is captured better by the vibration sensor. The magnitude of the vibrations generated by the interaction between the piston and the various elements of the cylinder wall of course declines with distance from the point of origin, and it is therefore an advantage to capture the vibration as close to the source as possible. The engine 200 may for instance comprise an engine vibration sensor 270 arranged in connection to a scavenging channel of the engine 200. It may also be advantageous to deploy several vibration sensors in connection to the cylinder wall of the engine, where each sensor covers a specific part of the cylinder wall, such as a specific cylinder wall port. Thus, the engine 200 optionally comprises a plurality of engine vibration sensors arranged at respective locations in connection to the cylinder wall 260 to sense respective vibrations by the engine 200. It is appreciated that strong vibration from external sources, such as vibrations generated by interaction between the power tool and a work object, may have a negative effect on the capability of the control unit to detect lubrication deficiency in the engine based on vibration generated during operation of the engine. To reduce the impact of vibrations from external sources, additional vibration sensors may be arranged separated from the engine and used to sense such external vibration. This data indicative of the external vibration can then be used to suppress interference generated by the external vibrations. In other words, the control unit 280 may be arranged to receive data indicative of an external vibration generated by an external source, where the control unit 280 is configured to detect lubrication deficiency in the engine 200 based on a combination of the sensed engine vibration and the external vibration. Figure 5 illustrates an example of a signal processing structure for suppressing external vibration. One or more vibration sensors are used to sense external vibration 510 at one or more locations physically separated from the combustion engine, or at least from the combustion engine cylinder wall. This data ^ is here input to an equalizer structure 520, such as an adaptive finite impulse response (FIR) filter, which has a number of equalizer taps in a known manner. The output from this equalizer structure 520 is removed 530 from the vibration sensor data captured by the vibration sensor 270 before it is fed to the detector function in the control unit 280. A time delay by a delay element 540 may be necessary to compensate for the delay incurred by the equalizer structure 520. The equalizer structure 520 may, e.g., be updated adaptively based on the orthogonality principle to make the difference signal ^ orthogonal or uncorrelated with the external vibration data ^. A least-mean-squares (LMS) update principle may, e.g., be adopted, where the ^:th equalizer tap is updated at discrete time instant ^ + 1 as ^^[^ + 1] = ^^[^] + ^ ^[^]^[^ − ^] where ^ is a step size that can be fixed or adaptively determined in a known manner. The adaptive equalizer structure 520 finds correlation between the external vibration data ^ and the engine vibration data ^, and proceeds to remove this correlated part. Thus, any interference from the external vibration data in the engine vibration data is effectively removed. Adaptive equalizer structures of this type are generally known and will therefore not be discussed in more detail herein. The techniques disclosed herein may also be described in terms of a method performed by a control unit 280 arranged in connection to a crankcase scavenged two-stroke engine 200, as illustrated by the flow chart in Figure 6. The method comprises obtaining Sa1 engine vibration data from a vibration sensor 270 arranged in connection to the cylinder wall 260 and detecting Sa2 lubrication deficiency in the engine 200 based on the engine vibration data, when the engine 200 is in the active mode of operation. According to some aspects, the method also comprises suppressing Sa21 vibration from an external source in the engine vibration data, based on data indicative of the vibration from the external source. Figure 7 illustrates an example engine vibration sensor 270 attached to a control unit 280 via interface wires that transmit the engine vibration signal 275 from the sensor 270 to the control unit 280. This engine vibration sensor 270 has been produced in a way that allows cost-efficient large scale production. Figure 8 is a flow chart that illustrates this method. Figure 8 shows a method for producing an engine vibration sensor 270, such as the sensor discussed above and illustrated in, e.g., Figure 2. The method comprises providing Sb1 an engine vibration sensor body 710. This engine vibration sensor body may be formed as a cup, i.e., a cylindrical hollow piece with a flat bottom and internal volume large enough to receive a piezoelectric element, such as a piezoelectric ceramic element. The sensor body may be formed in metal, such as brass, bronze, or some other material that can be used in a soldering process. A metal coated polymer or the like can also be used as engine vibration sensor body. The method comprises placing Sb2 a piezoelectric element 720 on the vibration sensor body 710, with solder paste 715 added inbetween the piezoelectric element 720 and the vibration sensor body 710. Any suitable solder paste can be used inbetween the piezoelectric element 720 and the vibration sensor body 710, such as the type of solder paste used in reflow ovens for production of printed circuit boards (PCB). The method also comprises placing Sb3 a counterweight element 730 onto the piezoelectric element 720, again with solder paste 725 added inbetween the piezoelectric element 720 and the counterweight element 730. The counterweight element 730 has dimensions and weight matched to the engine 110, which dimensions, and weight can be determined in a straightforward manner from practical laboratory experimentation and / or from computer simulation. The counter- weight element is thus designed to provide a strong and clear vibration signal output by the piezoelectric element, at the desired