DE102007061226B4 - Method for determining the combustion chamber pressure profile of an internal combustion engine - Google Patents

Abstract

Method for determining the combustion chamber pressure profile (pi(t), p(a)) of at least one cylinder (Zi) of an internal combustion engine (1) from a variable which represents the crankshaft speed (n), in particular a signal from a crankshaft or camshaft rotation angle sensor (SN) , and a filtered characteristic variable (CharGi) which represents the combustion chamber pressure (pi), in particular a filtered signal from a combustion chamber pressure sensor (DS), with the filtered characteristic variable (CharGi) of the combustion chamber pressure curve (pi(t) , p (a)) at different speeds (nj), and the relationship between speed (nj) and displacement (Δφj) of the characteristic variable (CharGi,j) of the combustion chamber pressure curve relative to top dead center (TDC) of the cylinder (Z) is determined, characterized in that the correction of the filtered characteristic quantity (CharGi) takes into account the thermodynamic loss angle, w where a position of a pressure maximum is corrected.

Description

State of the art

The present invention relates to a method, a device and a computer program for carrying out the method, with the aid of which the combustion chamber pressure profile of at least one cylinder of an internal combustion engine can be determined.

The worldwide increasing demands on modern internal combustion engines, especially diesel engines, with regard to pollutant emissions also require the development of new combustion processes for internal engine emission reduction. What many of these newly developed combustion processes have in common is that, for system-related reasons, the combustion process in each cylinder can be different even during stationary operation. This in turn leads to different pollutant and noise emissions for each individual cylinder, which is undesirable.

A tried and tested means of compensating for the cylinder-specific differences within an internal combustion engine consists in detecting the combustion chamber pressure profile of at least one cylinder of the internal combustion engine and taking it into account accordingly when controlling the internal combustion engine. In order to be able to use such a combustion-chamber-pressure-controlled regulation to optimize combustion, for example by shifting the injection or correcting quantity or actuator errors, the combustion-chamber pressure must be measured using a combustion-chamber pressure sensor. These pressure sensors have an analog output signal that is filtered with a hardware low-pass filter to avoid aliasing. At the same time or afterwards, an analog/digital conversion is required, which can be time-synchronous or crankshaft-synchronous. In the case of a time-synchronous display of the pressure profile, the pressure p(t) is displayed as a function of the time t. In an angle-synchronous display of the pressure profile, the pressure p (α) is displayed as a function of the crank angle α. Both representations are equivalent and can be converted into one another. For the sake of clarity, only one representation is described below, although both representations are equivalent in the context of the claimed invention.

The anti-aliasing filter delays the output signal of the pressure sensor. The delay depends on the filter corner frequency and the tolerances of the components used in the anti-aliasing filter.

From the publication of the patent specification DE 37 21 162 C3 a method for determining the combustion chamber pressure profile of an internal combustion engine is known. Within the scope of this method, it is known, for example, to correct an angular position assigned to the maximum cylinder pressure via the crankshaft speed with the aid of a correction value. A time delay caused by a detection system is added here.

Furthermore, the disclosure documents DE 102 37 221 A1 and DE 10 2007 006 666 A1 known.

Disclosure of Invention

The invention is based on the object of optimally compensating for the unavoidable delay between the actual combustion chamber pressure and the signal determined by the combustion chamber pressure sensor and then filtered, and thereby providing an improved input signal for a control unit of the internal combustion engine.

According to the invention, this object is achieved in a method for determining the combustion chamber pressure profile p _i (t), p _i (a) of at least one cylinder Z _i of an internal combustion engine from a variable which represents the crankshaft speed n, in particular a signal from a crankshaft or camshaft rotation angle sensor SN, and a filtered characteristic variable CharGi, which represents the combustion chamber pressure p _i , in particular a filtered signal from a combustion chamber pressure sensor DS, solved in that the filtered characteristic variable CharG _i,j of the combustion chamber pressure curve p _i (t), p _i (a) is detected at different speeds n _j , and that the connection between speed n _j and displacement Δφ _j of the characteristic variable CharG _i,j of the combustion chamber pressure curve p _i (t), p _i (α) is determined relative to top dead center TDC of cylinder Z _i becomes. Within the scope of this compensation, the filtered characteristic quantity is advantageous when determining the propagation time shift The thermodynamic loss angle is also taken into account. As a result, a position of the maximum pressure is corrected. The thermodynamic loss angle describes the displacement of the pressure maximum before TDC caused by internal engine losses of a real internal combustion engine. More information can be found in the article "Definition and properties of the thermodynamic loss angle of piston machines" by Günther Hohenberg in "Automobilindustrie, 4, 1976", to which reference is hereby made.

