Temperature monitoring during power system shutdown

Abstract

A method of determining whether a power system component has exceeded a predetermined temperature after being switched off at which damage of the component is likely to occur, such as a maximum temper

Description

FIELD The present disclosure relates to temperature monitoring in components of a power system. Specifically, it relates to determining if a predetermined temperature has been exceeded while a power system has been shut off. The power system may for example be an internal combustion engine, a fuel cell system or an electric motor. The power system may be a vehicle power system. BACKGROUND Internal combustion engines (also referred to simply as engines) are well known devices with moving parts configured to convert energy into motion. As part of their operation, engines may produce heat, and as a result, parts and / or areas of the engine may get hot. Temperature management may be in progress while the engine is running for one or more components. In order to manage temperature in the engine, cooling systems may be provided. In order to monitor temperature in the engine, one or more temperature sensors may be provided as part of the engine, for measuring temperature of one or more engine components. Temperature management may for example be achieved by providing coolant for cooling a component. Despite coolant use, the temperature of the component may be higher (e.g. around 10°C higher) than the temperature of the coolant. This may be due to heating effects from electronic components. When an engine experiences a shutdown event, such as a hot-shutdown event, temperature management processes may be halted as a result of the shutdown. A hot shut-down event in this context may be understood for example to be an event where the engine is shut down at substantially full load. Full load may be where the engine is using a full (e.g. maximum) fuel / energy supply under its given operating conditions. At full load, the temperatures in the engine, and of components in (and around) the engine may be higher than when the engine is at zero load (just enough fuel / energy supply to leave the engine idling). Therefore, after an engine shutdown event, internal component temperatures may continue to rise for several minutes while the component is powered off. This phenomenon may be significant, as the temperatures of one or more components could exceed a maximum temperature threshold while the engine is shut down. This exceeding of the maximum temperature threshold may go undetected, as the engine is shut down. This may be a risk for the performance of the component and / or the engine. A corresponding issue may also arise for other power systems such as fuel cell systems and electric motors (e.g. electric motors used to drive vehicle wheels). It is an object of the present disclosure to overcome disadvantages associated with temperature management during power system shutdown. SUMMARY According to an aspect of the current disclosure, there is provided a method of determining whether a power system component has exceeded a predetermined temperature after the power system has been switched off. The predetermined temperature is a temperature at which damage of the power system component is likely to occur. The predetermined temperature may be the maximum temperature rating of the power system component. The method comprises measuring a first temperature of the power system component before the power system has been switched off. A second temperature of the power system component is measured when the power system is next switched on. The method then identifies that the power system component has exceeded the predetermined temperature if the first temperature is above a first threshold temperature and the second temperature is below a second threshold temperature. The second threshold temperature is below the first threshold temperature. An advantage of the methods described herein is that it is possible to determine that a power system component has exceeded a predetermined temperature during a time when no temperature measurements are being taken. Optionally, the first threshold temperature may be below the predetermined temperature at which damage to the power system component is likely to occur. Optionally, the second threshold temperature may be at least 10°C below the first threshold temperature. Optionally, the predetermined temperature at which damage to the power system component is likely to occur may be in a range from 80 °C to 150 °C , for example 125 °C , 150°C, or more. Optionally, for a predetermined temperature of 150 °C, the first threshold temperature may be at least 140°C. Optionally, for a predetermined temperature of 150 °C, the first threshold temperature may be 145°C or more. Optionally, for a predetermined temperature of 125 °C, the first threshold temperature may be at least 115 °C. Optionally, for a predetermined temperature of 125 °C, the first threshold temperature may be 120°C or more. Optionally, the method may further comprise storing in a memory data indicating a number of times that the temperature of the power system component has exceeded the predetermined temperature. Optionally, the measurement of the temperature of the power system component before the power system has been switched off may be performed periodically, and wherein a measured temperature obtained immediately before the power system is switched off may be used as the first temperature. Optionally, the power system component may be an actuator, a bearing, or a bearing housing. The power system may be an internal combustion engine, a fuel cell system or an electric motor. According to another aspect of the current disclosure, there is provided a controller comprising a processor and a memory. The