frequency range. I.e., in a frequency range comprising the high frequencies discussed in connection to Figure 4A and the low frequencies discussed in connection to Figure 4B above. The counter-weight element may also be formed in brass, bronze, or some other solder-friendly material. A metal-coated polymer can also be used here, although the weight of the polymer may cause the counter-weight to be rather large. The whole engine vibration sensor 270 assembly is then heated S4 in other to fixedly attach the components to each other. A large amount of engine vibration sensors can be manufactured in this manner, e.g., using a pick-and- place machine and a reflow oven, similar to the production methods for producing large amounts of PCBs. The method may also comprise attaching Sb5 interface wires to the engine vibration sensor body 710 and to the counterweight element 730, although this step can also be performed when assembling the engine vibration sensor with an engine to be monitored for lubrication deficiency. The interface wires may be attached by soldering or by some other attachment mechanism, such as threaded fastening arrangements, interference fits, or friction-based connectors. Any space left between the piezoelectric element 720 and the counterweight element 730 and the vibration sensor body 710 may be filled up by some suitable filler 740, such as epoxy resin or the like. Figures 9A-B show an example crankcase scavenged two-stroke engine 900, suitable for a hand-held power tool of the type illustrated in Figure 1. The engine 900 can also be used in other types of tools and machines, such as floor saws. It is appreciated that the engine 900 is an example engine, and that the techniques disclosed herein are generally applicable to several different types of two-stroke combustion engines. The engine 900 comprises a spark plug 910 arranged to ignite a fuel and air mixture in the cylinder of the engine. The sparks are generated from electrical pulses transmitted from an ignition module 920 of the engine in a known manner. The electrical connection 925 between the ignition module 920 and the spark plug 910 is schematically indicated by the dash-dotted line in Figure 9A. The engine further comprises a flywheel 930 which also provides a cooling air flow to transport heat away from the engine 900. This flywheel is also used by the ignition module 920 to harvest energy by means of a magnet and coil arrangement. An ignition module is, generally, a component enclosed in a housing which is fixedly attached to the motor block 940 of the engine. The ignition module normally comprises a coil configured to provide electrical energy by means of a magnet attached to the flywheel of the engine, or to some other rotating component, in a known manner. The energy obtained via the coil is transmitted to the spark plug 910 in pulses, where each pulse generates a spark by the spark plug. The ignition module normally comprises circuitry configured to control the timing of the spark pulses transmitted to the spark plug 910. At least one vibration sensor 270 is optionally integrated with the ignition module 920, i.e., comprised in the same housing as the coil and the control circuitry for the electrical pulses that are transmitted to the spark plug 910 from the ignition module 920. The example in Figure 9A shows two vibration sensors integrated with the ignition module. The control unit arranged to detect lubrication deficiency based on the vibration signal from the vibration sensor 270 may also be integrated in the ignition module 920. The vibration sensor 270 integrated with the ignition module is fixed to the motor block 940 of the engine by the same fastening means as the ignition module 920. Vibrations from the engine propagates to the ignition module and thus reaches the vibration sensor 270. According to some aspects the ignition module 920 is arranged to control ignition of the engine 900 based on the lubrication deficiency detection by the control unit 280. The control circuitry in the ignition module may, e.g., refrain some transmitting some of the electrical pulses, i.e., to cut out some ignitions, thereby reducing the engine speed. The feature of integrating the engine vibration sensor with the ignition module is not inextricably linked to any of the other technical features discussed herein. With reference also to Figure 2, there is disclosed herein a crankcase scavenged two-stroke engine 200, 900 comprising a cylinder wall 260 with an intake port 250 and an exhaust port 251, a spark plug 910, and an ignition module 920 electrically connected to the spark plug 910. The engine 200, 900 comprises at least one engine vibration sensor 270 that is arranged to sense vibration by the engine 200, 900 and to output an engine vibration signal 275. The engine 200, 900 also comprises a control unit 280 that is configured to detect a lubrication deficiency in the engine 200, 900 based on the engine vibration signal 275. The engine vibration sensor is integrated with the ignition module 920 of the crankcase scavenged two-stroke engine 200, 900. The engine 900 further comprises scavenging channels 950 (also known as transfer channels) and a decompression valve 960. One or more engine vibration sensors 270 may be arranged in connection to a scavenging channel 950 of the engine 900 and / or in connection to the decompression valve 960. A scavenging channel 950 can be at least partly delimited by a scavenging channel lid, as illustrated in the example in Figures 9A-B. In this case at least one engine vibration sensor 270 may be attached to an exterior surface of the scavenging channel lid. This simplifies assembly of the engine 900. A decompression valve 960 is, generally, a manually operated valve that is configured to release pressure in a combustion chamber of the engine 200, 900 during a start procedure. At least one engine vibration sensor 270 may advantageously be attached to the engine 200, 900 in connection to the decompression valve 960, as illustrated schematically in Figure 9B. The control unit 280 is optionally arranged to obtain crankshaft angle data indicative of a crankshaft