The method according to the invention utilizes the fact that the propagation time shift of the sensor signal is constant over time within the filter and an A/D converter. At different speeds, this shift in transit time leads to different shifts in the characteristic variable relative to the top dead center of the relevant cylinder Z _i . Once it has been recorded, this linear relationship can be used in the control unit to compensate for the shift in transit time of the signal from the combustion chamber pressure sensor as a function of the speed of the internal combustion engine.

This makes it possible to more precisely control and regulate the control of the cylinder or cylinders of the internal combustion engine due to the better input signal and thereby improve the operating behavior of the internal combustion engine without additional hardware requirements. As a result, emission behavior, smooth running and fuel consumption improve in the desired way.

A further embodiment of the method according to the invention provides that the relationship between speed and displacement of the characteristic variable of the combustion chamber pressure curve relative to the top dead center of the cylinder or cylinders is determined by a regression analysis, in particular by a linear regression. The regression analysis is easy to implement and, like the entire method according to the invention, can be carried out at predetermined time intervals during operation of the internal combustion engine in order to compensate for any drift.

In a further advantageous addition to the invention, the relationship between the speed and the displacement of the characteristic variable relative to the top dead center of the cylinder is used to correct the signal of the filtered characteristic variable. As a result, it is possible to at least almost completely compensate for the shift in running time and thereby provide a significantly improved input signal for the combustion chamber pressure for the control unit.

Due to the comparatively simple functional relationship between the speed and the shift in the characteristic variable, the additional load on the control unit due to the compensation for the runtime shift is usually unproblematic and can be implemented without increased demands on the hardware of the control unit. It is therefore also possible to implement the method according to the invention in a software update for control devices that are already in series production and thereby to realize the advantages according to the invention even in control devices or internal combustion engines that are already in series production.

In a further advantageous embodiment of the method according to the invention, it is provided that after the correction of the filtered characteristic variable, an offset angle φ _offset between the top dead center and the angle φ _max at which the characteristic variable is present is determined, and that this offset angle is Correction of the position of a sensor wheel of the angle of rotation sensor is used.

This makes it possible to compensate for any angular errors of the sensor wheel relative to the crankshaft, which result, for example, from manufacturing tolerances or assembly inaccuracies. This offset angle is not subject to any changes over time, so that it is sufficient to carry out this compensation once when the internal combustion engine is started up and to store the offset angle φ _offset determined in the process in the control unit. Depending on the type of inaccuracy, the offset angle φ _offset can be different or the same for all cylinders of an internal combustion engine. Because of the improved accuracy, a cylinder-specific offset angle φ _Offset,i is preferably stored in the control unit.

The method according to the invention can advantageously be applied to all cylinders of an internal combustion engine. This applies in particular when all cylinders of an internal combustion engine are equipped with a combustion chamber pressure sensor, since the runtime shifts of the filters from cylinder to cylinder also show scatter due to the manufacturing tolerances of the components used in the anti-aliasing filters. The scattering of the anti-aliasing filter of an internal combustion engine can thus also be compensated for by the cylinder-specific application of the method according to the invention.

The maximum of the combustion chamber pressure is a particularly favorable characteristic variable of the combustion chamber pressure curve, because it is easy to detect and comparatively undemanding in terms of computing time. Of course, it is also possible as an alternative to determine other characteristic variables, such as the median of the integral over the pressure increase during the compression and power strokes.

In a further advantageous embodiment of the invention, an anti-aliasing filter, preferably a low-pass filter and particularly preferably a hardware low-pass filter, is used to filter the characteristic variable of the combustion chamber pressure sensor.

The advantages according to the invention can also be realized with a computer program and a control unit for an internal combustion engine.

Further advantages and advantageous configurations of the invention can be found in the following drawing, its description and the patent claims. All advantages disclosed in the drawing, its description and the patent claims can be essential to the invention both individually and in any combination with one another.

• 1 eine schematische Darstellung einer Brennkraftmaschine und
• 2 eine nach dem erfindungsgemäßen Verfahren gewonnene Ausgleichsgerade und
• 3 ein Ablaufdiagramm eines Ausführungsbeispiels des erfindungsgemäßen Verfahrens.

In the drawings show:

• 1 a schematic representation of an internal combustion engine and
• 2 a regression line obtained by the method according to the invention and
• 3 a flowchart of an embodiment of the method according to the invention.