controller is configured to communicate with a temperature sensor arranged to determine the temperature of a component of a power system. A first measured temperature of the power system component is received from the temperature sensor when the power system is switched on, and the controller stores the first measured temperature. ‘When the power system is switched on’ in this context may be understood to mean ‘while the power system is in an on-state’, and ‘before the power system is switched off’. A second measured temperature of the power system component is received from the temperature sensor when the power system is next switched on. The controller determines whether the first measured temperature is above a first threshold temperature. The controller determines whether the second measured temperature is below a second threshold temperature. The second threshold temperature is below the first threshold temperature. An output is provided, the output indicating that the power system component has exceeded a predetermined temperature if the first measured temperature is above the first threshold temperature and the second measured temperature is below the second threshold temperature. An advantage of the controller described herein is that it is possible to determine that a power system component has exceeded a predetermined temperature during a time when no temperature measurements are being taken. Optionally, the first threshold temperature may be below the predetermined temperature at which damage to the power system component is likely to occur. Optionally, the second threshold temperature may be at least 10°C below the first threshold temperature. Optionally, the predetermined temperature at which damage to the power system component is likely to occur may be 150°C or more. Optionally, the first threshold temperature may be at least 140°C. Optionally, the first threshold temperature may be 145°C or more. Optionally, the memory may be configured to store data indicating a number of times that the temperature of the power system component has exceeded the predetermined temperature. Optionally, the processor may be configured to cause a periodic measurement of the temperature of the power system component before the power system has been switched off, and to use as the first temperature a measured temperature obtained immediately before the power system is switched off is used. Optionally, the power system component may an actuator, a bearing, or a bearing housing. According to another aspect of the current disclosure there is provided a controller as described above, a temperature sensor, and a component of a power system. The power system is an internal combustion engine, a fuel cell system or an electric motor. BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 depicts a flow chart of a method for determining whether a component of a power system has exceeded a predetermined temperature after the power system has been switched off; Figure 2 depicts a block diagram of a hot shutdown detection feature; and Figure 3 depicts a block diagram of a cross-section of an actuator assembly of an internal combustion engine. DETAILED DESCRIPTION Temperature may be a variable of interest during operation of an internal combustion engine (which may be referred to simply as an engine). Temperature can be indicative of a failure of a component within the engine. For example, a spike in temperature may indicate a problem with a cooling system of the engine. Monitoring whether a damage threshold temperature has been exceeded, could be used as an indication of damage to an engine component and / or whether the lifetime of the engine component could be reduced. Excessive temperature can damage oil or other lubricants in the engine, and may thereby risk damage to bearings. Excessive temperature can also damage perishable seals such as O-rings. Excessive temperature can also damage components by way of, for example, thermal expansion or warp. Excessive temperature can also damage semiconductors within electronic components by way of permanently altering their physical state, for example to become fully electrically conducting. This can create unexpected electrical shorts and faults within the electrical circuits in which they reside. Excessive temperature may also cause damage to components of other power systems such as fuel cell systems or electric motors (e.g. electric motors used to drive vehicle wheels). It is therefore useful to be able to sense temperatures reached inside an engine or other power system during operation. It is also desirable to be able to monitor the temperature of the power system or power system components following shut down of the power system, when power supply to may be abruptly terminated, and temperature sensors may cease measuring / recording power system temperature data. Just after an engine shutdown event, such as a hot shutdown of the engine, internal temperatures of engine components, such as internal actuator temperatures, may continue to rise for several minutes while the engine is powered off. This phenomenon may be a significant concern since engine component temperatures could exceed the maximum acceptable temperature for that engine component, while the engine component is powered off, and this exceeding of the maximum acceptable temperature could go undetected. The maximum acceptable temperature may be referred to as the rated temperature of the engine component. The term “hot shutdown” may be interpreted as meaning shutdown of the engine after the engine has reached or surpassed a nominal operating temperature typical for its application. A shutdown of an engine may mean the engine is turned off (usually by the user switching off the engine). A corresponding issue may apply for components of other power systems such