The present invention relates to a method of operating piston engine in ac cordance with claim 1. The invention also concerns a piston engine as defined in the other independent claim.

Background of the invention

The requirements set for piston engines in terms of fuel efficiency, emissions, and performance are becoming stricter and stricter. At the same time, the abil- ity to operate engines using different fuels has become a more desired proper ty. However, the type and quality of the fuel can have a significant effect on the performance and emissions of an engine. In some cases, changes in the quali ty of the fuel can lead to a situation, where an engine cannot meet the required emission limits or the promised efficiency, or where exhaust temperatures ex- ceed allowed limits. There is thus a need for a method, which can adjust the operation of a piston engine to take into account changes in the quality of fuel.

Summary of the invention

An object of the present invention is to provide an improved method of operat- ing a piston engine. The characterizing features of the method according to the invention are given in claim 1. Another object of the invention is to provide an improved piston engine. The characterizing features of the engine according to the invention are given in the other independent claim.

The method according to the invention comprises the steps of monitoring pres- sure in a plurality of cylinders of the engine; based on the monitoring of cylin der pressure, determining the value of a first variable, which represents the development of cumulative heat release in a single cylinder of the engine dur ing one or more engine cycles up to a first reference point; comparing the val ue of the first variable to a predetermined first target value or target range, which represents desired development of the cumulative heat release in the single cylinder up to the first reference point; on the basis of the comparison, adjusting a first engine parameter, wherein the first engine parameter is a cyl inder specific parameter affecting start of combustion in the single cylinder; based on the monitoring of cylinder pressure determining the value of a sec ond variable, which represents the average development of cumulative heat release in a plurality of cylinders of the engine during one or more engine cy cles up to a second reference point, the second reference point being located at a later phase of combustion than the first reference point; comparing the value of the second variable to a predetermined second target value or target range, which represents desired average development of the cumulative heat release in the plurality of cylinders up to the second reference point; and on the basis of the comparison, adjusting a second engine parameter, which affects combustion in the plurality of cylinders.

In the method according to the invention, both an early phase of the combus tion and a later phase of the combustion are monitored and two different en gine parameters are controlled. By adjusting the first engine parameter, the start of combustion can be individually controlled in each cylinder. This allows quick adjustments of the combustion process. The adjustment of the second engine parameter allows slower control of the combustion and affects a later phase of the combustion. By the combination of the adjustment of the two en gine parameters, for instance variations in the fuel quality can be effectively compensated.

The piston engine according to the invention comprises means for determining cumulative heat release in a plurality of the cylinders of the engine during combustion and a control unit, wherein the control unit is configured to operate the engine in accordance with the method defined above.

According to an embodiment of the invention, the first reference point is se lected so that the value of the first variable describes an early phase of com bustion.

According to an embodiment of the invention, the first engine parameter is the timing of pilot fuel injection. Alternatively, the first engine parameter can be the injection amount of pilot fuel. Both alternatives allow cylinder specific adjust ment of the combustion process and affect particularly the early part of the combustion. The timing of pilot fuel injection affects especially the start of combustion.

According to an embodiment of the invention, the amount of the pilot fuel is ad justed by controlling the duration of pilot fuel injection and/or by controlling the injection pressure of the pilot fuel.

According to an embodiment of the invention, the first engine parameter is in take valve timing.

According to an embodiment of the invention, the first engine parameter is spark timing. In spark ignited engines, the spark timing is an effective way of adjusting the ignition timing.

According to an embodiment of the invention, the method comprises the steps of determining the average injection amount of pilot fuel of at least three earlier engine cycles, comparing the average injection amount to a predetermined upper limit, and in case the average injection amount has not reached the up per limit, adjusting the injection amount of the pilot fuel as the first engine pa rameter. The injection amount can thus be used as the primary way of control ling the early part of combustion, but by setting an upper limit for the injection amount, excessive emissions can be avoided.

