GB2501701A

GB2501701A - A method of evaluating the thermal fatigue of a cylinder head of an internal combustion engine

Info

Publication number: GB2501701A
Application number: GB1207569.3A
Authority: GB
Inventors: Michele Bilancia; John W Siekkinen; Stephen Paul Levijoki; Daniel A Bialas
Original assignee: GM Global Technology Operations LLC
Current assignee: GM Global Technology Operations LLC
Priority date: 2012-04-30
Filing date: 2012-04-30
Publication date: 2013-11-06
Also published as: GB201207569D0

Abstract

An embodiment of the invention provides a method of evaluating a thermal fatigue of a cylinder head 130 of an internal combustion engine 110, comprising the steps of: reÂceiving a first signal T(t) representative of a temperature of the cylinder head 130; filtering the first signal T(t) to obtain a second signal Tm(t) representative of a mean temperaÂture of the cylinder head 130; and calculating an evaluating index KTHD for the thermal fatigue of the cylinder head 130 according to the expression: Â Â Â Â Â Â Â Â Â t 1 K THD = « | T(t) T m (t) | d t, Â Â Â Â Â Â Â Â Â t 0 wherein t is the time, tl is the time instant at which the calculation is made, and t0 is a predetermined time instant that precedes the time instant t1.

Description

A METHOD OF EVALUATING A THERMAL FATIGUE OF A CYLINDER HEAD OF AN

INTERNAL COMBUSTION ENGINE

TECHNICAL FIELD

The present disclosure relates to a method of evaluating a thermal fatigue of a cylinder head of an internal combustion engine, for example an internal combustion engine of a motor vehicle, including a compression-ignition engine or a spark-ignition engine.

BACKGROUND

It is known that an internal combustion engine generally comprises an engine block hay-ing at least one cylinder which accommodates a reciprocating piston coupled to rotate a crankshaft. A cylinder head is fastened to the engine block so as to close the top of the cylinder and cooperate with the piston for defining a combustion chamber. A fuel-and-air mixture is periodically supplied into the combustion chamber and ignited, thereby produc-ing hot exhaust gasses whose expansion causes the movement of the piston.

Due to the combustion of the fuel-and-air mixture in the combustion chamber, the tem- perature of the engine block and of the cylinder raise up. In order to keep this tempera-ture below critical values, the internal combustion engine is conventionally equipped with an engine cooling system. The engine cooling system usually comprises a coolant pump that delivers a coolant, typically a mixture of water and antifreeze, from a coolant tank to a plurality of cooling channels internally located in the engine black and in the cylinder head, and a radiator for cooling down the coolant, once it has circulated through the cool-ing channels and before that it returns to the coolant tank.

Nevertheless, the temperature of the cylinder head is not a constant during the operation of the internal combustion engine, but generally increases and decreases cyclically, sub-jecting the cylinder head to a plurality of thermal cycles that follow one another.

The amplitude and the number of these thermal cycles are enhanced when the coolant pump of the engine cooling system is a switchable coolant pump, also conventionally re-ferred as switchable water pump (SWP). The switchable water pump may be a pump which is coupled to the engine crankshaft th(ough a clutch that is operated by an elec- tronic control unit (ECU) according to a predetemiined strategy. When the clutch is en-gaged, the pump is driven by the crankshaft to pump the coolant in the cooling circuit, so that the temperature of the cylinder head generally decreases. Conversely, when the clutch is disengaged, the pump is inactive and does not pump the coolant in the cooling circuit, so that the temperature of the cylinder head generally increases.

On the other hand, any variation of the cylinder head temperature causes a thermal strain of the metallic material which the cylinder head is made of. The thermal strain of the metallic material may be expressed by the following relation: 5TH = a(T-r)-a(7 -7p) wherein $TM is the thermal strain of the metallic material1 7 is a starting temperature, T is a current temperature, rf is a reference temperature, a is the thermal expansion coefficient of the metallic material at the starting temperature fl and a is the thermal ex-pansion coefficient of the metallic material at the current temperature 7.

In its turn, this thermal strain generates a stress in the metallic material of the cylinder head, which may be expressed according to the relation: a-tx = 5. srIi wherein a-'11 is the thermal stress and B is the Young Modulus of the metallic material of the cylinder head.

In view of the above, it follows that the thermal cycling of the cylinder head causes cycli-cal oscillations of the thermal stress. These oscillations cause a progressive structural damage of the cylinder head, conventionally referred as thermal fatigue, whose increase rate depends on the number and duration of the thermal cycles and on the amplitude of the temperature oscillation in each thermal cycle.