as fuel cells or electric motors. In order to address this potential problem it is proposed herein to provide a hot shutdown detection feature. This hot shutdown detection feature may determine if the power system component temperature has exceeded its max temperature rating after shutdown. This may for example be during an engine coolant heat soaking period. Proposed herein is to determine whether a maximum temperature has been exceeded by comparing the last known power system component temperature just before power system shutdown with the initial power system component temperature at the next power-on. Figure 1 depicts a flowchart of a method 100 of determining whether a power system component has exceeded a predetermined temperature after the power system has been switched off. The method may be a computer-implemented method. Figure 1 is described below in the context of an engine. However, the method is also applicable to other power systems such as fuel cell systems or electric motors. The predetermined temperature may be a temperature at which damage of the engine component is likely to occur. The method may comprise measuring 102 a first temperature of the engine component before the engine has been switched off. A second temperature of the engine component may be measured 104 when the engine is next switched on. The method may identify 106 that the engine component has exceeded the predetermined temperature if the first temperature is above a first threshold and the second temperature is below a second threshold. The second threshold temperature is below the first threshold temperature. The temperature of the engine component will increase after the engine has been switched off. This may be because coolant that removes heat from the engine component is no longer circulating and thus no longer transporting heat away from the engine component. The temperature of the engine component will reach a peak, following which the engine will cool. The method described herein advantageously is able to identify that the engine has passed through this peak of temperature (the temperature must have passed through the peak in order to reach the second threshold temperature). An advantage of this method may be that it is possible to determine that an engine component has exceeded a predetermined temperature during a time when no temperature measurements are being taken. The methods described herein may be performed by a hot shutdown detection feature 200, as depicted in figure 2. The feature 200 may be an apparatus, assembly, and / or system. The feature may comprise or be part of a controller 202. The controller may comprise one or more processors 204, and a non-transitory storage medium 206 (i.e. a memory). The memory may store instructions which, when executed by the processor(s), cause the processor(s) to perform the methods as described herein. The one or more processors may comprise at least one hardware processor. The memory may comprise at least some hardware memory located in the controller. The hot shutdown detection feature 200 may have the ability to communicate and connect with other electronic components inside and / or outside the power system (e.g. engine), for example for sending and / or receiving data. This communication ability is represented in figure 2 by a generic receiver 208 and transmitter 210. In practice, the connectivity may comprise any type of known connection systems, including wired and / or wireless connections. Connections to other elements (e.g. the power system component, temperature sensor, external processors, etc.) may be implemented in hardware and / or software components. Figure 2 is described below in the context of an engine. However, the method is also applicable to other power systems such as fuel cell systems or electric motors. The first threshold temperature may be below the predetermined temperature at which damage to the engine component is likely to occur. The second temperature may be below the predetermined temperature at which damage to the engine component is likely to occur. The second threshold temperature may be below the first threshold temperature, for example the second threshold temperature may be at least 10°C below the first threshold temperature. The second threshold temperature may be below a temperature at which the engine component would be expected to operate in a steady state, e.g. a temperature equilibrium reached after a start-up process has been completed. By having a second threshold temperature below a normal / expected steady state operating temperature, this may indicate that the temperature has fallen and has therefore gone through a peak. The hot shutdown detection feature 200 may advantageously identify that the engine has passed through a peak of temperature, as it is determined that the temperature must have passed through a peak to reach the second threshold temperature. The predetermined temperature at which damage to the engine component is likely to occur may be 150°C or more. The first threshold temperature may be at least 140°C, or at least 145 °C. The measurement of the temperature of the engine component before the engine has been switched off may be performed periodically. A measured temperature obtained immediately before the engine is switched off is used as the first temperature. The measurement of the temperature of the engine component may be obtained periodically while the engine is on, because it may not be known when engine shutdown may occur. That way the measured temperature obtained immediately before the engine is switched off may be used as the first temperature. ‘Immediately before’, in this context may mean for example within a predetermined period of time before the engine is switched off. Immediately before, may mean for example the last measurement