angle. In this case the control unit 280 can be configured to selectively obtain the engine vibration signal 275 as function of the crankshaft angle. This way the collection of vibration data from the vibration sensor 270 can be focused on the time instants where the piston passes the places of interest since the cylinder, such as the cylinder wall ports 250, 251. The crankshaft angles where the vibration data should be collected may be preconfigured in a LUT or the like. The crankshaft angle data can also be used in a weighting scheme to emphasize vibration data portions associated with certain crankshaft angle ranges, such as the angle ranges corresponding to passage over a cylinder wall port 250, 251. According to some aspects, a plane that is aligned with and intersects a center axis of a cylinder of the engine 900 divides the engine into a cold half and a hot half. The hot half is the side of the engine where the exhaust is, i.e., the hot half of the engine comprises an exhaust port of the cylinder. At least one engine vibration sensor 270 is attached to the cold half of the engine, where it will be less affected by heat from the engine. The special vibrations generated by an engine experiencing lubrication deficiency are relatively weak and may be difficult to pick up reliably. It is therefore advantageous to amplify the vibration signal from the vibration sensor close to the vibration sensor. Figure 10 schematically illustrates a vibration sensor design where a vibration signal amplifier has been arranged close to the vibration sensor in order to amplify the vibration signal that is output from the vibration sensor. In the design, the engine vibration sensor 270 comprises a piezoelectric element 1010 attached to a printed circuit board (PCB) 1020. The PCB carries a vibration signal amplifier 1030 configured to amplify an output signal from the piezoelectric element 1010. In the example shown in Figure 10, the piezoelectric element 1010 and the amplifier 1030 are attached to opposite sides of the PCB 1020 and connected by a signal conduit passing through the PCB. The PCB 1020 optionally also carries processing circuitry 1040, 1050 that is electrically connected to the vibration signal amplifier 1030, such as the control unit 280. The signal processing circuitry may comprise a field-programmable The different control units 280 discussed herein may also be configured to detect any of a faulty air filter, a malfunctioning engine bearing, and a damaged crankshaft based on the engine vibration signal 275. The detection can be implemented by known signal processing techniques, including machine learning where the detection algorithms have been trained using vibration data obtained from engines with various faults. Computer simulation can also be used to train the machine learning algorithms. Figure 11 shows an example three-dimensional (3D) vibration sensor. This sensor is arranged to measure vibration along three orthogonal axes, indicated as x, y and z in Figure 11. In the example of Figure 11, three piezoelectric elements 1100, 1110, 1120 are arranged such that the different axes x, y, z are normal to the faces of the piezoelectric sensors. The vibration data collected in this manner becomes more nuanced compared to one- dimensional vibration data. It is sometimes an advantage to obtain vibration data in more than one dimension, in particular if machine learning detection techniques are employed. Figure 12 schematically illustrates, in terms of a number of functional units, the general components of a control unit 280. Processing circuitry 1210 is provided using any combination of one or more of a suitable central processing unit CPU, multiprocessor, microcontroller, digital signal processor DSP, etc., capable of executing software instructions stored in a computer program product, e.g. in the form of a storage medium 1230. The processing circuitry 1210 may further be provided as at least one application specific integrated circuit ASIC, or field programmable gate array FPGA. Particularly, the processing circuitry 1210 is configured to cause the control unit 280 to perform a set of operations, or steps, such as the methods discussed herein. For example, the storage medium 1230 may store the set of operations, and the processing circuitry 1210 may be configured to retrieve the set of operations from the storage medium 1230 to cause the device to perform the set of operations. The set of operations may be provided as a set of executable instructions. Thus, the processing circuitry 1210 is thereby arranged to execute methods as herein disclosed. The storage medium 1230 may also comprise persistent storage, which, for example, can be any single one or combination of magnetic memory, optical memory, solid state memory or even remotely mounted memory. The device 280 may further comprise an interface 1220 for communications with at least one external device, such as a vibration sensor 270 or a plurality of vibration sensors 270. As such the interface 1220 may comprise one or more transmitters and receivers, comprising analogue and digital components and a suitable number of ports for wireline or wireless communication. The processing circuitry 1210 controls the general operation of the control unit 280, e.g., by sending data and control signals to the interface 1220 and the storage medium 1230, by receiving data and reports from the interface 1220, and by retrieving data and instructions from the storage medium 1230. The control unit 280 may comprise a frequency processing unit 1215 arranged to perform frequency selective processing of the engine vibration signal or signals 275 from the vibration sensors 270. The frequency selective processing may comprise, e.g., digital or analog filtering to separate out a specific frequency band from the engine vibration signal or signals. There is also disclosed herein a computer readable medium carrying a computer program comprising program code means for performing the methods discussed herein, when said program product is run on a computer. The computer readable medium and the code means may together form a computer program product.

