WO2022050841A1

WO2022050841A1 - Method for testing a decompression engine brake system; combustion engine and vehicle

Info

Publication number: WO2022050841A1
Application number: PCT/NL2021/050535
Authority: WO
Inventors: Remco STREEFKERK; Luc Henricus Maria AKKERMANS
Original assignee: Daf Trucks N.V.
Priority date: 2020-09-04
Filing date: 2021-09-03
Publication date: 2022-03-10
Also published as: EP4208629A1; NL2026412B1

Abstract

Method for testing a decompression engine brake, DEB, system (10) of a combustion engine (8), wherein the DEB system (10) comprises DEB actuators (5) to adjust opening characteristics of exhaust valves (4) of the combustion engine (8), wherein, while the engine (8) is running, the DEB system (10) is tested for a cylinder (1) under investigation by shutting off fuel injection for at least one cylinder (1), including the cylinder (1) under investigation, while at least one other cylinder (1) keeps the engine (8) running, activating the DEB for at least one of the cylinders (1) for which the fuel injection has been shut off, including the cylinder (1) under investigation, determining the momentary unbalance on the crank shaft by the cylinder (1) under investigation and determining the result of the test of the DEB system (10) for the cylinder (1) under investigation in dependence on the determined momentary unbalance on the crank shaft by the cylinder (1) under investigation.

Description

Title Method for testing a decompression engine brake system; combustion engine and vehicle

Description

The invention relates to a method for testing a decompression engine brake (DEB) system of a combustion engine, a combustion engine comprising a DEB system and a vehicle, in particular a heavy duty truck, comprising a combustion engine with a DEB system.

Decompression engine brakes (DEB) are one example of so-called primary retarders and can be found in many combustion engines, in particular in, but not limited to, heavy duty trucks.

Malfunctioning of the DEB system can prevent the engine from running smoothly, can add to wear and tear of the engine and/or the conventional brake system and may constitute a considerable safety risk. It is therefore of importance that the DEB system functions properly.

In order to provide and ensure a properly functioning DEB system, the DEB system should be tested on a regular basis and/or when a driver of the vehicle equipped with the DEB system realizes abnormal behavior of the DEB system and/or the engine.

Tests of the DEB system, where the combustion engine is driven by an external engine, are known from the state of the art. However, such tests require connecting the external engine to the combustion engine and are therefore tedious and time-consuming.

As an alternative, the combustion engine can be driven during the test by a starter motor of the engine. However, the starter motor can be too weak to drive the combustion engine at engine speeds close or equal to engine speeds at regular operation of the engine for a period long enough to perform the test and/or overheat during such a test.

The object of the present invention is therefore to provide a method for testing a DEB system of a combustion engine that overcomes the problems mentioned above. In particular, the object of the present invention is to provide a method for testing a DEB system that requires only little time and tests the DEB system at engine speeds close or equal to engine speeds at regular operation of the engine.

According to the invention, this object is achieved by a method for testing a decompression engine brake (DEB) system of a combustion engine according to claim 1, a combustion engine according to claim 15 and a vehicle according to claim 16.

According to the invention, a method for testing a decompression engine brake (DEB) system of a combustion engine is provided. The DEB system comprises DEB actuators to adjust opening characteristics of exhaust valves of the combustion engine. In particular, the DEB actuators are configured to open the exhaust valve at the end of the compression stroke, thereby performing negative work on the engine. As an example, the DEB actuators may comprise a dedicated cam and rocker for engine braking. As another example, the DEB actuators may comprise a brake lobe integrated in an exhaust cam. In both cases, it may be possible to actuate the DEB on a single valve, for example by a pin-through-bridge design.

The test of the DEB system is performed while the engine is running. In order to test the DEB system for a cylinder under investigation, the following steps are performed:

Fuel injection is shut off for at least one cylinder, including the cylinder under investigation. Meanwhile, at least one other cylinder keeps the engine running. Consequently, no other engine, like an external engine or a starter motor, is needed to keep the engine running. Therefore, testing the DEB system can be done both quickly, since there is no need to connect an external engine to the engine, and at engine speeds that are close to or equal to engine speeds at regular operation of the engine, since even one or a few cylinders are powerful enough to reach such engine speeds when there is no load on the engine.