During the operation of the internal combustion engine, if the thermal fatigue exceeds a maximum threshold, the cylinder head may crack and/or fracture.

For this reason, an object of an embodiment of the present invention is that of providing a reliable and effective index of the thermal fatigue of the cylinder head.

An object of another embodiment of the invention is that of providing a strategy of operat- ing a switchable water pump of an internal combustion engine, in order to monitor, quan-tify, and/or limit the thermal fatigue of the cylinder head.

Still another object is to attain these goals with a simple, rational and rather inexpensive solution.

SUMMARY

These andlor other objects are attained by the features of the embodiments of the inven-tion as reported in the independent claims. The dependent claims recite preferred and/or especially advantageous features of the embodiments of the invention.

More particularly, an embodiment of the invention provides a method of evaluating a thermal fatigue of a cylinder head of an internal combustion engine, comprising the steps of: -receiving a first signal T(t) representative of a temperature of the cylinder head, -processing the first signal T(t) to obtain a second signal Tm(t) representative of a mean temperature of the cylinder head, -calculating an evaluating index kTHo for the thermal fatigue of the cylinder head accord-ing to the expression: = 1T(t) -T(t)l dt wherein t is the time, t1 is the time instant at which the calculation is made, and t0 is a predetermined time instant that precedes the time instant t1.

Thanks to this solution, the evaluating index KTHD takes contemporaneously into account the temperature variation (delta temperature) and the duration of each thermal cycle, as well as the overall number of the thermal cycles, thereby providing a reliable evaluation of the thermal fatigue of the cylinder head.

According to an aspect of the invention, the time instant t0 may be the time instant at which the internal combustion engine has been started.

In this way, the evaluating index KTHD becomes a reliable evaluation of the overall ther-mal fatigue which the cylinder head has accumulated from the engine start.

Another aspect of the invention provides for reselling the evaluating index KTHO, for ex-ample at zero, any time the internal combustion engine is started.

This solution has the advantage that the evaluating index KIHO becomes representative of the thermal fatigue which the cylinder head accumulates during each single running.

According to still another aspect of the invention, the processing of the first signal T(t) may be performed by filtering the first signal Ta), for example by means of ow-pass filter such as a first-order filter, a second-order filter or a moving average filter properly tuned.

This aspect of the invention has the advantage of attenuating the harmonic components of the first signal T(t) that have high frequencies, thereby obtaining a second signal Tm(t) that follows the global variation of the first signal T) but in a smoother way, so as to pro-vide a reliable representation of the mean temperature of the cylinder head.

3D According to still another aspect of the invention, the calculation of the evaluating index Kmo is cyclically repeated for subsequent instants t1.

In this way, the evaluating index KTHD is cyclically updated, thereby allowing an effective monitoring of the thermal fatigue of the cylinder head over the time.

Another embodiment of the invention provides a method of operating a switchable water pump of an internal combustion engine, comprising the steps of: -evaluating a thermal fatigue of a cylinder head of the internal combustion engine ac-cording to the method disclosed above, and -keeping the switchable water pump switched-on, if the evaluating index KTHD exceeds a predetermined threshold value.

The threshold value may be a calibration value representative of a maximum allowable limit of the thermal fatigue that the cylinder head can endure without being damaged.

Thanks to this solution, when the evaluating index KTHO reaches the threshold value, the switchable water pump remains constantly switched-on to reduce the thermal cycling of the cylinder head, while always guaranteeing an effective cooling of the internal combus-tion engine.

The method according to the invention can be carried out with the help of a computer program comprising a program-code for carrying out all the steps of the method de-scribed above, and in the form of a computer program product on which the computer program is stored. The method can be also embodied as an electromagnetic signal, said signal being modulated to carry a sequence of data bits which represent a computer pro-gram to carry out all steps of the method.

Another embodiment of the invention provides an apparatus for evaluating a thermal fa- tigue of a cylinder head of an internal combustion engine, wherein the apparatus com-prises: -means for receiving a first signal T(t) representative of a temperature of the cylinder head, -means for processing the first signal T(t) to obtain a second signal Tm(t) representative of a mean temperature of the cylinder head, -means for calculating an evaluating index KT,-,,, for the thermal fatigue of the cylinder head according to the expression: ETUD = fIr&) -Tm(t)j dt wherein t is the time, t1 is the time instant at which the calculation is made, and t0 is a predetermined time instant that precedes the time instant t1.