obtained before the engine was shut down. The method may include storing memory data indicating a number of times that the temperature of the engine component has exceeded the predetermined temperature. The memory data may be stored in a non-volatile memory on and / or connected to the hot-shutdown detection feature. The memory data may be stored in an event log. The memory data may include an event code comprising information identifying the hot shutdown event. The memory data may include a counter indicating the number of times that the temperature of the engine component has exceeded the predetermined temperature. The memory data (e.g. the event log) may be available for download from the non-volatile memory to a separate memory for processing, e.g. for further analysis. The further analysis may for example occur during a future service event for the engine component. A high number of recorded hot shutdown events, e.g. indicated by a high number on the counter of the event log, may indicate a higher distress ranking for the engine component. Distress ranking may be an indicator for the likelihood of damage to the engine component. The further analysis may be performed for example by a manufacturer, a warranty provider, and / or a provider of maintenance or a service of the engine. An example implementation of a method for determining whether a predetermined temperature has been exceeded will now be described. A hot shutdown detection feature may be provided. The hot shutdown detection feature may have stored on it a predetermined temperature, a first threshold temperature, and a second threshold temperature, all for the engine component, as described above. The hot shutdown detection feature may periodically log the temperature of an engine component. One or more temperature sensors may measure temperature of an engine component. The measured temperature may be stored in a log. The time at which the temperature was measured may also be recorded and logged (e.g. stored in a memory). This stored temperature may be referred to as the last known value of the engine component’s current internal temperature. This last known value will be the value of the first temperature, when the engine shuts down. The engine may then experience a shutdown event. Temperature measurements and logs are halted while the engine is shut down. Upon the next power-on of the engine, the temperature sensor(s) may obtain a temperature measurement of the engine upon power-on. This temperature may also be recorded and logged by the hot shutdown detection feature as the temperature of the engine component upon start-up. This value will be taken as the second temperature for the hot shutdown detection feature. Once the first temperature and the second temperature are known, the hot shutdown detection feature will check three primary conditions 1) IF (first temperature >first threshold temperature) AND (second temperature <second threshold temperature)', 2) IF (first temperature >predetermined temperature) 3) IF (second temperature >predetermined temperature) If any of these three primary conditions are determined to be true, then the hot shutdown detection feature determines that a hot shutdown event has occurred, in which the engine component exceeded the predetermined temperature. In response, the feature may log the event in the memory (e.g. by storing the event code), and may increase the counter of the event by one (+1). The engine component may be an actuator of an engine. The actuator may be for example an electric motor, a hydraulic component, or a pneumatic component. The engine component may be a bearing, or a bearing housing. The damage threshold temperature (and therefore predetermined temperature) of a bearing and / or bearing housing may be higher than the damage threshold temperature of an actuator. The damage threshold temperature may be the maximum temperature rating for the bearing / bearing housing component, which may typically be listed on a datasheet provided by the manufacturer. If not provided, the damage threshold temperature may be determined for example based on testing and / or calculation. The algorithm for determining whether a bearing and / or bearing housing has exceeded a predetermined temperature may be performed by an engine control module (including processor(s) and memory), as a bearing / bearing housing may not have its own controller provided. The controller described herein may be provided as part of a component system. The component system may comprise a power system component, one or more temperature sensors, and a controller, configured to perform the methods described herein. An example of a component system will be described in relation to figure 3 below. In the depicted example the component system is an actuator assembly of an engine. In other embodiments the component system may be a different component system of an engine. In other embodiments the component system may be a component system of a fuel cell (or other power system). The component system of a fuel cell system may be a control circuit or other electrical system that controls or monitors operation of the fuel cell. The component system of a fuel cell system may be an electrical motor that drives a compressor or blower that blows gas (e.g. air) into the fuel cell. The fuel cell system may for example comprise a solid oxide fuel cell. A component of a solid oxide fuel cell may experience temperatures which correspond with the temperatures referred to in this description in connection with an engine. The fuel cell system may for example comprise a proton-exchange membrane fuel cell (PEM), which may alternatively be referred to as a polymer electrode membrane fuel cell. A component of a PEM fuel cell may experience temperatures which are lower than the temperatures referred