Then, DEB is activated by the DEB actuators for at least one of the cylinders for which fuel injection has been shut off, including the cylinder under investigation. That is, if the DEB system works properly for the cylinder under investigation, the cylinder under investigation should perform negative work on the engine.

With fuel injection shut off and DEB activated for the cylinder under investigation, the momentary unbalance on the crank shaft by the cylinder under investigation is determined. If the DEB system works properly, this momentary unbalance is caused for the most part by the negative work performed by the cylinder under investigation.

Finally, the result of the test of the DEB system for the cylinder under investigation is determined in dependence on the determined momentary unbalance on the crank shaft by the cylinder under investigation. If the momentary unbalance on the crank shaft by the cylinder under investigation is above a predetermined threshold, indicating negative work performed on the engine, the test indicates that the DEB system works properly for the cylinder under investigation. On the other hand, if the momentary unbalance on the crank shaft by the cylinder under investigation is below the predetermined threshold, a malfunctioning of the DEB system for the cylinder under investigation is detected.

DEB may be activated per single cylinder or per cylinder bank, depending, e.g., on the capabilities of the DEB actuators. It is important, however, that at least one cylinder keeps the engine running.

After the momentary unbalance on the crank shaft by the cylinder under investigation with activated DEB has been determined, DEB may be deactivated for the cylinder under investigation and the method may be repeated for another cylinder under investigation. In order to do so, it may also be necessary to turn on fuel injection for the cylinder under investigation and shut off fuel injection for the other cylinder under investigation. Preferably, this is done until the method has been repeated for all cylinders of the combustion engine.

Determining the momentary unbalance on the crank shaft may be performed by measuring a cylinder acceleration signal, i.e., the acceleration of the engine caused by a cylinder. For a cylinder with a properly working DEB, this acceleration is negative, i.e., a deceleration. Preferably, such a measurement of the cylinder acceleration signal is performed at a flywheel of the combustion engine or directly at the cylinder under investigation. To achieve a meaningful measurement of the cylinder acceleration signal, it is important to get a very accurate measurement of the engine speed. For a photoelectric tachometer, for example, this means that the number of holes in the rotating disc must be at least large enough so that the Nyquist frequency of the tachometer corresponds to the expected acceleration or deceleration, respectively.

The acceleration signal may be measured by comparing the engine speed of the cylinder under investigation to a previous engine speed. Here, the engine speed of a predetermined event is measured. The predetermined event is preferably an injection event, wherein the injection event for a cylinder for which the fuel injection has been shut off is defined as the event in the combustion cycle when fuel injection would occur if it had not been shut off. Another example for a predetermined event is the passing of a predetermined tooth of a cylinder on the crank gear. In particular, the predetermined tooth may be located somewhere at the expansion stroke of the cylinder. Yet another predetermined event may be the time taken for the expansion stroke of the respective cylinder. The previous engine speed is the engine speed of the predetermined event of a cylinder which precedes the cylinder under investigation in a cylinder firing sequence. In other words, the acceleration signal is determined as the engine acceleration from one predetermined event to the next predetermined event, wherein the latter predetermined event is the predetermined event of the cylinder under investigation. That way, the deceleration caused by the cylinder under investigation, mainly provided by the compression stroke and the subsequent dissipation of the compressed air, is included in the measurement of the cylinder acceleration signal and therefore of the momentary unbalance on the crank shaft by the cylinder under investigation. In order to increase the informative value of the test of the DEB system and to provide root cause failure analysis, the difference of the momentary unbalance on the crank shaft by the cylinder under investigation with DEB deactivated on the cylinder under investigation and the momentary unbalance on the crank shaft by the cylinder under investigation with DEB activated on the cylinder under investigation is determined. That is, the momentary unbalance on the crank shaft by the cylinder under investigation is measured once when fuel injection is shut off and DEB is deactivated and once when fuel injection is shut off and DEB is activated. If the absolute value of the difference between the two measurements is greater than a predetermined threshold, the DEB actuators for the cylinder under investigation are working properly. If, however, the absolute value of the difference between the two measurements is smaller than the predetermined threshold, there is some malfunctioning of the DEB actuators for the cylinder under investigation: either they fail to activate DEB on the cylinder under investigation or they permanently activate DEB on the cylinder under investigation, even when it is supposed to be turned off. Preferably, the measurement of the momentary unbalance on the crank shaft by the cylinder under investigation with DEB deactivated on the cylinder under investigation is performed after fuel injection has been shut off for the cylinder under investigation and before DEB is activated for the cylinder under investigation.