Thanks to this solution, the evaluating index K0 takes contemporaneously into account the temperature variation (delta temperature) and the duration of each thermal cycle, as well as the overall number of the thermal cycles, thereby providing a reliable evaluation of the thermal fatigue of the cylinder head.

According to another aspect of the invention, the apparatus may comprise means for re-setting the evaluating index KTHD, for example at zero, any time the internal combustion engine is started.

This solution has the advantage that the evaluating index KTHD becomes representative of the thermal fatigue which the cylinder head accumulates during each single running.

According to still another aspect of the invention, the means for processing the first sig-nal T(t) may include means for filtering the first signal T(t), for example a of low-pass filter such as a first-order filter, a second-order filter or a moving average filter properly tuned.

According to still another aspect of the invention, the means for calculating the evaluating index k0 are configured to cyclically repeat the calculation for subsequent instants f1.

In this way, the evaluating index KYHO is cyclically updated, thereby allowing an effective monitoring of the thermal fatigue of the cylinder head over the time.

Another embodiment of the invention provides an apparatus for operating a switchable water pump of an internal combustion engine, comprising: -the apparatus for evaluating a thermal fatigue of a cylinder head of the internal combus-tion engine as disclosed above, and -means for keeping the switchable water pump switched-on1 if the evaluating index KTHD exceeds a predetermined threshold value.

Thanks to this solution, when the evaluating index KTHD reaches the threshold value, the switchable water pump remains constantly switched-on to reduce the thermal cycling of the cylinder head, while always guaranteeing an effective cooling of the internal combus-tion engine.

Another embodiment of the invention provides an automotive system comprising an in-ternal combustion having a cylinder head, and an electronic control unit configure to: -receive a first signal T(t) representative of a temperature of the cylinder head, -process the first signal T(t) to obtain a second signal Tm(t) representative of a mean temperature of the cylinder head, -calculate an evaluating index KTHD for the thermal fatigue of the cylinder head according to the expression:

S

= Jirw -(I dt wherein t is the time, t1 is the time instant at which the calculation is made, and t0 is a predetermined time instant that precedes the time instant t.

In this way, the evaluating index K0 becomes a reliable evaluation of the overall ther-mal fatigue which the cylinder head has accumulated from the engine start.

Another aspect of the invention provides that the electronic control unit is configured to reset the evaluating index KTHD, for example at zero, any time the internal combustion engine is started.

According to still another aspect of the invention, the electronic control unit may be con-figured to process the first signal T(t) by filtering the first signal T(t), for example by means of low-pass filter such as a first-order filter, a second-order filter or a moving av-erage filter properly tuned.

This aspect of the invention has the advantage of attenuating the harmonic components of the first signal T(t) that have high frequencies, thereby obtaining a second signal Tm('t) that follows the global variation of the first signal T(t) but in a smoother way, so as to pro-vide a reliable representation of the mean temperature of the cylinder head.

According to still another aspect of the invention, the electronic control unit may be con-figured to cyclically repeating the calculation of the evaluating index KTHQ for subsequent instants t1.

Another embodiment of the invention provides a that the electronic control unit is further configured to keep the switchable water pump switched-on, if the evaluating index KYND exceeds a predetermined threshold value.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described, by way of example, with reference to the accompanying drawings.

Figure 1 is a schematic representation of an automotive system according to an embod-iment of the invention.

1 0 Figure 2 is a simplified representation of the section A-A indicated in figure 1.

Figure 3 is a schematic representation of a cooling system included in the automotive system of figure 1.

Figure 4 is a flowchart representing a strategy of controlling a switchable water pump of the cooling system of figure 3.

Figure 5 is a flowchart representing a method of evaluating a thermal fatigue of a cylinder head of the internal combustion engine belonging to the automotive system of figure 1.

Figure 6 is diagram representing two temperatures signals involved in the method of fig-ure 5.

DETAILED DESCRIPTION

Some embodiments may include an automotive system 1001 for example an automotive system of a motor vehicle (not shown). As shown in figures 1 and 2, the automotive sys- tem includes an internal combustion engine (ICE) 110 having an engine block 120 defin-ing at least one cylinder 125 having a piston 140 coupled to rotate a crankshaft 145. A cylinder head 130 cooperates with the piston 140 to define a combustion chamber 150.