According to an embodiment of the invention, in case the average injection amount has reached the upper limit, the injection timing of the pilot fuel and/or cylinder compression temperature is adjusted as the first engine parameter.

According to an embodiment of the invention, the compression temperature is adjusted by means of exhaust gas recirculation.

According to an embodiment of the invention, the compression temperature is adjusted by adjusting intake valve closing timing.

According to an embodiment of the invention, the first variable is the crank an gle at which a predetermined first cumulative amount of released heat is reached.

According to an embodiment of the invention, the predetermined first cumula tive amount of released heat is 2-40 percent of the total amount of heat re leased during one engine cycle in the single cylinder. According to an embodiment of the invention, the first variable is a heat re lease value representing cumulative heat release up to a first predetermined crank angle in the single cylinder.

According to an embodiment of the invention, the second engine parameter is the pressure of intake air. By controlling the intake air pressure, combustion in a plurality of cylinders is affected.

According to an embodiment of the invention, the second variable is the aver age crank angle, at which a predetermined second cumulative amount of re leased heat is reached.

According to an embodiment of the invention, the predetermined second cu mulative amount of released heat is 41-95 percent of the total amount of heat released during one engine cycle.

According to an embodiment of the invention, the predetermined second cu mulative amount of released heat is 50-95 percent of the total amount of heat released during one engine cycle.

According to an embodiment of the invention, the predetermined second cu mulative amount of released heat is at least 10 percentage units greater than the predetermined first cumulative amount of released heat.

According to an embodiment of the invention, the predetermined second cu mulative amount of released heat is at least 20 percentage units greater than the predetermined first cumulative amount of released heat.

According to an embodiment of the invention, the second variable is a heat re lease value representing average cumulative heat release up to a predeter mined second crank angle.

According to an embodiment of the invention, the second variable represents the average development of cumulative heat release during several engine cy cles. By monitoring the heat release over several engine cycles, slow changes in the combustion can be better taken into the account.

According to an embodiment of the invention, the first variable represents the development of cumulative heat release during a single engine cycle. This al lows quick adjustments in the early part of the combustion process. According to an embodiment of the invention, the method comprises the steps of calculating the value of a third variable, which is the average of the values of the first variables of at least three earlier engine cycles, comparing the value of the third variable to a predetermined third target value or target range, which represents desired average development of the cumulative heat release up to the first reference point, and on the basis of the comparison, adjusting a third engine parameter, which is different than the first engine parameter. This al lows further adjustment of the early part of combustion.

According to an embodiment of the invention, the third engine parameter is dif ferent than the second engine parameter.

According to an embodiment of the invention, the third engine parameter is the amount of recirculated exhaust gas and/or the timing of pilot fuel injection.

According to an embodiment of the invention, the method comprises the steps of determining the average value of the first variables of all cylinders of the en gine, comparing the value of the first variable of a single cylinder of the engine to the average value of the first variables, and in case the value of the first var iable of the single cylinder is not within predetermined limits from the average value of the first variables, adjusting a fourth engine parameter, which is differ ent than the first engine parameter. Differences in the combustion processes of different cylinders can thus be reduced.

According to an embodiment of the invention, the fourth engine parameter is intake valve timing.

According to an embodiment of the invention, the method comprises the steps of determining the difference between the value of the first variable and the value of the second variable, comparing the difference to a predetermined tar get value or target range, and in case the difference is not in a desired range, adjusting a fifth engine parameter.

According to an embodiment of the invention, the fifth engine parameter is dif ferent than the first engine parameter. Brief description of the drawings

Embodiments of the invention are described below in more detail with refer ence to the accompanying drawings, in which

Fig. 1 shows schematically a piston engine, Fig. 2 shows schematically one cylinder of a piston engine,

Fig. 3 shows as a flowchart a method according to an embodiment of the in vention,

Fig. 4 shows as a flowchart part of a method according to an embodiment of the invention, Fig. 5 shows as a flowchart part of a method according to another embodiment of the invention,

Fig. 6 shows as a flowchart part of a method according to still another embod iment of the invention,

Fig. 7 shows an example of the selection of reference points in a method ac- cording to the invention,

Fig. 8 shows another example of the selection of the reference points, and

Fig. 9 shows as a flowchart part of a method according to still another embod iment of the invention.

