Technical field of the invention

The present invention relates to a method of detecting abnormal closing timing or malfunction of one or more intake valves of a cylinder of a piston engine in accordance with claim 1 . The invention also concerns an arrangement for de- tecting abnormal closing timing or malfunction of one or more intake valves of a cylinder of a piston engine as defined in the other independent claim.

Background of the invention

Each cylinder of a four-stroke piston engine is typically provided with two in- take valves and two exhaust valves. A conventional way of operating the in- take and exhaust valves of a piston engine is the use of one or more cam- shafts. The intake and exhaust valves are mechanically connected to the cams of the camshafts. Mechanical, cam-operated actuating mechanisms are relia- ble. However, the opening and closing crank angles of the intake and exhaust valves are usually fixed. Variable valve timing has many benefits for example in terms of engine performance, emissions and fuel consumption. Therefore, valve actuating systems allowing variable valve timing are becoming more and more common. Variable valve timing can be implemented in many different ways. Typically hydraulic and/or electrical components are utilized, but also purely mechanical systems exist.

A drawback of valve actuating systems that allow variable valve timing is that compared to simple mechanical systems with fixed valve timing, the increased complexity increases the risk of malfunctions and failures. The cylinders with abnormal valve timing can be difficult to identify.

Summary of the invention

An object of the present invention is to provide a method of detecting abnormal closing timing or malfunction of one or more intake valves of a cylinder of a pis- ton engine. The characterizing features of the method according to the inven- tion are given in claim 1 . Another object of the invention is to provide an ar- rangement for detecting abnormal closing timing or malfunction of one or more intake valves of a cylinder of a piston engine. The characterizing features of the arrangement are given in the other independent claim.

The method according to the invention comprises the steps of measuring cyl- inder pressure in the cylinder at least at two different crank angles falling within a crank angle range between a first crank angle and a second crank angle, on the basis of measured cylinder pressures, determining average cylinder pres- sure for the crank angle range between the first crank angle and the second crank angle, determining estimated average cylinder pressure in the cylinder in the crank angle range between the first crank angle and the second crank an- gle, and comparing the estimated average cylinder pressure to the average cylinder pressure determined on the basis of the measured cylinder pressures. The arrangement according to the invention comprises means for determining the crank angle of the engine, pressure measurement means for measuring pressure in the cylinder, and data processing means, wherein the data pro- cessing means are configured to receive pressure measurement data from the pressure measurement means, on the basis of the pressure measurement da- ta, determine average cylinder pressure for a crank angle range between a first crank angle and a second crank angle, determine estimated average cylinder pressure in the cylinder in the crank angle range between the first crank angle and the second crank angle, and compare the estimated average cylinder pressure to the average cylinder pressure determined on the basis of the pres- sure measurement data.

With the method and arrangement according to the invention, abnormal intake valve timing or failures of a valve actuating arrangement can be detected.

According to an embodiment of the invention, the average cylinder pressure is estimated based on the intended closing crank angle of the intake valves, pressure of the intake air, predetermined polytropic exponent, and change of cylinder volume over the crank angle range between the first crank angle and the second crank angle.

According to an embodiment of the invention, the estimated average cylinder pressure is retrieved from a look-up table, which comprises estimated average cylinder pressures for a plurality of combinations of intake air pressures and in- tended closing crank angles of the intake valves. This simplifies the compari- son between the estimated average cylinder pressure and the measured aver- age cylinder pressure, since the estimated pressure does not need to be calcu- lated in real-time.

According to an embodiment of the invention, the cylinder pressure is meas- ured at least at the second crank angle.

According to an embodiment of the invention, the second crank angle is after the intended closing crank angle of the intake valves. If the second crank angle is before the intended closing crank angle of the intake valves, only too early closing of the intake valves or a failure preventing opening of the intake valves can be detected. By measuring the cylinder pressure after the intended closing crank angle, also too late closing of the intake valves can be detected.