The cylinder head 130 may be made in aluminium or other metallic material. A fuel and air mixture (not shown) is disposed in the combustion chamber 150 and ignited, resulting in hot expanding exhaust gasses causing reciprocal movements of the piston 140. The fuel is provided by at least one fuel injector 160 and the air through at least one intake pod 210. The fuel is provided at high pressure to the fuel injector 160 from a fuel rail 170 in fluid communication with a high pressure fuel pump 180 that increases the pressure of the fuel received from a fuel source 190. Each of the cylinders 125 has at least two valves 215. The valves may be electrically actuated valves or may be actuated by a camshaft 135 rotating in time with the crankshaft 145. The valves 215 selectively allow air into the combustion chamber 150 from the port 210 and alternately allow exhaust gases to exit through at least one exhaust port 220. In some examples, a cam phaser may selectively vary the timing between the camshaft 135 and the crankshaft 145.

The air may be distributed to the air intake port(s) 210 through an intake manifold 200.

An air intake pipe 205 may provide air from the ambient environment to the intake mani-fold 200. In other embodiments, a throttle body 330 may be provided to regulate the flow of air into the manifold 200. In still other embodiments, a forced air system such as a tur-bocharger 230, having a compressor 240 rotationally coupled to a turbine 250, may be provided. Rotation of the compressor 240 increases the pressure and temperature of the air in the intake pipe 205 and manifold 200. An intercooler 260 disposed in the intake pipe 205 may reduce the temperature of the air. The turbine 250 rotates by receiving ex-haust gases from an exhaust manifold 225 that directs exhaust gases from the exhaust ports 220 and through a series of vanes prior to expansion through the turbine 250. This example shows a variable geometry turbine (VGT) with a VGT actuator 290 arranged to move the vanes to alter the flow of the exhaust gases through the turbine 250. In other embodiments, the turbocharger 230 may be fixed geometry andlor include a waste gate.

The exhaust gases exit the turbine 250 and are directed into an exhaust system 270.

The exhaust system 270 may include an exhaust pipe 275 having one or more exhaust aftertreatment devices 280. The aftertreatnient devices may be any device configured to change the composition of the exhaust gases. Some examples of aftertreatment devices 280 include, but are not limited to, catalytic converters (two and three way), oxidation catalysts, lean NO traps, hydrocarbon adsorbers, selective catalytic reduction (SCR) systems, and particulate filters.

Other embodiments may include an exhaust gas recirculation (EGR) system 300 coupled between the exhaust manifold 225 and the intake manifold 200. The EGR system 300 may include an EGR cooler 310 to reduce the temperature of the exhaust gases in the EGR system 300. An EGR valve 320 regulates a flow of exhaust gases in the EGR sys-tem 300.

The automotive system 100 may further include an engine cooling circuit 500 for cooling the ICE 110, as shown in figure 3. The engine cooling circuit 500 schematically compris-es a coolant pump that delivers a coolant, typically a mixture of water and antifreeze, from a coolant tank 510 to a plurality of cooling channels (not shown) internally defined in the engine block 120 and in the cylinder head 130, and a radiator 520 for cooling down the coolant, once it has circulated through the cooling channels and before that it returns to the coolant tank 510.

The coolant pump may be a switchable coolant pump (SWP) 505, which is able to selec- tively allow or prevent the coolant to circulate in the engine cooling circuit 500. In this ex-ample, the switchable coolant pump 505 is coupled to the engine crankshaft 145 through a clutch 525, which is selectively engaged or disengaged by an electric actuator 530.

When the clutch 525 is engaged, the coolant pump 505 is driven directly by the ICE 110, thereby pumping the coolant to circulate in the coolant circuit 500. When the clutch 525 is disengaged, the switchable coolant pump 505 is inactive, so that the coolant does not circulate in the coolant circuit 500. Those skilled in the art will recognize that other kind of switchable coolant pump may be used instead of the one here disclosed, for example an electric pump (i.e. purely electric pump without a clutch).

The automotive system 100 may further include an electronic control unit (ECU) 450 in communication with one or more sensors and/or devices associated with the ICE 110.

The ECU 450 may receive input signals from various sensors configured to generate the signals in proportion to various physical parameters associated with the ICE 110. The sensors include, but are not limited to, a mass airflow and temperature sensor 340, a manifold pressure and temperature sensor 350, a combustion pressure sensor 360, coolant and oil temperature and level sensors 380, a fuel rail pressure sensor 400, a crankshaft position sensor 420, exhaust pressure and temperature sensors 430, an EGR temperature sensor 440, and an accelerator pedal position sensor 445. As shown in fig- ure 2, the sensor may also include a temperature sensor 410 for sensing the tempera-ture of the cylinder head 130, namely the temperature of the metallic material which the cylinder head 130 is made of.