Description of embodiments of the invention

Figure 1 shows schematically a piston engine 1. The engine 1 can be a large internal combustion engine, such as a main or an auxiliary engine of a ship or an engine that is used at a power plant for producing electricity. The expres sion“large internal combustion engine” refers to an engine of which cylinder bore is at least 150 mm. The engine 1 is a four-stroke engine 1. The engine 1 comprises a plurality of cylinders 2. Four cylinders 2 are shown in figure 1 , but the engine 1 can comprise any reasonable number of cylinders 2. The engine 1 can be, for instance, an in-line engine, as shown in figure 1 , or a V-engine. The engine can be, for instance, a gas engine, a dual-fuel engine or a multi fuel engine. A dual-fuel engine or a multi-fuel engine can be selectively oper ated using two or more different main fuels, for instance a liquid main fuel and a gaseous main fuel. In a gas engine or in a dual-fuel or multi-fuel engine that is operated in a gas mode, a gaseous main fuel can be ignited using for exam ple liquid pilot fuel or spark plugs for triggering combustion of the gaseous main fuel.

The engine 1 of figure 1 is provided with a turbocharger 5, which comprises a turbine 5a and a compressor 5b. The engine 1 could also be provided with two or more turbochargers. The turbochargers could be arranged in series and/or in parallel. Intake air of the engine is pressurized by the compressor 5b of the turbocharger 5 and introduced via an intake duct 8 into the cylinders 2 of the engine 1. Exhaust gas is introduced via an exhaust duct 9 into the turbine 5a of the turbocharger 5. The intake duct 8 comprises branches 8a that are con nected to the cylinders 2. Similarly, the exhaust duct 9 comprises branches 9a that are connected to the cylinders 2. The intake duct 8 on the downstream side of the compressor 5b of the turbocharger 5 can be called as an intake or inlet receiver or intake or inlet manifold.

Figure 2 shows schematically one cylinder 2 of a piston engine according to an embodiment of the invention. The cylinder 2 shown in figure 2 belongs to a gas engine or a dual-fuel or multi-fuel engine that can be operated in a gas mode and which is provided with pilot fuel injection system. However, apart from the description of the pilot fuel injection system, the features discussed below could be present also in other types of engines.

Each cylinder 2 of the engine 1 is provided with a piston 4, which is configured to move in a reciprocating manner within the cylinder 2. The piston 4 is con nected via a connecting rod 6 to a crankshaft 16. A flywheel 17 is attached to one end of the crankshaft 16. Together with the walls of the cylinder 2 and a cylinder head 7, the piston 4 delimits a combustion chamber 10. Each cylinder 2 of the engine 1 is provided with at least one intake valve 3. According to an embodiment of the invention, each cylinder 2 is provided with two intake valves 3. The intake valves 3 are used for opening and closing fluid communication between the intake duct (intake receiver) 8 and the combustion chamber 10. Each cylinder 2 of the engine 1 is provided with at least one exhaust valve 1 1. According to an embodiment of the invention, each cylinder 2 is provided with two exhaust valves 1 1 . The exhaust valves 1 1 are used for opening and clos ing fluid communication between the combustion chamber 10 and the exhaust duct 9.

