FIELD OF THE INVENTION

The present invention relates to a method for determining the compression pressure in a combustion chamber of at least one cylinder in a combustion engine, and a device for detecting said compression pressure. The invention also relates to a computer program and a computer program product comprising computer program code for the implementation of a method according to the invention, and an electronic control device and a motor vehicle. BACKGROUND

The compression pressure in a combustion chamber of a cylinder means the pressure in the combustion chamber, when the cylinder's piston has compressed the gas. The compression pressure for a cylinder in a combustion engine is normally unambiguously determined at so-called "dragging" of the engine, that is to say when the engine is dragged around by external means, without any fuel supply. The compression pressure depends partly on the charge air pressure, that is to say the pressure, with which the cylinder is "charged" via an inlet pipe during a suction stroke in the cylinder, partly by when the valves open and close and partly by the cylinder's compression ratio, that is to say the ratio between the largest and the smallest possible volume in the cylinder's combustion chamber. The largest volume is achieved at the piston's bottom turning point, also called the bottom dead centre. The compression ratio for a cylinder in a combustion engine is thus determined by the design of the cylinder. Normally, the compression ratio is within an interval of between 10-30. Determining the compression pressure during the operation of a combustion engine traditionally requires that a pressure sensor is fitted in the combustion chamber of the cylinder, in which the gas pressure is to be determined. The compression pressure is then measured with the help of the pressure sensor. US5168854 shows the use of a pressure sensor to measure the gas pressure in a combustion chamber of a cylinder in a combustion engine.

However, fitting a pressure sensor in the combustion chamber is expensive, and requires that a hole be bored in the cylinder wall. Pressure sensors fitted inside the combustion chamber may also cause disruptions in the operation of the combustion engine. Therefore, it is not common for the compression pressure to be measured or determined during the normal operation of a combustion engine.

SUMMARY OF THE INVENTION

The objective of the present invention is to provide a manner, which is improved in at least one aspect, of determining the compression pressure of at least one cylinder in the combustion chamber in a combustion engine.

This objective is achieved with the method and the device according to the enclosed claims. In a first aspect of the invention a method is provided, which comprises: - determining a first gas pressure in the combustion chamber when a piston in the cylinder is at its bottom dead centre;

- during a compression stroke in the cylinder, detecting a pressure change in the combustion chamber in relation to the first gas pressure, wherein the detection is carried out by detecting movements generated by said pressure change in a cylinder head, belonging to said cylinder or in parts adjacent thereto in the engine; and

- based upon the first gas pressure and the detected pressure change, calculating the compression pressure.

With the help of the innovative method, it is possible to determine a compression pressure in the cylinder following production, and to verify that such compression pressure is maintained and that the compression pressure in the cylinder is such that for the relevant operating condition it corresponds to the setpoint value, which is provided among other by the compression ratio determined at production. According to the invention, this is achieved by determining the gas pressure in the combustion chamber when the piston is at its bottom dead centre, that is to say when the combustion chamber's volume is at its maximum, and by detecting the relative pressure change in the combustion chamber during a compression stroke of the cylinder, that is to say when the piston moves from its bottom dead centre to its top dead centre, while the cylinder's valves are closed and/or kept closed. By monitoring the compression pressure, potential errors in the cylinder may be detected, such as leakages from valves, deformations, leaky gaskets etc. The method according to the invention allows this to be carried out at normal operation of the combustion engine, and potential errors may therefore be discovered and corrective measures may be taken at an early stage. The innovative method also has the advantage that it may be carried out without any holes for pressure sensors having to be bored in the combustion chamber. In addition to a cost saving thus achieved, thanks to the invention disruptions that may be caused by pressure sensors fitted in the cylinder chamber are also avoided.

The movements which may be detected with the innovative method, and which are generated by pressure changes in a combustion chamber, may for example be vibrations, noise, that is to say gas movements, and various types of shape changes, such as protrusions, in said cylinder head or in parts adjacent thereto in the engine.