The measurement of the momentary unbalance on the crank shaft by the cylinder under investigation with DEB deactivated on the cylinder under investigation and fuel injection shut off for the cylinder under investigation may be compared to the measured momentary unbalance on the crank shaft by the cylinder under investigation with DEB deactivated on the cylinder under investigation and fuel injection turned on for the cylinder under investigation. If the difference in momentary unbalance on the crank shaft by the cylinder between fuel injection turned on and shut off is below a predetermined threshold, a faulty behavior of the cylinder under investigation, in particular with its fuel injection, is assumed and the test of the DEB system is aborted.

In order to further increase the informative value of the test of the DEB system, the difference of the momentary unbalance on the crank shaft by the cylinder under investigation with DEB activated and/or deactivated on the cylinder under investigation and an average momentary unbalance on the crank shaft by all cylinders may be determined. Said difference is, for example, indicative of whether DEB is always activated or always deactivated by the DEB actuators for the cylinder under investigation.

While said method is performed, the average speed of the combustion engine may be held constant. This leads to more robust and significant results. Holding the average speed of the combustion engine constant may be achieved by use of a feedback control, adjusting the engine speed, e.g., by the amount of injected fuel to the cylinders for which fuel injection has not been shut off.

The method may be performed under repeatable test circumstances. The repeatable test circumstances may include one or more of the following: a constant engine speed; a given variable turbine geometry (VTG) position; a given back pressure (BP) valve position; and a given exhaust gas recirculation (EGR) valve position. Repeatable test circumstances enable an even better determination of the proper functioning or malfunctioning of the DEB system. In particular, repeatable test circumstances provide an accurate quantification of malfunctioning of the DEB system, which may help in determining the reason for the malfunctioning, e.g., by comparing the measured values to a look-up table containing previously determined values and/or predetermined thresholds and the corresponding reasons for malfunctioning of the DEB system. Furthermore, using pre-defined VTG, BP valve and/or EGR valve positions, the effect of potentially malfunctioning VTG, BP valves and/or EGR valves on the test result can be minimized. The reason is that a malfunctioning of the VTG, BP valve and/or EGR valve can also cause reduced engine braking performance, which may erroneously lead to the assumption of a malfunctioning of the DEB system.

In order to ensure reliable test results, a safe execution of the method and/or to prevent damage to the combustion engine, one or more entry conditions may be defined. Before the engine is started and/or before fuel injection is shut off for the first cylinder under investigation, said entry conditions are checked and the method is only performed if all of the entry conditions are fulfilled. The entry conditions may include one or more of the following: the engine is idling; a throttle pedal is not pressed; a vehicle comprising the combustion engine is standing still; no other failures are detected; no other engine test is in progress; for an automatic gearbox, the gearbox is in neutral position; and oil pressure, oil temperature, boost pressure and/or brake circuit air pressure are above calibrate-able thresholds. Oil pressure and oil temperature, for example, have to be in a range where the DEB system should function normally, since, e.g., at a very low oil temperature, the DEB system will not perform properly and will therefore be disabled, leading to the erroneous assumption of a malfunctioning DEB system. Also, the brake circuit air pressure has to be above a certain threshold to ensure that the compressor is not activated, which may cause additional unbalance to the crank shaft and thereby influence the test results.

In order to abort the method if reliable test results cannot be ensured anymore and/or if unsafe conditions arise, one or more exit conditions may be defined. Said exit conditions may be checked continuously while the method is performed and the method may be aborted if one or more of the exit conditions are fulfilled. The exit conditions may include one or more of the following: a vehicle comprising the combustion engine is not standing still; a throttle pedal is pressed; for an automatic gearbox, the gearbox is not in neutral position; other engine tests are in progress; the test does not finish in a calibrate-able duration of time; engine speed is out of a calibrate-able threshold band; and boost pressure, oil pressure, oil temperature and/or brake circuit air pressure are equal to or below a calibrate-able threshold and/or their reading is not valid.