Furthermore, the ECU 450 may generate output signals to various control devices that are arranged to control the operation of the ICE 110, including, but not limited to, the fuel injectors 160, the throttle body 330, the EGR Valve 320, the VGT actuator 290, the cam phaser 155, and the electric actuator 530 operating the clutch 525 of the switchable coolant pump 505. Note, dashed lines are used to indicate communication between the ECU 450 and the various sensors and devices, but some are omitted for clarity.

Turning now to the ECU 450, this apparatus may include a digital central processing unit (CPU) in communication with a memory system 460 and an interface bus. The memory system 460 may include various storage types including optical storage, magnetic stor- age, solid state storage, and other non-volatile memory. The interface bus may be con-figured to send, receive, and modulate analog and/or digital signals to/from the various sensors and control devices. The CPU is configured to execute instructions stored as a program in the memory system 460, and send and receive signals toifrom the interface bus. The program may embody the methods disclosed herein, allowing the CPU to car-ryout out the steps of such methods and control the ICE 110.

One of the tasks performed by the ECU 450 is that of controlling the operation of the switchable coolant pump 505. In this example, the switchable coolant pump 505 is con-trolled by the ECU 450 according to the procedure that is illustrated in figure 4.

As soon as the ICE 110 is started (block 600), the procedure provides for evaluating and cyclically updating an index KTHD (black 605) representative of the thermal fatigue which the cylinder head 130 is progressively subjected to.

The index Krno is then cyclically compared with a threshold value Ui thereof (block 610).

The threshold value th may be a calibration value representing a maximum level of ther- mal fatigue that the cylinder head 130 can endure without being damaged, possibly re- duced by a safety factor. The threshold value Ui may be determined during an experi-mental activity on a test bench, and then stored in the memory system 460.

As long as the index KTHD is below the threshold value Ui, the procedure provides for controlling the switchable coolant pump 505 according to a conventional control strategy (block 615). The conventional control strategy generally provides for switching the pump on or off, namely to engage of disengage the clutch 525, on the basis of a plurality of pa-rameters including, but not limited to, engine temperature, engine speed and engine load. By way for example, during the warm-up phase of the ICE 110, the conventional control strategy may keep the coolant pump 505 switched off, in order to speed up the heating of the ICE 110. When the warm-up phase is over, the conventional control strat-egy may cyclically switch the coolant pump 505 on and off, on the basis of the engine speed, engine load and other parameters, in order to reduce the fuel consumption.

Due to this cyclical activation and deactivation of the switchable coolant pump 505, the temperature of the cylinder head 130 is affected by significant oscillations over the time, which increase its thermal fatigue.

For this reason, when the index KTHD exceeds the threshold value Ui, and form that time on, the procedure provides for the ECU 450 to maintain the coolant pump 505 constantly switched-on (block 620). In this way, the temperature oscillations of the cylinder head are strongly reduced, while guaranteeing an effective cooling of the ICE 110.

It should be understood that the index K0 may be reset each time the ICE 110 is stopped or, equivalently, each time the ICE 110 is restarted. More specifically, the KTHD may be reset to an initial value smaller than the threshold value Ui, for example it may be reset to zero.

Turning now to the index KTHD, its value is determined and cyclically updated according to the procedure illustrated in figure 5. This procedure uses as input a first electrical sig- nal T(t) representative of a temperature of the cylinder head 130. The first electrical ig-nal T(t) may be provided by the temperature sensor 410, which is generally arranged to sense the temperature of the metallic material of the cylinder head 130 and to generate an electrical signal proportional thereto. An explanatory example of the first electrical signal T(t) is depicted in the diagram of figure 6.

The procedure further provides for processing (block 700) the first electrical signal T(t), in order to obtain a second electrical signal Tm(t) representative of a mean temperature of the cylinder head 130. The second electrical signal Tm(t) should follow the global varia-tion over the time of the first electrical signal T(t), disregarding the harmonic components at higher frequency. An explanatory example of second electrical signal Tm(t) corre-sponding to the first electric signal T(t) is depicted in the diagram of figure 6.

In greater details, the second electrical signal Tm(t) may be obtained by filtering first sig- nal T(t) with a low-pass filter, such as for example a first-order filter or a seyond-order fil- ter properly tuned. The tuning of the filter may be determined during an experimental ac-tivity on a test bench.