The intake valves 3 are connected to intake valve actuating means 12. The in take valve actuating means 12 are used for opening and closing the intake valves 12. The intake valve actuating means 12 can be configured to allow var iable intake valve timing. The crank angle at which the intake valves 3 are opened and/or closed can thus be varied. The intake valve actuating means 12 can be implemented in many alternative ways. The intake valve actuating means 12 can comprise an electrical, hydraulic or mechanical actuator. The in take valve actuating means 12 could also be any combination of electrical, hy draulic and/or mechanical means. For instance, the intake valves 3 can be opened by means of a camshaft. The closing force for closing the intake valves 3 can be created by means of one or more springs, such as helical springs and/or air springs. The closing of the intake valves 3 can be delayed by means of a hydraulic system. Alternatively, both the opening and closing timing could be determined by means of an electrical actuator, such as a solenoid. Alterna tively, the intake valves 3 could be both opened and closed hydraulically. The intake valve actuating means 12 are connected to a control unit 14, which is configured to transmit a control signal to the intake valve actuating means 12 for determining the opening and/or closing timing of the intake valves 3.

In the embodiment of figure 2, the exhaust valves 1 1 are connected to similar actuating means 13 as the intake valves 3. However, the exhaust valves 1 1 could also be provided with different actuating means. For instance, it is not necessary that the actuating means 13 of the exhaust valves 1 1 allow variable valve timing. In the embodiment of figure 2, also the exhaust valve actuating means 13 are connected to the control 14 unit which is configured to transmit a control signal to the exhaust valve actuating means 13 for determining the opening and/or closing timing of the exhaust valves 1 1 . Instead of the ar rangements described above, both the intake valves 3 and the exhaust valves 1 1 could be cam-controlled. The valve timings could be either variable or fixed.

The engine of figure 2 can be operated in a gas mode. Each cylinder 2 of the engine is provided with a gas admission valve 20. Via the gas admission valves 20, gaseous main fuel can be introduced into the cylinders 2 of the en gine 1 . The gaseous fuel can be, for instance, natural gas. In the embodiment of figure 2, the gas admission valve 20 is arranged to introduce the gaseous main fuel between the intake receiver 8 and the intake valves 3. The gaseous fuel is mixed with the intake air during the intake stroke and the compression stroke to form a homogenous mixture. Each cylinder 2 of the engine could also be provided with an additional gas admission valve for introducing part of the gaseous main fuel into a prechamber.

In the embodiment of figure 2, each cylinder 2 of the engine 1 is provided with a pilot fuel injector 21. Via the pilot fuel injector 21 , liquid pilot fuel can be in jected into the cylinder 2. The pilot fuel is injected directly into the combustion chamber 10. The ignition of the pilot fuel triggers combustion of the gaseous main fuel. The pilot fuel can be, for instance, light fuel oil. Instead of pilot fuel injection, each cylinder 2 of the engine 1 could be provided with a spark plug for controlling ignition timing. The engine could also be provided with an addi tional fuel injection system for injecting liquid main fuel into the cylinders 2 when operated in a liquid fuel mode.

The gas admission valves 20 and the pilot fuel injectors 21 are connected to the control unit 14. The injection timings and durations of both the pilot fuel in jection and the main fuel injection can thus be individually controlled in each cylinder 2 of the engine 1.

Each cylinder 2 of the engine 1 is provided with a cylinder pressure sensor 15. The cylinder pressure sensor 15 is arranged to measure pressure in the cylin der 2. The engine 1 is provided with data processing means, such as a control unit 14, which receives measurement data from the cylinder pressure sensors 15. The engine 1 further comprises a crank angle sensor 18 or other means for determining the angular position of the crankshaft 16. In the embodiment of figure 2, the crank angle sensor 18 monitors the angular position of the fly wheel 17. On the basis of the angular position of the flywheel 17, the position of the piston 4 in each cylinder 2 can be determined. In addition to the pres sure measurement data, the control unit 14 also receives measurement data from the crank angle sensor 18. The cylinder pressure can thus be determined in respect of crank angle.

The engine 1 is further provided with intake air pressure sensor 19. The intake air pressure sensor 19 monitors the pressure of the intake air. The intake air pressure sensor 19 is connected to the control unit 14, which receives meas urement data from the intake air pressure sensor 19.

The control unit 14 can be arranged to receive measurement data from many other sensors and to control different devices. For instance, the control unit 14 can monitor and control intake air temperature, monitor and control pilot fuel in jection pressure, monitor exhaust gas temperature, and/or control intake air pressure.