Claims

CLAIMS 1. A crankcase scavenged two-stroke engine (200, 900) comprising a cylinder wall (260) with an intake port (250) and an exhaust port (251), where the engine (200, 900) is associated with an idle mode of operation and an active mode of operation, where a speed (^) of the engine (200, 900) is higher in the active mode of operation compared to the idle mode of operation, the engine (200, 900) comprising at least one engine vibration sensor (270) arranged to sense vibration by the engine (200, 900) and to output an engine vibration signal (275), and a control unit (280) configured to detect a lubrication deficiency in the engine (200, 900) based on the engine vibration signal (275) in at least one delimited frequency band.

2. The engine (200, 900) according to claim 1, where the lubrication deficiency detection by the control unit (280) is conditioned on that the engine (200, 900) is operating in the active mode of operation.

3. The engine (200, 900) according to claim 1 or 2, where the at least one engine vibration sensor (270) comprises a piezoelectric element.

4. The engine (200, 900) according to any previous claim, where the at least one predetermined frequency band comprises a first frequency band below 20 kHz and preferably below 15 kHz, where the control unit (280) is configured to detect lubrication deficiency in the engine (200, 900) based on a decrease in vibration magnitude in the first frequency band.

5. The engine (200, 900) according to claim 4, where the first frequency band comprises frequencies above 5 Hz and preferably above 10 Hz.

6. The engine (200, 900) according to any previous claim, where the at least one predetermined frequency band comprises a second frequency band above 50 kHz and preferably above 100 kHz, where the control unit (280) is configured to detect lubrication deficiency in the engine (200, 900) based on an increase in engine vibration magnitude in the second frequency band.

7. The engine (200, 900) according to claim 6, where the second frequency band comprises frequencies below 500 kHz and preferably below 450 kHz.

8. The engine (200, 900) according to any previous claim, where at least one engine vibration sensor (270) is arranged in connection to a port (250, 251, 252) in the cylinder wall (260).

9. The engine (200, 900) according to any previous claim, where at least one engine vibration sensor (270) is arranged in connection to a scavenging channel (950) of the engine (200, 900).