According to an embodiment of the invention, the estimated average cylinder pressure is determined on the basis of a polynomial that approximates ex- pected average cylinder pressure as a function of intake air pressure and in- tended closing crank angle of the intake valves. This simplifies calculation of the average pressure.

According to an embodiment of the invention, an alarm is triggered in case the difference between the estimated average cylinder pressure and the average cylinder pressure determined on the basis of the measured cylinder pressures exceeds a predetermined threshold.

A piston engine according to the invention comprises an arrangement defined above.

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 the engine of fig. 1 , Fig. 3 shows as a flowchart the method according to the invention, and

Fig. 4 shows cylinder pressure during part of intake and compression strokes as a function of crank angle.

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 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 turbo- charger 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 connected 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 receiv- er or intake or inlet manifold.

Figure 2 shows schematically one cylinder 2 of the engine of figure 1 . 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 connect- ed 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 cylin- der 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 em- bodiment 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 be- tween 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 are configured to allow varia- ble 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 be an electrical, hydraulic or mechanical actuator. The intake valve actuating means 12 could also be any combination of electrical, hydraulic 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 hy- draulic system. Alternatively, both the opening and closing timing could be de- termined by means of an electrical actuator, such as a solenoid. Alternatively, 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 config- ured to transmit a control signal to the intake valve actuating means 12 for de- termining 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 3 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.

A benefit of a cam-operated completely mechanical valve actuating system with fixed valve timing is its reliability. Variable valve timing usually increases the risk of malfunctions and failures. Therefore, there is a need for a method that allows detection of abnormal intake valve timing and failures of valve ac- tuating means. Abnormal intake valve timing can be detected by estimating the closing crank angle of the intake valves 3 and by comparing the estimated in- take valve timing to the intended intake valve timing. The expression "closing crank angle" refers to the crank angle at which the intake valves 3 of the cylin- der 2 in question are closed. "Intended valve timing" refers to a set closing crank angle. In practice, the closing of the intake valves 3 is not instantaneous, but the intake valves are closed gradually over a certain crank angle range, which depends on the intake valve actuating means 12. The closing crank an- gle is the crank angle at which the intake valves 3 are completely closed.

In a four-stroke engine, the crank angle of the engine 1 can range from -360 degrees to 360 degrees, where angles -360, 0 and 360 are top dead center (TDC) positions of the piston 4 and angles -180 and 180 are bottom dead cen- ter (BDC) positions of the piston 4. Alternatively, the crank angles can be pre- sented as ranging from 0 degrees to 720 degrees, where angles 0, 360 and 720 are TDC positions and angles 180 and 540 BDC positions. A common way is to denote the start of the intake stroke as crank angle -360 degrees. The in- take valves 3 are often closed near bottom dead center, i.e. around crank an- gle of -180 degrees. Alternatively, the intake valves 3 can be closed early dur- ing the intake stroke. This is called as Miller timing. In Miller timing, the intake valves can be closed for example 20 to 80 degrees before BDC, i.e. in the crank angle range of -260 - -200 degrees. Depending on whether a conven- tional or Miller timing is used, the intended closing crank angle of the intake valves 3 is thus typically in the range of -260 - -180 degrees. However, also earlier or later closing timings can be used.

The method according to the invention is shown as a flowchart in figure 3. In a first step of the method according to the invention, cylinder pressure in the cyl- inder 2 is measured 101 . The cylinder pressure is measured at least at two dif- ferent crank angles, which fall within a crank angle range between a first crank angle θι and a second crank angle θ2. Both the first crank angle θ1 and the second crank angle θ2 belong either to the intake stroke or the compression stroke. Both endpoints of the range should be before ignition in the cylinder 2. At least the second crank angle θ2 is preferably after the intended closing tim- ing Give of the intake valves 3. The first crank angle θ1 can be before the in- tended closing timing Give of the intake valves 3. According to an embodiment of the invention, the cylinder pressure is measured at least at the second crank angle θ2. According to an embodiment of the invention, the cylinder pressure is measured at least at the first crank angle θ1 . A suitable crank angle range for the cylinder pressure measurements can be determined experimentally. The crank angle range is chosen so that signal-to-noise ratio can be maximized.