Figure 3 shows as a flowchart a method according to an embodiment of the in vention. In the method according to the invention, pressure in a plurality of cyl inders 2 of the engine 1 is monitored 101 . Preferably, the pressures of all cyl inders 2 of the engine 1 are monitored. The cylinder pressures can be moni tored by means of the pressure sensors 15. The cylinder pressure in each cyl inder 2 is measured several times at least during each power stroke.

On the basis of the cylinder pressure measurements, the value of a first varia ble CAi , HRi is determined 102. The first variable CAi , HRi represents the de velopment of cumulative heat release in a single cylinder 2 of the engine 1 dur ing one or more engine cycles up to a first reference point Ri. The first refer ence point Ri is a predetermined value, which is located at an early phase of the combustion. The value of the first variable CA-i , HRi thus describes an ear ly phase of the combustion.

Figures 7 and 8 show two examples of the selection of the first reference point Ri and the determination of the value of the first variable CAi , HR-i. In both fig ures the curve with a dashed line shows an example of the development of the cumulative heat release in a single cylinder 2 of the engine 1 as the function of crank angle. The curve can represent either a single engine cycle or the aver age of a plurality of engine cycles. The horizontal axis represents the crank angle and“0” refers to top dead center, i.e. the beginning of the power stroke.

In figure 7, the first reference point Ri is a first cumulative amount of released heat, i.e. a predetermined heat release percentage. The first cumulative amount of released heat could be for example in the range of 2^10 percent of the total heat release during one engine cycle. The value of the first variable is the crank angle CAi at which the predetermined first cumulative amount of re leased heat is reached in the single cylinder 2 during a single engine cycle, or the average crank angle at which the predetermined first cumulative amount of released heat is reached in the single cylinder 2 during a plurality of engine cy cles.

Alternatively, the first reference point Ri can be a predetermined crank angle, as shown in figure 8. The first variable is thus a heat release value HRi repre senting cumulative heat release up to a first predetermined crank angle Ri.

Referring again to figure 3, the value of the first variable CAi, HRi is compared to a predetermined first target value or target range 103. The first target range represents desired development of the cumulative heat release in the single cylinder 2 up to the first reference point Ri. If the first reference point Ri is a certain crank angle, as shown in figure 8, the cumulative amount of released heat HRi at that crank angle is thus compared to a desired heat release value or range. If the first reference point Ri is a certain heat release percentage, as shown in figure 7, the crank angle CAi at which the predetermined heat re lease amount is reached is compared to a desired crank angle or crank angle range.

On the basis of the comparison, a first engine parameter is adjusted 104. The first engine parameter is a cylinder specific parameter affecting start of com bustion in the single cylinder 2. Adjustment of the parameter does thus not af fect the combustion in the other cylinders 2 of the engine 1. The first engine parameter is changed only in case the value of the first variable CA-i, HRi is not within a desired range. For instance, if the value of the first variable CAi, HRi is outside the first target range or there is a certain predetermined devia tion between the value of the first variable CAi, HRi and the first target value, the first engine parameter is changed. If the value CAi, HRi is within the de sired range, the prevailing value of the first engine parameter is maintained. The monitoring of cylinder pressures continues.

The determination of the value of the first variable CAi, HRi 102, the compari son step 103, and the adjustment of the first engine parameter 104 can be done simultaneously for a plurality of cylinders 2 or for all cylinders 2 of the en gine 1. The first engine parameter can thus be adjusted individually in each cylinder 2 of the engine 1.