The combustion engine may be an internal combustion engine of a vehicle, such as a heavy duty truck. In particular, the vehicle may be at rest while the method is performed, e.g., the method may be performed in a workshop. This yields the most reliable and time effective test results, since the test conditions can be controlled and there is no need to drive the vehicle on the road.

The results of the test of the DEB system may be visualized, in particular on a per cylinder basis. That way, it is easy to determine which DEB actuators need to be fixed. The visualization may be a simple pass/fail visualization, a numeric or graphic representation of the DEB braking power per cylinder or even a graphic indication of the parts of the DEB actuators that might be broken.

The method may be performed automatically, i.e., with little or no human interaction. In particular, the method may be performed by an engine control unit (ECU) of the combustion engine. Preferably, the method is started and controlled by a workshop tool connected to the ECU, e.g., via an OBD interface. Said workshop tool may be a generic workshop tool equipped with specific software. Also, determining the reason for malfunctioning may be performed by the workshop tool, wherein the look-up table with the previously determined values and/or predetermined thresholds and the corresponding reasons for malfunctioning of the DEB system is stored in the workshop tool. Since there is no need to hook up external equipment besides the workshop tool before performing the method, the method can be performed very quickly.

Moreover, a combustion engine comprising a decompression engine brake (DEB) system and an electronic control unit (ECU) is provided, wherein the ECU is designed to perform the method according to the above description. This provides a quick test of the DEB system at engine speeds close to or equal to regular operating speeds. Finally, a vehicle, in particular a heavy duty truck, is provided, comprising a combustion engine according to the above description. Vehicles benefit greatly from the short time needed to perform the DEB system test, since this means little downtime for the vehicle.

The invention will further be elucidated by description of some specific embodiments thereof, making reference to the attached drawings. The detailed description provides examples of possible implementations of the invention, but is to be regarded as illustrative without being restrictive on the invention. In the drawings:

Figures la - le show a cylinder of a combustion engine at different moments of a decompression engine brake cycle;

Figure 2 shows a rocker arrangement for a decompression engine brake system;

Figure 3 schematically shows a combustion engine with a decompression engine brake system and an electronic control unit;

Figure 4 a flowchart depicting a method for testing a decompression engine brake system and

Figure 5 exemplary data from a test of a decompression engine brake system.

In Figures la - le, a cylinder 1 of a combustion engine is depicted at different moments of a decompression engine brake (DEB) cycle. The cylinder 1 comprises, inter alia, a piston 2 which can move freely inside the cylinder 1, an intake valve 3 and an exhaust valve 4.

During an intake stroke, as shown in Fig. la, the exhaust valve 4 is in a closed position and the intake valve 3 in an open position. The piston 2 moves downward and fresh air flows through the intake valve 3 into the cylinder 1.

After the piston 2 has reached the lowest point in the cylinder 1, the intake valve 3 is closed and the piston 2 starts moving upward again. During this compression stroke, depicted in Fig. lb, the air inside the cylinder 1 is compressed. In order to do this, work has to be performed.

Contrary to the combustion cycle, no fuel has been injected into the cylinder 1 and there is no ignition of an air-fuel mixture. Instead, as depicted in Fig. 1c, the exhaust valve 4 is opened and the compressed air is released from the cylinder 1 through the exhaust valve 4. The work used to compress the air inside the cylinder 1 is therefore dissipated.

During the following so-called power stroke, which refers to the power generated in a regular combustion cycle, depicted in Fig. Id, both the intake valve 3 and exhaust valve 4 are closed and the piston 2 moves downward, creating a vacuum in the cylinder 1. Since the compressed air had been released in the previous step, no expansion power is generated by the cylinder 1.

In the final stroke, the exhaust stroke, depicted in Fig. le, the exhaust valve 4 is opened and the piston 2 moves upward. The remaining air inside the cylinder 1 leaves the cylinder 1 through the exhaust valve 4. Afterwards, the cycle starts over again.