10. The engine (200, 900) according to any previous claim, where the scavenging channel (950) is at least partly delimited by a scavenging channel lid, where at least one engine vibration sensor (270) is attached to an exterior surface of the scavenging channel lid.

11. The engine (200, 900) according to any previous claim, comprising a spark plug (910), and an ignition module (920) electrically connected to the spark plug (910) and fixedly attached to a motor block (940) of the engine (200, 900), where at least one engine vibration sensor (270) is integrated in the ignition module (920).

12. The engine (200, 900) according to claim 11, where the ignition module (920) is arranged to control ignition of the engine (200, 900) to reduce engine speed in response to lubrication deficiency detection by the control unit (280).

13. The engine (200, 900) according to any previous claim, comprising a decompression valve (960) configured to release pressure in a combustion chamber of the engine (200, 900) during a start procedure, where at least one engine vibration sensor (270) is attached to the engine (200, 900) in connection to the decompression valve (960).

14. The engine (200, 900) according to any previous claim, where at least one engine vibration sensor (270) is arranged in connection to the cylinder wall (260).

15. The engine (200, 900) according to any previous claim, where a plane that is aligned with and intersects a center axis of a cylinder of the engine (200)divides the engine into a cold half and a hot half, where the hot half of the engine comprises an exhaust port of the cylinder, where at least one engine vibration sensor (270) is attached to the cold half of the engine (200, 900).

16. The engine (200, 900) according to any previous claim, where the control unit (280) is arranged to obtain crankshaft angle data indicative of a crankshaft angle of the engine (200, 900), where the control unit (280) is configured to selectively obtain and / or process the engine vibration signal (275) as function of the crankshaft angle.

17. The engine (200, 900) according to any previous claim, where the control unit (280) is configured to detect lubrication deficiency in the engine (200, 900) based on a machine learning function.

18. The engine (200, 900) according to any previous claim, comprising a plurality of engine vibration sensors arranged at respective locations in connection to the cylinder wall (260) to sense respective vibrations by the engine (200, 900).

19. The engine (200, 900) according to any previous claim, where the control unit (280) is arranged to receive data indicative of an external vibration generated by an external source (510), where the control unit (280) is configured to detect lubrication deficiency in the engine (200, 900) based on a combination of the sensed engine vibration and the external vibration.

20. The engine (200, 900) according to any previous claim, where the control unit (280) is arranged to limit maximum engine speed or prevent engine operation in response to detecting lubrication deficiency.

21. The engine (200, 900) according to any previous claim, where the control unit (280) is arranged to control a variable oil dispensing system in response to detecting lubrication deficiency.

22. The engine (200, 900) according to claim 21, where the variable oil dispensing system comprises a sealed oil reservoir.

23. The engine (200, 900) according to any previous claim, where the control unit (280) is arranged to be powered by a Peltier element attached to the engine (200, 900).

24. The engine (200, 900) according to any previous claim, where at least one engine vibration sensor (270) comprises a piezoelectric element (1010) attached to a printed circuit board, PCB, (1020), where the PCB carries a vibration signal amplifier (1030) configured to amplify an output signal from the piezoelectric element (1010).

25. The engine (200, 900) according to claim 24, where the piezoelectric element (1010) and the vibration signal amplifier (1030) are mounted on opposite sides of the PCB (1020) 26. The engine (200, 900) according to claim 24 or 25, where the PCB (1020) also carries processing circuitry (1040, 1050) electrically connected to the vibration signal amplifier (1030).

27. The engine (200, 900) according to any previous claim, where the control unit (280) is configured to detect any of a faulty air filter, a malfunctioning engine bearing, and a damaged crankshaft based on the engine vibration signal (275).

28. The engine (200, 900) according to any previous claim, where at least one engine vibration sensor (270) is a two- or three-dimensional vibration sensors configured to sense vibration along two or three non-parallel axes.

29. A hand-held power tool comprising an engine (200, 900) according to any previous claim.

30. A method performed by a control unit (280) arranged in connection to a crankcase scavenged two-stroke engine (200, 900) comprising a cylinder wall (260) with an intake port (250) and an exhaust port (251), where the engine (200, 900) is associated with an idle mode of operation and an active mode of operation, where a speed (^) of the engine (200, 900) is higher in the active mode of operation compared to the idle mode of operation, the method comprisingobtaining (Sa1) engine vibration data from a vibration sensor (270) arranged in connection to the cylinder wall (260), and detecting (Sa2) lubrication deficiency in the engine (200, 900) based on the engine vibration data, conditioned on that the engine (200, 900) is operating in the active mode of operation.