According to one embodiment of the invention, the step of determining the first gas pressure comprises measuring a charge air pressure, also called a charge pressure. This is usually carried out during a suction stroke with the help of a pressure sensor in an inlet pipe to the cylinder, and may thus be carried out without a pressure sensor having to be fitted in the combustion chamber itself. The gas pressure in the cylinder's combustion chamber, just before the compression stroke following after the suction stroke, corresponds to the charge air pressure. When the engine drags, the charge air pressure is normally the same as, or somewhat lower than the atmospheric pressure, and the first gas pressure may in these cases be set at the atmospheric pressure, or at a somewhat lower measured and/or modelled pressure. According to one embodiment of the invention, movements in the form of vibrations are detected, it has been shown that the pressure changes that arise during the compression stroke in the combustion chamber in turn give rise to vibrations in the cylinder head and in adjacent parts in the engine. By reading these vibrations, a measuring value of the pressure change in the combustion chamber is obtained in a reliable manner.

According to one embodiment of the invention, movements in the form of strains in screws of the engine are detected, such as screws in main bearings or in connecting rod bearings, and in cylinder head bolts. The appearance of such movements depends on pressure changes in the combustion chamber, and may for example be detected by applying strain-sensors on such screws.

According to one embodiment of the invention, movements in the form of strains in an engine block part or in a cylinder head of the engine are detected, and according to another embodiment of the invention, movements in the form of strains in the injector- or glow plug fixings are detected. These embodiments provide additional possibilities of determining the pressure change in the combustion chamber during the compression stroke. According to one embodiment of the invention, the compression is determined by way of calculating the compression, based on a pressure detected at the piston's bottom dead centre in the combustion chamber, and on a detected pressure at the piston's top dead centre in the combustion chamber. The piston's bottom and top dead centres, respectively, may for example be determined by way of reading a signal from a crank angle sensor, and then determining the respective dead centres based upon this, or by determining the piston's respective dead centres solely based upon the measured gas pressure in the combustion chamber.

According to one embodiment of the invention, the pressure change used to calculate the compression pressure is the total pressure change when the piston moves from its bottom dead centre to its top dead centre. This provides a reliable value for the compression pressure in the combustion chamber.

According to one embodiment of the invention, the detection of the pressure change occurs when the combustion engine is dragged around without any fuel supply, that is to say during so- called dragging of the combustion engine. There is no combustion in the cylinder at this stage, which means that the compression pressure may be determined relatively easily based on the measured pressure changes. Also, it may be determined with great certainty whether the determined compression pressure differs from the normal, since the normal compression pressure for these operating conditions is unambiguously determined. Instead it is of course possible to determine the compression pressure during a working cycle in the cylinder, when fuel is supplied and combustion occurs, but the calculation of the compression pressure then becomes more complex. It is also possible to use a combination of determining the compression pressure at dragging, at a low load and at a high load, in order to be able to determine one reason for the compression pressure's divergence from the expected pressure based upon a comparison of the results,.

According to one embodiment of the invention, the detection of the pressure change occurs separately for the respective cylinder in the combustion engine. It is then possible to determine directly in which cylinder there are potential problems. This is especially applicable in multi-cylinder combustion engines, where each cylinder has a separate cylinder head. For multi-cylinder combustion engines, where several cylinders share one common cylinder head, the detection may occur either separately or jointly - but with a joint detection, information about the compression pressure of individual cylinders is usually lost. According to one embodiment of the invention, the detection of the pressure change occurs by way of detecting movements with a frequency of < 250 Hz, 0,5 Hz-250 Hz or 0,5 Hz-200 Hz. The movements detected are thus movements with a relatively low frequency, and the basic frequency of the variation of the gas pressure in the combustion chamber, which is the same as the combustion engine's engine speed, lies within these intervals. Typically, this may be 60 revolutions per minute (1 Hz) for a marine diesel engine and as high as 12 000 revolutions per minute (approximately 200 Hz) for an Otto engine in a motorbike. By detecting movements within these frequency areas, information about the main variation of the gas pressure during the cylinder's compression stroke may be extracted.

According to one embodiment of the invention, the method comprises the steps: - comparing the calculated compression pressure with an alarm criterion, determined for the relevant operating condition of the engine; and

- provided that the alarm criterion is met, generating an error code.

In this manner, the compression pressure in the combustion chamber may be monitored during normal operation of the combustion engine, and as soon as this differs from the normal, an error code may be set and an error signal may be generated, indicating that there is a cylinder error. Errors resulting in diverging compression pressure may thus be discovered and corrective measures may be taken at an early stage. The error signal may, for example, be in the form of an electronic message, a sound and/or a light signal.