Simultaneously with the determination of the value of the first variable CAi, HR-i, the value of a second variable CA2, HR2 is determined 202. The second variable CA2, HR2 represents the average development of cumulative heat re- lease in a plurality of cylinders 2 during one or more engine cycles up to a sec ond reference point Fte. In figures 7 and 8, the curve with the solid line shows an example of the average development of cumulative heat release in a plurali ty of cylinders 2. The curve can represent either a single engine cycle or the average of a plurality of engine cycles. The second reference point R2 is locat ed at a later phase of combustion than the first reference point R1. The second variable CA2, HR2 thus describes a late phase of combustion. The second var iable CA2, HR2 can be based on the heat release in all the cylinders 2 of the engine. Alternatively, the second variable CA2, HR2 could be based for in stance on the heat release in the cylinders 2 of one bank of a V-engine.

Like the first reference point R-i , also the second reference point R2 can be a certain heat release percentage, as shown in figure 7. The value of the second variable can thus be the average crank angle CA2, at which a predetermined second cumulative amount of released heat is reached in the plurality of cylin ders 2. The predetermined second cumulative amount of released heat could be for example in the range of 41 -95 percent of the total heat release during one engine cycle, or in the range of 50-95 percent of the total heat release during one engine cycle. The predetermined second cumulative amount of re leased heat should be at least 10 percentage units greater than the predeter mined first cumulative amount of released heat. The predetermined second cumulative amount of released heat could optionally be at least 20 percentage units greater than the predetermined first cumulative amount of released heat. As an example, if the predetermined first cumulative amount of released heat is 40 percent of the total heat release during one engine cycle, the predeter mined second cumulative amount of released heat should be at least 50 per cent of the total heat release during one engine cycle.

Alternatively, the second reference point R2 can be a certain crank angle, as shown in figure 8. The second variable is thus a heat release value HR2 repre senting average cumulative heat release in a plurality of cylinders 2 up to a second predetermined crank angle R2.

The second variable can represent the development of cumulative heat re lease during several engine cycles.

The value of the second variable CA2, HR2 is compared to a predetermined second target value or target range 203. The second target value or target range represents desired average development of the cumulative heat release in the plurality of cylinders 2 up to the second reference point R2.

Based on the comparison, a second engine parameter is adjusted 204. The second engine parameter affects combustion in the plurality of cylinders 2. The second engine parameter can thus affect the combustion in all cylinders 2 of the engine or for example in the cylinders of one bank in case of a V-engine. The second engine parameter can be, for instance, the pressure of the intake air. The pressure of the intake air can be adjusted for example by controlling the operation of one or more turbochargers 5.

The engine 1 is operated with a closed-loop control. The cylinder pressures are thus continuously monitored, and both the first engine parameter and the second engine parameter are adjusted as long as the first variable and the second variable are outside the desired ranges.

There are various alternatives for what can be used as the first engine parame ter. The first engine parameter can be, for instance, the timing of pilot fuel in jection, the injection amount of pilot fuel, intake valve timing or spark timing. In case the amount of pilot fuel injection is adjusted, the amount can be controlled by controlling the duration of pilot fuel injection and/or by controlling the injec tion pressure of pilot fuel. The pilot fuel injection is adjusted individually in each cylinder 2 of the engine 1 .

It is also possible that the first engine parameter to be adjusted is different in different situations. The flowchart of figure 4 shows an example of using two different parameters as the first engine parameter. The flowchart shows only part of the method according to the invention, i.e. the adjustment of the first engine parameter. The second engine parameter can be adjusted in the way described above. In the embodiment of figure 4, the value of the first variable is determined 102 and compared to a target value or target range 103 in the same way as in the embodiment of figure 3. In case adjustment of the first var iable is needed, the amount of pilot fuel injected during one or more earlier en gine cycles is determined 105. The pilot fuel amount to be determined can be an average injection amount of three or more earlier engine cycles. The actual (average) injection amount is compared to a predetermined upper limit 106. In case the (average) injection amount has not reached the predetermined upper limit, the injection amount is adjusted as the first engine parameter 104a. If the injection amount has already reached the upper limit, another parameter is ad justed as the first engine parameter 104b. This prevents excessive emissions due to excessive pilot fuel injection amounts. The other parameter can be, for instance, the injection timing of the pilot fuel or cylinder compression tempera ture. Alternatively, both the injection timing and the compression temperature can be adjusted. The expression“compression temperature” refers to the cyl inder temperature during the compression stroke. The temperature varies dur ing the compression stroke, and therefore the compression temperature can be defined as an average temperature over the compression stroke or as a momentary temperature at a predetermined crank angle during the compres sion stroke. The compression temperature can be adjusted for example by controlling exhaust gas recirculation (EGR) and/or by controlling the closing timing of intake valves. The engine can be provided with an internal EGR sys tem, where exhaust gas flow from the exhaust duct 9 back into the combustion chamber 10 can be individually controlled in each cylinder 2 of the engine. The steps of the method can be carried out also in a different order. For instance, the actual pilot fuel amount can be determined simultaneously with the deter mination of the value of the first variable.