Figure 2 shows one exemplary embodiment of a DEB actuator 5. The person skilled in the art appreciates that the present invention is not limited to this specific DEB actuator 5, but can be applied to a wide variety of DEB actuators 5.

The DEB actuator 5 comprises an exhaust rocker 6 and a brake rocker 7, wherein the brake rocker 7 can be actuated by a dedicated cam, which is not shown here. The exhaust rocker 6 and brake rocker 7 are arranged in a pin-through-bridge design such that both the exhaust rocker 6 and the brake rocker 7 actuate the exhaust valve 4. In other embodiments, which are not shown here, there may be two or more exhaust valves 4 per cylinder 1 and the rockers may operate one or more exhaust valves 4. Such embodiments are known to a person skilled in the art and the adaptation of the method presented here to such embodiments is straight forward. Figure 3 schematically shows a combustion engine 8 according to the invention. The combustion engine 8 comprises several cylinders 1 with fuel injectors 9, where only one cylinder 1 is shown in order to improve clearness. The combustion engine 8 further comprises a DEB system 10 with DEB actuators 5. It further comprises an electronic control unit (ECU) 11 and at least one engine speed sensor 12. A workshop tool 13 is connected to the ECU 11.

Upon request of a test of the DEB system 10 by the workshop tool 13, the ECU 11 first checks whether entry conditions are fulfilled for a safe execution of the test and in order to prevent damage to the combustion engine. The entry conditions are that the combustion engine 8 is idling; that a throttle pedal is not pressed; that a vehicle comprising the combustion engine 8 is standing still; that no other failures are detected; that no other engine test is in progress; that an automatic gearbox is in neutral position; and that oil pressure, oil temperature, boost pressure and/or brake circuit air pressure are above a predetermined threshold. If these conditions are fulfilled, the ECU 11 starts the test of the DEB system 10.

First, fuel injection is shut off for one cylinder 1, the cylinder 1 currently under investigation. Alternatively, fuel injection can be shut off for a cylinder bank, including the cylinder 1 under investigation. The remaining cylinders keep the combustion engine 8 running.

With fuel injection shut off, the momentary unbalance on the crank shaft by the cylinder 1 under investigation is determined by measuring a cylinder acceleration signal using the engine speed sensor 12. To do so, engine speed of an injection event of the cylinder 1 preceding the cylinder 1 under investigation in the firing sequence is measured. Also, the engine speed of the injection event of the cylinder 1 under investigation is measured, and the difference between the two said engine speeds is the acceleration signal. Instead of measuring the engine speed of the injection event, the engine speed of another predetermined event may be measured. Further examples for such a predetermined event are the passing of a predetermined tooth of a cylinder on the crank gear, in particular of a tooth located somewhere at the expansion stroke of the cylinder, or the time taken for the expansion stroke of the respective cylinder. In particular, the predetermined event is chosen such that the measured acceleration signal includes the effects of the compression and the subsequent dissipation of compressed air of the cylinder under investigation on the engine speed.

Then, DEB is activated for the cylinder 1 under investigation or for the cylinder bank including the cylinder 1 under investigation. Again, the momentary unbalance on the crank shaft by the cylinder 1 under investigation is determined, as described above.

The difference of the momentary unbalance on the crank shaft by the cylinder 1 under investigation with DEB deactivated and DEB activated is compared to a predetermined threshold value stored in the ECU 11 or the workshop tool 13. If it exceeds the threshold value, the DEB system is working properly for the cylinder 1 under investigation. However, if the difference is below the threshold value, the DEB system is malfunctioning for the cylinder 1 under investigation.

In the latter case, it is beneficial to compare the momentary unbalance on the crank shaft by the cylinder 1 under investigation with DEB activated and/or deactivated to an average momentary unbalance on the crank shaft by all cylinders 1. This difference is indicative of whether DEB is not activated at all for the cylinder 1 under investigation or DEB is activated permanently for the cylinder 1 under investigation.

Having finished the test of the DEB system 10 for the cylinder 1 under investigation, the test is repeated for the next cylinder 1 until it has been performed for all cylinders 1 of the combustion engine 8.