According to a second aspect of the invention, the above mentioned objective is achieved with a device adapted to detect the compression pressure in a combustion chamber of at least one cylinder in a combustion engine, characterised in that it comprises at least one device to determine a first gas pressure in the combustion chamber when a piston in the cylinder is at its bottom dead centre, at least one sensor element adapted to be arranged separately from the combustion chamber on a part of a cylinder head belonging to the cylinder or on parts adjacent thereto in the engine, and adapted to detect movements in said cylinder head or in the parts adjacent thereto in the engine, which movements have been generated by a pressure change in the combustion chamber, and it also comprises a device adapted to calculate the compression pressure, based on the detected movements and the first gas pressure. The function of such a device and embodiments thereof, defined in non-independent claims, and the possibilities which they offer are explained in the discussion above of the innovative method.

The invention is not limited to any specific type of combustion engine, but encompasses Otto engines as well as compression ignited engines, nor to any specific fuel, non-exhaustive examples of which may comprise fuel in the form of petrol, ethanol, diesel and gas.

Likewise, the invention comprises combustion engines intended for all types of use, such as in industrial applications, in crushing machines and various types of motor vehicles, wheeled motor vehicles as well as trucks and buses, and boats and crawlers or similar vehicles.

The invention also relates to a computer program, a computer program product, an electronic control device, and a motor vehicle arranged to function as set out above.

BRIEF DESCRIPTION OF THE DRAWINGS

Below are descriptions of example embodiments of the invention, with reference to the enclosed drawings, in which:

Fig. 1 a is a schematic view illustrating a part of a combustion engine, in which a device according to one embodiment of the invention is arranged, Fig 1 b shows a possible location of a sensor element, is a diagram, which schematically shows the gas pressure in and volume of the combustion chamber of a cylinder in a combustion engine, as a function of time during part of a working cycle of the cylinder with and without combustion, respectively, is a flow chart showing a method according to one embodiment of the invention, and is a fundamental diagram of an electronic control device for the implementation of a method according to the invention. DETAILED DESCRIPTION OF EMBODIMENTS ACCORDING TO THE INVENTION

Fig. 1 a very schematically illustrates a combustion engine 1 , in which a device adapted for determining the compression pressure according to one embodiment of the invention is arranged. The combustion engine is arranged in an implied motor vehicle 2, for example a truck. The engine 1 is equipped with a device 3, indicated with a dashed line, adapted to detect operating conditions in the engine, and such device has a schematically drawn device 4, which is adapted to detect the pressure in the combustion chambers 5 of the combustion engine's cylinders 6, of which there are six in this case, but of which there may be any number. Each cylinder 6 comprises a piston 14, arranged to operate a crankshaft 12, an inlet valve 10, controlling the gas in-flow from an inlet channel 1 5 to the cylinder's combustion chamber 5, and an exhaust valve 1 1 , controlling the outflow of exhausts via an exhaust channel 16. In the inlet channel 15, a pressure sensor 17 is arranged.

The device 4 has, in order to be able to detect said pressures in the combustion chambers 5, one sensor element 7 per cylinder 6, and this is arranged separately from the associated combustion chamber 5 on the respective cylinder's cylinder head 8. The sensor elements 7 in this case are piezo resistive sensors, adapted to detect, in the form of vibrations, movements propagated in the cylinder head 8, generated by pressure changes in the relevant combustion chambers 5. The sensor element 7 may also be a piezo electric element or an optical sensor. The device 3 also comprises a unit 9, which may consist of the vehicle's electronic control device, adapted to receive information about the detected movements from the sensor elements 7, and to compare such information, or information calculated based on such sensor information, with values stored in relation to the desired operating conditions in the engine.

Fig. 1 b shows another possible placement of the sensor element 7. The sensor element is here placed on a section adjacent to the cylinder head. In this example, the sensor element is placed on the engine, specifically on the engine block. The sensor element may here be placed on the engine, in an area adjacent to the outlet of the exhaust channel from a cylinder. For example it may be placed on a surface on the engine block next to the outlet, on the engine, of the exhaust channel from a cylinder e. The surface where the sensor is placed may be substantially vertical. The sensor may be arranged to detect movements, which are perpendicular to the movements of the piston. The sensor may also be arranged to detect movements, which are perpendicular both in relation to the piston's direction of movement and in relation to the engine's longitudinal direction. In one embodiment, the sensor is located on the engine's long side. The sensor may be arranged to detect movements in a direction, which is perpendicular in relation to the surface on which it is placed.

In another embodiment (not displayed) the sensor element 7 may be placed in a corresponding manner as when placed on the engine at the outlet of the exhaust channel from a cylinder, but instead placed in a corresponding location on the engine, at the suction channel's inlet to a cylinder.