In a second step of the method, average cylinder pressure PAMeas for the crank angle range between the first crank angle θ1 and the second crank angle θ2 is determined on the basis of measured cylinder pressures 102. The greater the number of the cylinder pressure measurements in the measurement step 101 is, the more accurately the average cylinder pressure PAMeas can be deter- mined in the second step 102 of the method.

In a third step of the method, average cylinder pressure P _AESt in the cylinder 2 in the crank angle range between the first crank angle θ1 and the second crank angle θ2 is estimated 103. The estimated average cylinder pressure P _AESt can be determined in real-time. Alternatively, the estimated average cylinder pres- sure P _AESt can be predetermined for a plurality of combinations of intake air pressures Prec and intended closing crank angles Give of the intake valves 3. The predetermined values can be stored as a look-up table.

In a fourth step of the method, the estimated average cylinder pressure P _AESt is compared to the average cylinder pressure PAMeas determined on the basis of the measured cylinder pressures 104.

If a certain deviation between the estimated cylinder pressure P _AESt and the measured cylinder pressure PAMeas is detected, it is an indication of abnormal intake valve closing timing.

The estimation of the average cylinder pressure P _AESt can be based on the in- tended closing crank angle Give of the intake valves 3, pressure Prec of the in- take air, predetermined polytropic exponent γ, and change of cylinder volume over the crank angle range between the first crank angle θ1 and the second crank angle θ2.

The average cylinder pressure P _AESt can be estimated in a simple way by mak- ing certain assumptions.

First, it can be assumed that during the intake stroke when the intake valves 3 are open, the cylinder pressure Pivo is constant and equals the pressure of the intake air Prec. The following equation thus applies: The intake air pressure P _rec refers here to the pressure in the intake duct 8 downstream from the compressors 5b of the turbochargers 5 of the engine 1 , i.e. to the pressure in the intake receiver. In practice, the assumption does not fully correspond to the reality. Usually the pressure in the cylinder 2 is lower than the pressure of the intake air. In addition, the pressure is not necessarily constant, but may decrease as the piston 4 moves towards bottom dead cen- ter. However, despite that the estimation may be accurate enough for intake valve diagnostics.

Second, it can be assumed that compression of gas in the cylinder 2 is a poly- tropic process. In a polytropic process, ΡV ^y = constant. In case of an Otto combustion process, the gas is a mixture of intake air and fuel. In case of a Diesel combustion process, the gas in the cylinder is intake air.

A third assumption is that the intake valves 3 are closed immediately and the cylinder pressure is not affected by any gradual closing of the intake valves 3. With these three assumptions, the cylinder pressure Ρ _θ at any crank angle θ during the intake stroke and the compression stroke before start of combustion can be calculated taking the crank angle Give at which the intake valves 3 are closed as a reference point. From the first assumption it follows that the cylin- der pressure at the closing crank angle Give of the intake valves 3 is the same as the intake air pressure Prec. The cylinder volume corresponding to the crank angle at which the intake valves 3 are closed can be marked with Vive. Poly- tropic exponent γ for the compression process depends on the properties of the gas mixture in the cylinder 2. The polytropic exponent γ of the air-fuel mix- ture can be known or it can be determined experimentally. The pressure Ρ _θ during compression can be expressed by the following equation:

Based on the assumptions presented above, the cylinder pressure Ρ _θ while the intake valves 3 are open and throughout the intake stroke and the compres- sion stroke can be determined as follows:

The average pressure P _AESt between the first crank angle θ1 and the second crank angle θ2 is thus

Assuming that the first crank angle θ1 is before closing of the intake valves 3 and the second crank angle θ2 is after closing of the intake valves 3, the aver- age pressure can be expressed as

By inserting equation (3) into equation (5) the following equation results:

Since Prec does not depend on the crank angle, the equation can be simplified to equation (7) and further to equation (8).