31. The method according to claim 30, further comprising suppressing (Sa21) vibration from an external source in the engine vibration data, based on data indicative of the vibration from the external source.

32. A crankcase scavenged two-stroke engine (200, 900) comprising a cylinder wall (260) with an intake port (250) and an exhaust port (251), the engine (200, 900) comprising at least one engine vibration sensor (270) arranged in connection to the cylinder wall (260) to sense vibration by the engine (200, 900), and a control unit (280) configured to detect a lubrication deficiency in the engine (200, 900) based on the sensed engine vibration, where the engine vibration sensor (270) comprises a piezoelectric element.

33. A crankcase scavenged two-stroke engine (200, 900) comprising a cylinder wall (260) with an intake port (250) and an exhaust port (251), the engine (200, 900) comprising a control unit (280) and at least one engine vibration sensor (270) arranged in connection to the cylinder wall (260) to sense vibration by the engine (200, 900), where the control unit (280) is configured to detect lubrication deficiency in the engine (200, 900) based on a decrease in vibration magnitude in a first frequency band below 20 kHz and preferably below 15 kHz.

34. A crankcase scavenged two-stroke engine (200, 900) comprising a cylinder wall (260) with an intake port (250) and an exhaust port (251), the engine (200, 900) comprising a control unit (280) and at least one engine vibration sensor (270) arranged in connection to the cylinder wall (260) to sense vibration by the engine (200, 900),where the control unit (280) is arranged to receive data indicative of an external vibration generated by an external source, and where the control unit (280) is configured to detect lubrication deficiency in the engine (200, 900) based on a combination of the sensed engine vibration and the data indicative of the external vibration.

35. A method for producing an engine vibration sensor (270), the method comprising providing (Sb1) an engine vibration sensor body (710), placing (Sb2) a piezoelectric element (720) on the vibration sensor body (710), with solder paste added inbetween the piezoelectric element (720) and the vibration sensor body (710), placing (Sb3) a counterweight element (730) on the piezoelectric element (720), with solder paste added inbetween the piezoelectric element (720) and the counterweight element (730), and heating (Sb4) the engine vibration sensor (270) assembly.

36. The method according to claim 35, further comprising attaching (Sb5) interface wires to the engine vibration sensor body (710) and to the counterweight element (730), where the interface wires are arranged to convey an engine vibration signal (275) to a control unit (280).

37. An engine vibration sensor (270) comprising an engine vibration sensor body (710), a piezoelectric element (720) soldered to a surface of the vibration sensor body (710), a counterweight element (730) soldered to the piezoelectric element (720) on a side opposite to the vibration sensor body (710), and interface wires arranged to convey an engine vibration signal (275) to a control unit (280).

38. A crankcase scavenged two-stroke engine (200, 900) comprising a cylinder wall (260) with an intake port (250) and an exhaust port (251), a sparkplug (910), and an ignition module (920) electrically connected to the spark plug (910), the engine (200, 900) comprising an engine vibration sensor (270) arranged to sense vibration by the engine (200, 900) and to output an engine vibration signal (275), and a control unit (280) configured to detect a lubrication deficiency in the engine (200, 900) based on the engine vibration signal (275), where the engine vibration sensor is integrated with the ignition module (920) of the crankcase scavenged two-stroke engine (200, 900).

39. A crankcase scavenged two-stroke engine (200, 900) comprising a cylinder wall (260) with an intake port (250) and an exhaust port (251), the engine (200, 900) comprising at least one engine vibration sensor (270) arranged to sense vibration by the engine (200, 900) and to output an engine vibration signal (275), and a control unit (280) configured to detect a lubrication deficiency in the engine (200, 900) based on the engine vibration signal (275), where the control unit (280) is configured to detect lubrication deficiency in the engine (200, 900) based on a decrease in vibration magnitude in a first frequency band and on an increase in engine vibration magnitude in a second frequency band above the first frequency band.

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