1 1 shows a greatly simplified internal combustion engine 1 which is restricted to what is essential for the claimed invention. The internal combustion engine 1 has a cylinder bank with a total of four cylinders Z ₁ , Z ₂ , Z ₃ and Z ₄ . A combustion chamber pressure sensor DS ₁ , DS ₂ , DS ₃ and DS ₄ is assigned to each of the four cylinders Z ₁ to Z ₄ .

These combustion chamber pressure sensors DS _{i usually} work analogously and therefore record the pressure curves p _i (t), p _i (a) in the combustion chambers of the internal combustion engine 1 with high temporal resolution. The method according to the invention can also be used if there is only one pressure sensor DS ₁ per cylinder bank is available. In this case, the signal delay for all cylinders Z ₁ is corrected by the value determined using the lead cylinder Z ₁ .

On a crankshaft of the internal combustion engine (not shown), a sensor wheel G is attached in a torque-proof manner and interacts with a rotation angle sensor SN, which is attached to the crankcase of the internal combustion engine 1 . Angle of rotation sensors SN with a transmitter wheel G have long been prior art and are well known to a person skilled in the field of motor control, so that their description in connection with the claimed invention can be omitted.

The output signals of the rotation angle sensor SN are fed to a control unit 3 of the internal combustion engine via a signal line (no reference number). There they are processed and the "tooth times" of the sensor wheel D are converted into a rotation angle φ and/or a speed n of the crankshaft.

When installing the sensor wheel G on the crankshaft, it must of course be ensured that the sensor wheel G is installed in the desired position with the greatest possible accuracy. If the sensor wheel G is turned relative to the crankshaft, then there is an angular error between the angle of rotation φ detected by the angle of rotation sensor SN and the actual position of the crankshaft relative to the crankcase. This systematic error means, for example, that all angle specifications are incorrect. As a result, all controls of the internal combustion engine that are dependent on the angle of rotation are naturally subject to a systematic error, which negatively affects the precision of the control and thus the operating behavior of the internal combustion engine 1 .

A hardware filter 5.1, 5.2, 5.3 and 5.4 is arranged between each of the combustion chamber pressure sensors DS ₁ to DS ₄ . The hardware filters 5.1. to 5.4 are designed as anti-aliasing filters and are used to filter the analog signal of the sensors DS ₁ to DS ₄ to avoid aliasing. At the same time, an analog-to-digital conversion is carried out, which can take place either synchronously in time or synchronously with the crankshaft angle.

A disadvantage of the anti-aliasing filtering of the output signals of the combustion chamber pressure sensors DS ₁ to DS ₄ is that the output signal of the combustion chamber pressure sensors DS ₁ to DS ₄ is delayed as a result of the filtering. The amount of this delay depends, among other things, on the filter corner frequency of the anti-aliasing filter 5 .

As a result, when the time-synchronously or angle-synchronously recorded pressure values of the combustion chamber pressure sensors DS are assigned to the respective crankshaft position, there is a shift that is linearly dependent on the engine speed. This linear dependency is due to the fact that the delay in the output signal of the combustion chamber pressure sensors DS in the downstream filters 5 is constant over time.

In other words: Since the runtime shift within the filter is a constant, the runtime shift can be quantified via the gradient of a regression line and then corrected accordingly in the control unit.

In 2 this mentioned linear relationship between the rotational speeds of the crankshaft and the crankshaft angle φ is shown in diagram form.

The speed range relevant for diesel internal combustion engines is plotted on the x-axis of this diagram as an example.

The displacement angle Δφ is plotted on the Y-axis. In the method according to the invention, it is now provided to determine the relationship between the shift Δφ as a function of different speeds n and to calculate a regression line from the pairs of values determined between Δφ and speed n. The line of best fit is in 2 provided with the reference number 7. Various pairs of values are indicated in the form of points around the regression line 7 . The regression line 7 can be obtained from these pairs of values by means of a linear regression.

An important variable of the straight line 7 is the slope m. The slope m of the straight line 7 is shown in the drawing by a slope triangle 9 . The gradient m of the regression line 7 depends on the design of the filter 5 . In addition, it depends on the individual tolerances of the electronic components used in the filters 5.1, 5.2, 5.3 and 5.4 and must therefore be determined individually for each filter 5. Furthermore, the slope of the regression line 7, as well as its position relative to the coordinate origin, is subject to a certain drift.