As an example, comparison of the value of the first variable and the first target value or target range can show that the heat release in the early part of com bustion is too slow. A certain heat release percentage is thus reached at a later crank angle than desired, or at a certain crank angle the cumulative heat re lease percentage is too low. A correcting action can be increasing of the amount of pilot fuel injection. The amount of the pilot fuel injection can be in creased in steps and monitoring of the cylinder pressure is continued. If the in jection amount reaches an upper limit and the heat release rate is still too low, the injection timing is advanced until the value of the first variable is within ac ceptable range.

Figure 5 shows part of a method according to another embodiment of the in vention. In the embodiment of figure 3, the value of a third variable is deter mined 302. The first variable can be determined and the first engine parameter adjusted in the same way as in the embodiment of figure 3 or figure 4. Also the second variable can be determined and the second engine parameter adjusted in the same way as in the embodiment of figure 3. The value of the third varia ble is determined on the basis of the value of the first variable. In this case, the value of the first variable is based on a single engine cycle. The third variable is the average of the values of the first variables of at least three earlier engine cycles. The value of the third variable is compared to a predetermined third target value or target range 303. The third target value or target range repre sents desired average development of the cumulative heat release in a single cylinder 2 of the engine 1 up to the first reference point Ri. On the basis of the comparison, a third engine parameter is adjusted 304. The third engine pa rameter is different from the first engine parameter. Preferably the third engine parameter is also different from the second engine parameter. If the value of the third variable is not within the third target range or the deviation between the value of the third variable and the target value exceeds a predetermined limit, the third engine parameter is changed. The third engine parameter can be adjusted for example by controlling the amount of recirculated exhaust gas and/or by controlling the timing of pilot fuel injection.

Figure 6 shows part of a method according to still another embodiment of the invention. Compared to any of the embodiments shown in figures 3 to 5, the method of figure 6 comprises an additional step of determining the average value of the first variables CAi, HRi of a plurality of cylinders 2 402. The value of the first variable of a single cylinder 2 is then compared to the average value 403. In case the value of the first variable CAi, HRi of the single cylinder 2 is not within predetermined limits from the average value, a fourth engine param eter is adjusted 404. The fourth engine parameter is different than the first en gine parameter. The fourth engine parameter is preferably also different from the second engine parameter and/or the third engine parameter. Also the fourth engine parameter is a cylinder specific parameter, which affects com bustion in the single cylinder. The fourth parameter can be, for instance, intake valve timing. The fourth parameter is adjusted individually for each cylinder 2.

Figure 9 shows part of a method according to still another embodiment of the invention. Also this embodiment can be combined with the other embodiments. In the embodiment of figure 9, the difference between the values of the first variable is determined. The value is compared to a target value or target range. If the value is within the target range or within predetermined limits from a target value, monitoring of the cylinder pressures is continued without adjust ing engine parameters. In case the value is outside the target range, a fifth en gine parameter is adjusted. The fifth engine parameter can be, for instance, charge air pressure, charge air temperature or intake valve timing. It will be appreciated by a person skilled in the art that the invention is not lim ited to the embodiments described above, but may vary within the scope of the appended claims. For instance, the features of the different embodiments can be combined.