to in this description in connection with an engine. The component may for example be rated to handle temperatures of up to 80°C, but may experience temperatures of 100°C or more (which may damage the component). In general, any fuel cell system may experience a temperature that cause damage to a component when a hot shutdown occurs. In other embodiments the component system may be a component system of an electric motor (e.g. used to drive a wheel or wheels of an electric vehicle). The component system of an electric motor may be a control circuit or other electrical system that controls or monitors operation of the electric motor. A component of an electric motor may experience temperatures which are lower than the temperatures referred to in this description in connection with an engine. In general, any electric motor may experience a temperature that cause damage to a component when a hot shutdown occurs. Figure 3 depicts a simplified block diagram which shows a cross-section side view of an actuator assembly 300. The controller 302 may be mounted within a casing or housing 304. The housing 304 may substantially enclose the actuator 306, which may be a motor. The housing 304 may also include a cooling channel 310. The cooling channel 310 may comprise a fluid pathway which may be sealed save for two valve connectors. Seals 312 may be provided to the housing, for example to seal the coolant into the cooling channel. An inlet valve connector may be connected to a coolant source (not depicted). An outlet valve connector may be connected to a coolant sink (not depicted). Coolant may pass from the source and may flow through the housing 304 via the cooling channel 310 into the coolant sink. The housing 304 may thereby provide the fluid pathway 310 for coolant to flow through. As coolant flows through the housing 304, the controller 302 and actuator 306 may be cooled. The controller 302 and actuator 306 may be cooled as the result of the coolant removing heat from the controller 302 and actuator 306. Despite this, in steady state conditions and / or following a hot-shutdown event, the actuator 306 temperature may typically be around 10°C higher than the coolant temperature due to heating effects from electronic components. During operation, coolant may be circulated through the housing 304 by a pump (not shown). During operation, coolant may be circulated from the coolant source through the cooling channel 310 of the housing 304, through the outlet valve connector and into the coolant sink. The pump which circulates coolant may be the same pump as that used to circulate coolant through for example an engine and / or a turbocharger. The housing may have a connector 308 for connecting the controller 302 to features external to the housing 304. These may for example include components and / or other controllers, processors, and / or memory. The housing 304 may be attached to a mounting 320 for attaching the actuator assembly 300 to the engine. The actuator 306 may be linked to a shaft 322 for actuating the engine. The controller 302 may comprise integrated circuits and electronic components, which may be mounted on a printed circuit board (PCB) 316. The controller 302 may include a processor and memory which may be mounted to the PCB 316. The controller 302 may be connected to other electronic components which may be used to sense parameters of the engine during operation. The processor, memory, and sensing components 318 such as temperature sensors are all examples of electronic components. The electronic components can be mounted to the controller 302 or be located separately from and connected to the controller 302 on the PCB 316. Electronic components may be mounted elsewhere around the engine component, engine, and / or vehicle. The controller 302 may be supplied with power by a vehicle battery, or any other power supply provided to the engine component. The power may be supplied via a wired connection, which may be connected to the controller and / or a vehicle The electrical cable 308 may be connected to the controller 302 and / or the vehicle battery by, for example, a plug and socket connection. The controller described herein, may be present in / on the engine component itself. For example the engine component may be an actuator and the actuator may form part of an actuator assembly comprising the actuator and an actuator controller. In some implementations, the controller may be part of an engine control module for the engine. The processor 302 is a processing unit. In one non-limiting example, the processor 302 requires a 3.3V and 0.060A power supply to function. Any other type of known processor suitable for use in the methods described herein may be used herein. The functions of the processor 302 may include reading, processing and writing data. Data may include variables like temperature, obtained using sensing components such as a thermocouple. The processor 302 may process the data by, for example, performing calculations. In some implementation, the data may be transmitted to another, external processor, for further analysis, such as calculations. The processor 302 may, for example, read temperature data obtained from a thermocouple periodically, before a shutdown event, and after the next power-up. The processor 302 is also able to write data to the memory. Memory may be non-volatile memory. Non-volatile memory may be beneficial because the memory is not wiped and the data not erased after the memory is power cycled. That is to say, if the apparatus within which the memory is incorporated is powered down, the memory will retain the data which is written to it. This is beneficial as the data is retained in the event of a system shut down or power loss. The non-volatile memory may be flash memory. The memory is able to store data such as