Equation (8) can be expressed in a numerical form as follows:

Average cylinder pressure for the same crank angle range based on the cylin- der pressure measurement can be calculated by the following equation:

Figure 4 shows an example of the development of the cylinder pressure during the intake and compression strokes. The pressure curve is based on equation (3). In the example of figure 4, Miller timing is used. The intake valves 3 are thus closed during the intake stroke before the piston 4 reaches bottom dead center. In the example of figure 4, the intake valves 3 are fully closed at crank angle of -230 degrees, i.e. 50 degrees before bottom dead center. The intake air pressure in the intake receiver is 2 bar. Until the closing of the intake valves 3, the cylinder pressure equals the intake air pressure. After closing of the in- take valves 3, the pressure in the cylinder 2 starts to decrease. When the compression stroke begins at crank angle -180 degrees, the cylinder pressure starts to increase in accordance with equation ΡV ^y = constant.

If the intake valves 3 are closed later than the expected valve timing, the pres- sure in the cylinder 2 at the end of the intake stroke and during the compres- sion stroke is higher than expected. The average cylinder pressure PAMeas de- termined on the basis of the cylinder pressure measurements is thus higher than the estimated average cylinder pressure PAest. A certain difference be- tween the measured pressure PAMeas and the estimated pressure PAest can thus be an indication of abnormal intake valve timing. Because of the inaccuracies of the pressure measurements and the simplifying assumptions made for esti- mating the cylinder pressure, the estimated cylinder pressure PAest never per- fectly matches with the measured pressure PAMeas. Therefore, a certain thresh- old for the difference between PAest and PAMeas can be determined. If the differ- ence between PAest and PAMeas exceeds the predetermined threshold, an alarm can be triggered. The threshold does not need to be fixed, but it can depend on different factors, such as the intended intake valve closing timing, intake air pressure, engine load, etc.

An arrangement for detecting abnormal valve timing is described next. 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 cylinder 2. The engine 1 is provided with data processing means, such as a control unit 14, which receives measurement data from the cylinder pressure sensor 15. The engine 1 further comprises a crank angle sensor 18 or other means for de- termining the angular position of the crankshaft 16. In the embodiment of figure 2, the crank angle sensor 18 monitors the angular position of the flywheel 17. On the basis of the angular position of the flywheel 17, the position of the pis- ton 4 in each cylinder 2 can be determined. In addition to the pressure meas- urement data, the control unit 14 also receives measurement data from the crank angle sensor 18. The cylinder pressure can thus be determined in re- spect of crank angle.

The valve actuating means 12, 13 are connected to the control unit 14. The control unit 14 thus controls the opening and closing timing of the intake valves 3 and exhaust valves 1 1 . The engine 1 is further provided with intake air pressure sensor 1 9. The intake air pressure sensor 19 monitors the pressure of the intake air. The intake air pressure sensor 1 9 is connected to the control unit 14, which receives meas- urement data from the intake air pressure sensor 1 9. The crank angle sensor 1 8, the cylinder pressure sensor 1 5 and the control unit 14 are used for determining the average cylinder pressure PAMeas in the crank angle range θ1-θ2. In the control unit 14, the measured average cylinder pressure PAMeas is compared to the estimated average cylinder pressure P _AESt . The estimated average cylinder pressure P _AESt can be calculated for example using equation (9). The calculation can take place in real-time. Alternatively, estimated average cylinder pressure P _AESt can be predetermined for a plurality of combinations of intake air pressures P _rec and intended closing crank angles Give of the intake valves 3. The predetermined values can be stored as a look, up table for example in a memory of the control unit 14. Instead of using equation (9), the estimated average cylinder pressure P _AESt can be determined on the basis of a polynomial that approximates expected average cylinder pressure as a function of intake air pressure P _rec and intend- ed closing crank angle Give of the intake valves 3. This simplifies calculation of the average cylinder pressure. 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.