The signal detected by the sensor element 7 may be treated in various ways. For example, the following signal treatment steps may be carried out. First the sensor's electrical signal is entered into a control device/signal treatment device. The signal is filtered with a bandpass filter in order to remove superfluous information, which does not belong to the frequency range around which information is required. The signal is evened out by way of filtering, averaging or by being replaced with one or several continuous function(s) with good likeness. Subsequently, the signal is scaled, e.g. with the help of the correlation between pressure and volume at compression. Subsequently, (a) suitable part(s) of the signal is/are transformed to the pressure domain. Supplemental modelling closes gaps in the signal's reliability, in order to form a pressure curve. The thus formed pressure curve is used to calculate different values at engine control. In some embodiments one or several of the steps above may be omitted.

As schematically shown in Fig. 2, for a cylinder 6 which is dragged, that is to say dragged around without any fuel supply and without any combustion occurring, the volume V of and the gas pressure P in the respective combustion chamber 5 varies as a function of time t, during a part of a working cycle of the relevant cylinder 6. The piston 14 here moves between a bottom dead centre BDC and a top dead centre TDC. The gas pressure, in the absence of combustion in the cylinder, is displayed as a solid line. At combustion in the cylinder 6, the gas pressure in the combustion chamber 5 is at a higher level than in the absence of combustion. The gas pressure during an expansion stroke with combustion is displayed schematically as a dashed line.

During a method according to one embodiment of the invention, illustrated with a flow chart in Fig. 3, in a step S1 a first gas pressure P1 is determined in the combustion chamber, when the piston 14 is at its bottom dead centre. This gas pressure is suitably determined by way of measuring the charge air pressure with the help of the pressure sensor 17 in the inlet channel 15, which may occur continuously during a suction stroke in the cylinder 6. The measured charge air pressure corresponds to the first gas pressure P1 . During the subsequent compression stroke, the gas is compressed in the combustion chamber 5, which results in a pressure increase in the combustion chamber 5. The pressure increase generates movements in the cylinder head 8 belonging to the cylinder 6, wherein the sensor element 7 is fitted. The sensor element 7 registers these movements during one step S2. During a subsequent step S3, the control device 9 calculates the combustion chamber's 5 compression pressure P2, based on the movements detected in step S2, the gas pressure P1 determined at step S1 and the cylinder's known compression ratio. The compression pressure P2 is thus provided by P2=P1 +ΔΡ, where ΔΡ is the pressure increase detected with the help of the sensor element 7. During a step S4 following the step S3, the calculated compression pressure P2 may be compared with an alarm criterion, determined for the relevant operating condition in the engine. The alarm criterion may, for example, be set so that it is met if the compression pressure exceeds or falls short of a setpoint value. Provided that the alarm criterion is met, in step S5 an error code may be set and an error signal may be generated. If the alarm criterion is not met, no measure is taken. If desired, the method may be iterated for the following working cycle of the cylinder, alternatively the method may be carried out again in a later working cycle.

A computer program code for the implementation of a method according to the invention is suitably included in a computer program, loadable into the internal memory of a computer, such as the internal memory of an electronic control device of a combustion engine. Such a computer program is suitably provided via a computer program product, comprising a data storage medium readable by an electronic control device, which data storage medium has the computer program stored thereon. Said data storage medium is e.g. an optical data storage medium in the form of a CD-ROM, a DVD, etc., a magnetic data storage medium in the form of a hard disk drive, a diskette, a cassette, etc., or a Flash memory or a ROM, PROM, EPROM or EEPROM type memory.

Fig. 4 very schematically illustrates an electronic control device 9 comprising execution means 20, such as a central processor unit (CPU), for the execution of computer software. The execution means 20 communicates with a memory 21 , e.g. a RAM memory, via a data bus 22. The control device 9 also comprises a data storage medium 23, e.g. in the form of a Flash memory or a ROM, PROM, EPROM or EEPROM type memory. The execution means 20 communicates with the data storage means 23 via the data bus 22. A computer program comprising computer program code for the implementation of a method according to the invention is stored on the data storage medium 23.

The invention is obviously not limited in any way to the embodiments described above, but numerous possible modifications thereof should be obvious to a person skilled in the area, without such person departing from the spirit of the invention as defined by the appended claims. For example, the method may be carried out for a two stroke engine as well as for a four stroke engine as described above.