an event log and counter, as described above. Data can be retrieved from the memory for example for further analysis by an authorised user (e.g. owner and manufacturer of the engine component). The memory may also store other software, such as for example application software for controlling the actuator 306. The memory may be EEPROM. In some instances, memory may be separate from the controller i.e. the memory may be positioned elsewhere in the engine or vehicle. Some or all of the memory that is separate from the controller may be connected to the controller via a wired and / or a wireless connection. The memory may be used to store drive-time data i.e. data recorded whilst the engine is running. The hot shutdown detection feature may have a number of possible inputs and outputs. The inputs may comprise set parameters as well as measured values. The set parameters depend on the type and nature of the engine component to be monitored. Examples of set parameters for an engine component to be provided as an input to the hot shutdown detection feature may include the predetermined temperature, the first threshold temperature, and the second threshold temperature. The inputs may further comprise measurement values, such as the temperature measurements by the sensor(s) for the engine component. Further data related to the engine component may be provided, such as for example the lifetime of the engine component. The output values may depend on parameters and measurements provided as input to the hot shutdown detection feature. Example outputs include an estimated hot shutdown temperature, and some or all content of the event log and / or event counter of the hot shutdown detection feature. The predetermined temperature may be a temperature at which damage of the engine component may be likely to occur. The predetermined temperature may be the maximum temperature rating of the engine component, which may be listed on a datasheet provided by the manufacturer of the component. The predetermined temperature will depend on the nature of the engine component. Engine components may be made to different industry standards / ratings. For example, some components may be made to a temperature rating of 150°C (e.g. suitable for a diesel engine), some components may be made to a temperature rating of 125°C. Components with a lower temperature rating may be chosen in some instances for example because they may have a lower cost compared to the higher temperature rated components. The first threshold temperature may be set below the predetermined temperature at which damage of the engine component is likely to occur, for example in order to form a safety buffer. The predetermined temperature may be determined based on known data of the engine component, it may be determined experimentally, it may be based on estimations, and / or based on simulations. The first threshold temperature may also be referred to as a hot shutdown detection high temperature threshold, as it may be used as an upper limit to a temperature range. The first temperature threshold may set an upper limit to the temperature measured before shutdown. If the temperature measured immediately before shutdown is below the first threshold temperature, no hot shutdown will be detected by the hot shutdown detection feature. The second threshold temperature may also be referred to as a hot shutdown detection low temperature threshold, as it may be used as a lower limit to a temperature range. The second temperature threshold may set a lower limit to the temperature measured immediately after start-up. If the temperature measured immediately after start-up is above the second threshold temperature, no hot shutdown will be detected by the hot shutdown detection feature. In an example scenario, an actuator temperature just before engine shutdown (a first temperature) was recorded at 145°C and the initial temperature at the next power-on (a second temperature) was measured at 120°C. The feature further assumes that an amount of time has elapsed that is long enough for the actuator to become heat soaked. The feature assumes that the general physics underlying heat transfer supported by data observation of the heat soaking phenomenon will prevail, and that the continuous thermal path that started with an initial temperature of 145 °C and ended with a final reduced temperature of 120°C had to pass through a peak temperature exceeding the maximum rating of the part. Heat soaked may involve the temperature rising another 10°C - 20°C above the first temperature, during the power-off period. In this case, the feature will determine that the actuator exceeded its max temperature rating of 150°C. Furthermore, if the temperature recorded before shutdown or the temperature measured after power-on exceed 150°C then the feature will also record a hot shutdown event. If a hot shutdown event is detected, the feature may record an event code in an event log for the actuator. The feature may increment a counter in the event log. The event log may be stored in non-volatile memory. Embodiments of the invention have been described in connection with an engine. However, embodiments of the invention are applicable to other power systems. The power system may for example be a fuel cell system or an electric motor. The power system may provide power for a vehicle, and may be referred to as a vehicle power system. In the case of an engine, the power system converts chemical energy into mechanical power which may be used to drive vehicle wheels. In the case of a fuel cell system, the fuel cell converts chemical energy into electricity which may be used to drive vehicle wheels (via an electric motor). In the case of an electric motor, the electric motor converts electricity into mechanical power which may be used to drive vehicle wheels.