Since there is neither external interaction nor an external engine necessary to perform the test of the DEB system 10, the test can be performed very quickly, reducing downtime of the vehicle in the workshop. Also, since the remaining cylinders 1 of the combustion engine 8 keep the engine 8 running while the test is performed for the cylinder 1 under investigation, the engine 8 can run at engine speeds close to or equal to engine speeds during regular operation of the engine 8. Figure 4 shows in more detail the steps of a test of the DEB system 10. First, the test is started 401. After the start of the test 401, entry conditions are checked 402 and the engine 8 is conditioned for the test 403, e.g., by setting the engine speed to a predetermined constant value, by adjusting a variable turbine geometry (VTG) position, a back pressure (BP) valve position and/or an exhaust gas recirculation (EGR) valve position.

Then, an initial engine diagnostic is performed 410. This includes, in particular, a measurement of an averaged unbalance signal on all cylinders 1.

Once this measurement is completed, a first cylinder 1 of the engine 8 is chosen as cylinder 1 under investigation 420. For this cylinder 1 under investigation, injection is cut off 430 and the unbalance on the crank shaft by the cylinder 1 under investigation is measured 431, as described above.

Then, the DEB system 10 is activated for the cylinder 1 under investigation 440 and the unbalance on the crank shaft by the cylinder 1 under investigation with DEB activated is measured 441.

To finalize the test of the DEB system 10 for the first cylinder 1 under investigation, the DEB system 10 is deactivated 450 and fuel injection is turned back on 451 for the cylinder 1 under investigation.

Then, it is evaluated 460 whether the DEB system 10 has been tested for all cylinders 1 of the engine 8. If not 461, the test is repeated for the next cylinder 1.

If the DEB system 10 has been tested for all cylinders 1 of the engine 8, 462, the measurements are evaluated and reported 470 and the test is ended 480.

While the test is performed, exit conditions are continuously checked in order to abort the test 465 if unsafe conditions arise. The exit conditions comprise, e.g., whether a vehicle comprising the combustion engine 8 is not standing still; whether a throttle pedal is pressed; whether other engine tests are in progress; whether the engine speed is out of a calibrate-able threshold band; and whether boost pressure, oil pressure and/or oil temperature are equal to or below a calibrate-able threshold and/or their reading is not valid.

Finally, Figure 5 shows some exemplary graphs of measured engine speed 501 and unbalance on the crank shaft by the cylinder 1 under investigation 502, depending on the fuel injection status 503 of the cylinder 1 under investigation and the DEB actuator 5 status 504 of the cylinder 1 under investigation as a function of time t.

After conditioning the engine for the test 403, the measurement of an averaged unbalance signal 410 on all cylinders 1 is performed.

Then, fuel injection is cut off 430 on the cylinder 1 under investigation and the unbalance on the crank shaft by the cylinder 1 under investigation is measured 431.

Afterwards, the DEB system 10 is activated for the cylinder 1 under investigation 440 and the unbalance on the crank shaft by the cylinder 1 under investigation with DEB activated is measured 441.

Then, the DEB system 10 is deactivated 450 and fuel injection is turned back on 451 for the cylinder 1 under investigation.

For the exemplary data of Figure 5, there is a clear difference between the unbalance on the crank shaft by the cylinder 1 under investigation with DEB deactivated and with DEB activated. This implies that the DEB system 10 works properly for the cylinder 1 under investigation.

It is thus believed that the operation and construction of the present invention will be apparent from the foregoing description and drawings appended thereto. For the purpose of clarity and a concise description, features are described herein as part of the same or separate embodiments, however, it will be appreciated that the scope of the invention may include embodiments having combinations of all or some of the features described. It will be clear to the skilled person that the invention is not limited to any embodiment herein described and that modifications are possible which may be considered within the scope of the appended claims. In the claims, any reference signs shall not be construed as limiting the claim.