Having the first electric signal T(t) and the second electrical signal Tm(t), the procedure provides for calculating (block 705) the index KTMD according to the following relation: KTHD = JIT(t) -T;(eI at wherein t is the time, t1 is the time instant at which the calculation is made, and t0 is a predetermined time instant that precedes the instant t1.

More particularly, the time instant t0 may be the instant at which the ICE 110 has been started, whereas the time instant t1 may be determined by a timer 710 that constantly counts for the time passing from the instant to.

Referring to the diagram of figure 6, the index KTHD is represented by the sum of the shaded areas delimited between the first electric signal T(t) and the second electrical signal Tm(t). In this way, the index KTHD takes into account the number of the thermal cy-cles (namely of each temperature oscillation about the mean value) which the cylinder head 130 is subjected to, and takes also into account the temperature variation (delta temperature) and the duration of each of these thermal cycles. As a consequence, the index KTHD represents a reliable and effective evaluation of the thermal fatigue that the cylinder head 130 has accumulated from the initial time instant t0 to the current time in-stant t1.

In order to keep the index KTHD constantly updated, the calculation thereof is cyclically repeated at subsequent time instants t1 which are counted by the timer 710.

While at least one exemplary embodiment has been presented in the foregoing summary and detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration in any way. Rather, the forgoing summary and detailed description will provide those skilled in the art with a convenient road map for implementing at least one exemplary embodiment, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope as set forth in the appended claims and in their legal equivalents.

REFERENCES

automotive system internal combustion engine 120 engine block cylinder cylinder head camshaft piston 145 crankshaft combustion chamber cam phaser ISO fuel injector fuel rail 180 fuelpump fuelsource intake manifold 205 air intake pipe 210 intake port 215 valves 220 exhaust port 225 exhaust manifold 230 turbocharger 240 compressor 250 turbine 260 intercooler 270 exhaust system 275 exhaust pipe 280 aftertreatment devices 290 VGT actuator 300 exhaust gas recirculation system 310 EGR cooler 320 EGR valve 330 throttle body 340 mass airflow and temperature sensor 350 manifold pressure and temperature sensor 360 in-cylinder pressure sensor 380 coolant and oil temperature and level sensors 400 fuel rail pressure sensor 410 cylinder head temperature sensor 420 crankshaft position sensor 430 exhaust pressure and temperature sensors 440 EGR temperature sensor 445 accelerator pedal position sensor 450 ECU 460 memory system 500 engine cooling circuit 505 switchable coolant pump 510 coolant tank 520 radiator 525 clutch 530 electric actuator 600 block 605 block 610 block 615 block 620 block 700 block 705 block 710 timer k0 index th threshold value T(t) first electric signal Tm(t) second electrical signal time instant time instant

Claims

CLAIMS1. A method of evaluating a thermal fatigue of a cylinder head (130) of an internal combustion engine (110), comprising the steps of: -receiving a first signal T(t) representative of a temperature of the cylinder head (1 30), -processing the first signal T(t) to obtain a second signal Tm(t) representative of a mean temperature of the cylinder head (130), -calculating an evaluating index KTHO for the thermal fatigue of the cylinder head (130) according to the expression: = 117(0 wherein t is the time, t1 is the time instant at which the calculation is made, and t0 is a predetermined time instant that precedes the time instant t.
2. A method according to claim 1, wherein the time instant t0 is the time instant at which the intemal combustion engine (110) has been started.
3. A method according to any of the preceding claims, comprising the step of reset-ting the evaluating index KTHID any time the internal combustion engine (110) is started.
4. A method according to any of the preceding claims, wherein the processing of the first signal T(t) is performed by filtering the first signal T(t).
5. A method according to any of the preceding claims, wherein the calculation of the evaluating index KTHD is cyclically repeated for subsequent instants t1.
6. A method of operating a switchable water pump (505) of an internal combustion engine (110), comprising the steps of: -evaluating a thermal fatigue of a cylinder head (130) of the internal combustion engine (110) according to claim 5, -keeping the switchable water pump (505) switched-on, if the evaluating index KTHD ex-ceeds a predetermined threshold value.
7. A method according to claim 6, wherein the threshold value is a calibration value
8. A computer program comprising a computer code suitable for performing the method according to any of the preceding claims.
9. A computer program product on which the computer program of claim B is stored.
10. An electromagnetic signal modulated as a carrier for a sequence of data bits repre-senting the computer program according to claim B.