Claims

1. A method of determining whether a power system component has exceeded a predetermined temperature after shutdown of the power system, the predetermined temperature being a temperature at which damage of the power system component is likely to occur, the method comprising:measuring a first temperature of the power system component before the power system has been switched off;measuring a second temperature of the power system component when the power system is next switched on; andidentifying that the power system component has exceeded the predetermined temperature if the first temperature is above a first threshold temperature and the second temperature is below a second threshold temperature, the second threshold temperature being below the first threshold temperature.

2. The method of claim 1, wherein the first threshold temperature is below the predetermined temperature at which damage to the power system component is likely to occur.

3. The method of claim 1 or claim 2, wherein the second threshold temperature is at least 10°C below the first threshold temperature.

4. The method of any preceding claim, wherein the predetermined temperature at which damage to the power system component is likely to occur is 150°C or more.

5. The method of any preceding claim, wherein the first threshold temperature is at least 140°C.

6. The method of claim 5, wherein the first threshold temperature is 145°C or more.

7. The method of any preceding claim, further comprising storing in a memory data indicating a number of times that the temperature of the power system component has exceeded the predetermined temperature.

8. The method of any preceding claim, wherein the measurement of the temperature of the power system component before the power system has been switched off is performed periodically, and wherein a measured temperature obtained immediately before the power system is switched off is used as the first temperature.

9. The method of any preceding claim, wherein the power system component is an actuator.

10. The method of any preceding claim, wherein the power system is an internal combustion engine, a fuel cell system or an electric motor.

11. A controller comprising a processor and a memory, wherein the controller is configured to:communicate with a temperature sensor arranged to determine the temperature of a component of power system;receive and store a first measured temperature of the power system component from the temperature sensor when the power system is switched on;receive a second measured temperature of the power system component from the temperature sensor when the power system is next switched on;determine whether the first measured temperature is above a first threshold temperature, and determine whether the second measured temperature is below a second threshold temperature, the second threshold temperature being below the first threshold temperature; andprovide an output indicating that the power system component has exceeded a predetermined temperature if the first measured temperature is above the first threshold temperature and the second measured temperature is below the second threshold temperature.

12. The controller of claim 11, wherein the first threshold temperature is below the predetermined temperature at which damage to the power system component is likely to occur.

13. The controller of claim 11 or claim 12, wherein the second threshold temperature is at least 10°C below the first threshold temperature.

14. The controller of any of claims 11 to 13, wherein the predetermined temperature at which damage to the power system component is likely to occur is 150°C or more.

15. The controller of any of claims 11 to 14, wherein the first threshold temperature is at least 140°C.

16. The controller of claim 15, wherein the first threshold temperature is 145°C or more.

17. The controller of any of claims 11 to 16, wherein the memory is configured to store data indicating a number of times that the temperature of the power system component has exceeded the predetermined temperature.

18. The controller of any of claims 11 to 17, wherein the processor is configured to cause the periodic measurement of the temperature of the power system component before the power system has been switched off, and to use as the first temperature a measured temperature obtained immediately before the power system is switched off is used.

19. The controller of any of claims 11 to 18, wherein the power system component is an actuator.

20. An component system comprising a controller according to any of claims 11 to 19, a temperature sensor, and a component of a power system.

21. A power system comprising the component system of claim 20, wherein the power system is an internal combustion engine, a fuel cell system or an electric motor.

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