Claims

Claims Method for testing a decompression engine brake, DEB, system (10) of a combustion engine (8), wherein the DEB system (10) comprises DEB actuators (5) to adjust opening characteristics of exhaust valves (4) of the combustion engine (8), wherein, while the engine (8) is running, the DEB system (10) is tested for a cylinder (1) under investigation by shutting off fuel injection for at least one cylinder (1), including the cylinder (1) under investigation, while at least one other cylinder (1) keeps the engine (8) running, activating the DEB for at least one of the cylinders (1) for which the fuel injection has been shut off, including the cylinder (1) under investigation, determining the momentary unbalance on the crank shaft by the cylinder (1) under investigation and determining the result of the test of the DEB system (10) for the cylinder (1) under investigation in dependence on the determined momentary unbalance on the crank shaft by the cylinder (1) under investigation. Method according to claim 1, wherein DEB is activated per single cylinder (1) or per cylinder bank. Method according to any of the previous claims, wherein after determining the momentary unbalance on the crank shaft by the cylinder (1) under investigation with activated DEB, DEB is deactivated for the cylinder (1) under investigation and the method is repeated for another cylinder (1) under investigation, preferably until the method has been repeated for all cylinders (1) or cylinder banks of the combustion engine (8). Method according to any of the previous claims, wherein determining the momentary unbalance on the crank shaft is performed by measuring a cylinder (1) acceleration signal, preferably at a flywheel of the combustion engine (8) or at the cylinder (1) under investigation. Method according to the previous claim, wherein the acceleration signal is measured by comparing the engine speed of a predetermined event of the cylinder (1) under investigation to a previous engine speed of the predetermined event of a cylinder (1) which precedes the cylinder (1) under investigation in a cylinder (1) firing sequence, wherein the predetermined event is preferably an injection event, the passing of a predetermined tooth of a cylinder on the crank gear and/or the entire expansion stroke. Method according to any of the previous claims, wherein the difference of the momentary unbalance on the crank shaft by the cylinder (1) under investigation with DEB deactivated on the cylinder (1) under investigation and the momentary unbalance on the crank shaft by the cylinder (1) under investigation with DEB activated on the cylinder (1) under investigation is determined. Method according to any of the previous claims, wherein the difference of the momentary unbalance on the crank shaft by the cylinder (1) under investigation with DEB deactivated on the cylinder (1) under investigation and fuel injection shut off for the cylinder (1) under investigation and the momentary unbalance on the crank shaft by the cylinder (1) under investigation with DEB deactivated on the cylinder (1) under investigation and fuel injection turned on for the cylinder (1) under investigation is determined.

. Method according to any of the previous claims, wherein the difference of the momentary unbalance on the crank shaft by the cylinder (1) under investigation with DEB activated and/or deactivated on the cylinder (1) under investigation and an average momentary unbalance on the crank shaft by all cylinders (1) is determined. . Method according to any of the previous claims, wherein the average speed of the combustion engine (8) is held constant while the method is performed, in particular by use of a feedback control.

10. Method according to any of the previous claims, wherein the method is performed under repeatable test circumstances, including one or more of the following: constant engine (8) speed; given variable turbine geometry, VTG, position; given back pressure, BP, valve position; given exhaust gas recirculation, EGR, valve position.

11. Method according to any of the previous claims, wherein one or more entry conditions are defined for a safe execution of the method and/or to prevent damage to the combustion engine (8) and wherein, before the engine (8) is started and/or before fuel injection is shut off for the first cylinder (1) under investigation, the entry conditions are checked and the method is only performed if all of the entry conditions are fulfilled.

12. Method according to any of the previous claims, wherein one or more exit conditions are defined for abortion of the method if unsafe conditions arise and wherein, while the method is performed, the exit conditions are checked continuously and the method is aborted if one or more of the exit conditions is fulfilled.

13. Method according to any of the previous claims, wherein the combustion engine (8) is an internal combustion engine (8) of a vehicle and in particular the vehicle is at rest while the method is performed.

14. Method according to any of the previous claims, wherein the results of the test of the DEB system (10) are visualized, in particular on a per cylinder (1) basis.

15. Method according to any of the previous claims, wherein the method is performed automatically, in particular by an engine control unit (11), ECU, of the combustion engine (8).

16. Combustion engine comprising a decompression engine brake, DEB, system (10) and an electronic control unit (11), ECU, wherein the ECU (11) is designed to perform the method according to any of the previous claims.

17. Vehicle, in particular heavy duty truck, comprising a combustion engine (8) according to the previous claim.