An offset angle φ offset, which corresponds to the intercept of the regression line 7 on the Y axis, is a measure of the deviation of the sensor wheel G from the actual position of the crankshaft. This deviation can be caused by assembly errors and/or manufacturing tolerances. Since this deviation or the offset angle φ _offset is also recognized and quantified by the method according to the invention, it can then be used in the control unit 3 to correct the angle detection of the rotation angle sensor SN.

The offset angle φ _offset is preferably determined and stored for each individual cylinder. Alternatively, it is also possible to detect the offset angle φ _offset for a lead cylinder and to use this offset angle φ _offset for all cylinders.

If the gradient M and the offset angle φ _offset of the best fit line are now determined with the help of the best fit line 7, the group delay time and from this the filter delay time of the relevant filter can be converted from the slope using the following equation: $$G\mathrm{right}\mathrm{and}ppenla\mathrm{and}f\mathrm{e.g}eit=\frac{\mathrm{\Delta }{\phi }_{\text{pmax},\text{corrected}}[°]}{\mathrm{\Delta }n\left[1/\text{at least}\right]}•\frac{{10}^6\left[\mu s/s\right]•60\left[s/\text{at least}\right]}{360°}$$

Δφpmax,korrigiert:

Änderung der um den thermodynamischen Verlustwinkel korrigierten Lage des Druckmaximums [°KW]

Δn:

Änderung der Drehzahl n [1/min]

With:

Δφpmax, corrected:

Change in the position of the maximum pressure corrected by the thermodynamic loss angle [°CA]

Δn:

Change in speed n [1/min]

Mit:

τneu:

Zeit mit der die Signal-/Datenerfassung korrigiert wird

τalt:

Zeit mit der die Signal-/Datenerfassung bei der vorangegangenen Korrektur korrigiert wurde

α:

Lernfaktor

τaktuell:

Aktuell angelernte Zeit

Each time a new value for the group delay is available, it is then used to compensate for the filter delay. This can be done with the help of an adaptation algorithm, for example. $${\text{T}}_{\text{New}}={\text{T}}_{\text{old}}+\mathrm{a}·{\text{T}}_{\text{currently}}$$

With:

τnew:

Time with which the signal/data acquisition is corrected

τold:

Time with which the signal/data acquisition was corrected in the previous correction

a:

learning factor

τcurrent:

Current learned time

However, if the compensation is accurate enough, the compensation can also take place without adaptation. This correction is taken into account in the data acquisition of the next working cycle of the internal combustion engine.

During the crankshaft-angle-synchronous sampling or the crankshaft-angle-synchronous A/D conversion of the output signals of the combustion chamber pressure sensors 5, the learned group delay time is converted into a correction _{angle ΔφCorr} using the current speed n. This can be taken into account accordingly during data acquisition, in that the integral part is taken into account by shifting the sampled values, and the non-integer part is taken into account by interpolation between the sampling points.

In the case of time-synchronous sampling, the compensation is achieved by shifting the time stamp of the individual sampling values.

In 3 the flowchart of an exemplary embodiment of the method according to the invention is shown in a greatly simplified form as a block diagram.

The maximum of the combustion chamber pressure p _max is particularly advantageous as a characteristic variable of the combustion chamber which is suitable because it can be detected easily and with great accuracy and represents the combustion chamber pressure.

If the sensor wheel G is fastened to the crankshaft without tolerances and the pressure in the combustion chamber follows the position of the piston in the cylinder without a time delay and the filter 5 also does not cause a delay in the signal from the combustion chamber pressure sensors DS, the maximum combustion chamber pressure p _max,i im Top dead center OT occur.

However, since the pressure in the combustion chamber follows the position of the piston and thus also the angle of rotation of the crankshaft with a certain delay due to the polytropic change in state that the gas in the combustion chamber experiences due to the compression and expansion movement of the piston, this influence is due to the so-called to compensate for thermodynamic loss angle.

Furthermore, inaccuracies in the assembly of the sensor wheel G on the crankshaft, which are reflected in an offset angle (φ _offset ), must be compensated.

Taking these effects into account, it is easily possible with the method according to the invention to detect the propagation delays caused by the filters 5 and to compensate accordingly.

In 3 an exemplary embodiment of a flow chart according to the invention is shown. For reasons of simplicity, only the combustion chamber pressure sensor DS1 and the associated filter 5.1 are considered. In a first function block 11, the combustion chamber pressure is recorded with a high temporal resolution by the first combustion chamber pressure sensor DS1. Then, in a second function block 13, the analog output signal of the combustion chamber pressure sensor DS1 is filtered by a low-pass filter, so that only the low frequencies of the output signal of the combustion chamber pressure sensor DS1 pass through the filter. In addition, the signal is analog/digital (A/D) converted.

The output signal processed in this way is processed in the control unit 3 to form a characteristic variable of the combustion chamber pressure, preferably the position φ _max of the maximum of the combustion chamber pressure p max . This step takes place in a third function block 15. Alternatively, other characteristic variables can also be obtained from the output signal of the combustion chamber pressure sensor DS.

Parallel to the temporally high-resolution detection of the combustion chamber pressure, the rotational angle of the crankshaft is also detected with high resolution at the rotational angle sensor SN. The output signal of the rotation angle sensor SN is also transmitted to the control unit 3 and processed there into a rotation angle φ and/or a speed n of the internal combustion engine. The method according to the invention is then carried out in a function block 21 with the aid of the output signals of the signals of the angle of rotation sensor SN and the combustion chamber pressure sensor DS or the associated anti-aliasing filter 5 which have been processed in this way.

In a first partial step of the method according to the invention, the angular displacements Δφ associated with the maximum of the combustion chamber pressure are preferably determined in overrun mode of the internal combustion engine at different speeds of the internal combustion engine. The regression line 7 is then calculated from these measured pairs of values (see 2 ) determined and depending on a slope m of the regression line 7 and the offset angle φ _offset in a further function block 23 the correction of the output signal of the filter 5 or a correction of the information supplied by the rotation angle sensor SN about the rotation angle φ of the crankshaft.

The further control of the internal combustion engine takes place with the curve of the combustion chamber pressure corrected according to the invention and, because of the better quality of the curve of the combustion chamber pressure, leads to improved operating behavior of the internal combustion engine.

Claims (9)

Method for determining the combustion chamber pressure profile (p _i (t), p (a)) of at least one cylinder (Z _i ) of an internal combustion engine (1) from a variable which represents the crankshaft speed (n), in particular a signal from a crankshaft or camshaft rotation angle sensor ( SN), and a filtered characteristic variable (CharG _i ), which represents the combustion chamber pressure (p _i ), in particular a filtered signal from a combustion chamber pressure sensor (DS), with the filtered characteristic variable (CharG _i ) of the combustion chamber pressure curve during overrun operation of the internal combustion engine (1). (p _i (t), p (a)) at different speeds (n _j ) is recorded, and the relationship between speed (n _j ) and shift (Δφ _j ) of the characteristic variable (CharG _i,j ) of the combustion chamber pressure curve relative to Top dead center (OT) of the cylinder (Z) is determined, characterized in that when correcting the filtered characteristic variable (CharG _i ) of the thermodynamic loss angle is taken into account, with a position of a pressure maximum being corrected. procedure after claim 1 , characterized in that the relationship between speed (n _j ) and displacement (Δφ _j ) of the characteristic variable (CharG _i,j ) of the combustion chamber pressure curve (p _i (t), p (a)) relative to top dead center (TDC) of the Cylinder (Zi) is determined by a regression analysis, in particular a linear regression. procedure after claim 1 or 2 , characterized in that the relationship between the speed (n _j ) and the displacement (Δφ _j ) of the characteristic variable (CharG _i,j ) of the combustion chamber pressure curve (pi(t), p (a)) relative to top dead center (TDC) of the cylinder (Zi) to correct the signal of the filtered characteristic quantity (CharG _i,j ). Method according to one of the preceding claims, characterized in that after the correction of the filtered characteristic variable (CharG _i,j ) an offset angle (φ _offset ) between the top dead center (OT, φ = 0) and the angle (φ _max ) is determined, and that the offset angle (φ _offset ) to correct the position of a transmitter wheel (G) of the rotation angle sensor (SN) is used. Method according to one of the preceding claims, characterized in that it is applied to all cylinders (Z _i ) of the internal combustion engine (1). Method according to one of the preceding claims, characterized in that the position (φ _max ) of the maximum of the combustion chamber pressure (p _{i ,max} ) with respect to top dead center (OT ) is being used. Method according to one of the preceding claims, characterized in that an anti-aliasing filter, preferably a low-pass filter and particularly preferably a hardware low-pass filter, is used to filter the characteristic quantity (CharG _i,j ). Computer program with program code for carrying out all the steps according to one of the preceding claims when the computer program is executed by a computer. Device, in particular control device for an internal combustion engine (1), characterized in that it operates according to one of the preceding method claims.

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