FR2828237A1 - Method of calibrating a pressure sensor used in a motor vehicle internal combustion engine fuel injection system involves detecting a reference value and determining theoretical value for comparison - Google Patents

Abstract

The method of calibrating a pressure sensor (8) involves detecting a reference value for the fuel pressure in one condition of the injector. A theoretical value is calculated for this condition and the error is determined. A final value is calculated using the derived pressure value as a correcting factor. The control value is determined prior to detection of the reference value.

Description

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 The invention relates to a method for calibrating a pressure sensor of an injection device for an internal combustion engine, comprising the following operations: detecting a reference measurement value of the fuel pressure in a first state of defined operation of the injection device, calculating a theoretical value of the fuel pressure in the first defined operating state, using a known prefixed dependence of the fuel pressure on the state operating mode of the injection device, determining the difference between the reference measurement value and the theoretical value of the fuel pressure in the first defined operating state, detecting at least one subsequent measurement value of the fuel in a subsequent operating state of the injection device and to correct the subsequent measurement value according to the deviation determined in the first ier working condition.

 Conventional injection devices for internal combustion engines usually include a low pressure pump which draws fuel from a fuel tank and sends it to a high pressure pump located downstream which produces the fuel pressure necessary for the injection of fuel into the engine combustion chambers. There is then provided, arranged in the low pressure zone located between the low pressure pump and the high pressure pump, a low pressure regulating valve which, when crossed upwards a fuel pressure prefixed in the low pressure zone, diverts part of the fuel flow delivered by the low pressure pump and returns it to the fuel tank. On the output side, the high pressure pump is connected to a pressure accumulator (common rail or "common rail") from which the injectors associated with the different combustion chambers of the engine draw the fuel to be injected. The control of the fuel pressure in the pressure accumulator can for example take place by means of a high pressure regulating valve which, when crossed upwards a fuel pressure prefixed in the pressure accumulator, diverts part of the fuel flow delivered by the high pressure pump and returns it to the low pressure zone or to the fuel tank.

However, for controlling the fuel pressure in the pressure accumulator, it is also possible to use a high pressure pump controlled by volume flow, the flow of which can be adjusted in order to adapt the fuel pressure. However, in both cases, the current fuel pressure in the pressure accumulator must be measured, in order to be able to regulate the high pressure regulating valve or the high pressure pump controlled in volume flow appropriately. For this purpose, a pressure sensor is used, connected to the pressure accumulator, which delivers a pressure-dependent voltage signal.

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 One drawback is that the pressure sensor has its own error tolerance which is generally dependent on the temperature, which can lead to faulty regulation of the fuel pressure in the pressure accumulator.

A fact which is particularly important in this case is that the error bandwidth of the pressure sensor is reported as an absolute error to the final value of the pressure sensor, so that, in the case of low pressures of fuel, for example during starting of the internal combustion engine, there may be considerable relative error deviations. In particular during engine start-up, the error tolerance of the pressure sensor therefore causes considerable defective control of the fuel pressure in the pressure accumulator.

 Document DE 100 03 906 A 1, benefiting from a later publication date, discloses a method for calibrating a pressure sensor according to which the fact that the fuel pressure in the high pressure zone is prefixed shortly afterwards starting the internal combustion engine by means of the regulating set pressure of the low pressure regulating valve. The known calibration process therefore detects the fuel pressure shortly after starting the engine as a reference measurement value and determines the measurement error from the difference between the reference measurement value and the reference pressure for regulating the low pressure regulating valve. The measured values detected by the pressure sensor are then corrected during normal engine operation in a manner corresponding to the determined measurement error.

 A disadvantage of this known calibration method is the fact that the fuel pressure after starting the engine is not always exactly equal to the regulating set pressure of the low pressure regulating valve, which leads to a faulty determination of measurement error.

 From DE 199 59 678 A 1, another method of calibrating a pressure sensor is known, according to which the fact that the fuel pressure at a prefixed reference instant is also known. The known calibration process therefore measures the fuel pressure at the reference instant and compares it to the prefixed known fuel pressure, in order to deduce the measurement error therefrom. In this calibration process, the reference instant used to determine the measurement error can for example be given by the pressureless state of the high pressure accumulator.

 A drawback of this calibration method also consists in that the actual fuel pressure at the reference instant can deviate from the theoretical fuel pressure supposed to be known, which leads to a faulty determination of the measurement error.

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 Finally, from DE 199 08 411 A1, there is known a method for calibrating a pressure sensor according to which the fact that the regulation set pressure of the low pressure regulation valve is also known. The fuel pressure is therefore measured shortly after starting the internal combustion engine and compared with the known setpoint pressure of the low pressure control valve in order to determine the measurement error.

 A disadvantage of this method is that the actual fuel pressure during the calibration measurement can deviate from the fuel pressure assumed to be known, which leads to a false determination of the measurement error.

 This is why the invention aims to provide a method for calibrating a pressure sensor for which a faulty determination of the measurement error is avoided.

 To this end, the subject of the invention is a method, of the generic type defined in the introduction, characterized in that before the detection of the reference measurement value, a measurement value for controlling the fuel pressure is to first detected and is controlled by means of prefixed conditions, the detection of the reference measurement value only taking place when the control of the control measurement value has been carried out in a positive manner, and / or that the time interval between the last shutdown of the internal combustion engine and the subsequent switching on of the low pressure fuel pump is measured, the correction of the measured values only taking place when this interval time exceeds a fixed minimum duration.

 Thus, the invention is based on general technical teaching consisting in automatically calibrating the pressure sensor in order to compensate for the own errors of the pressure sensor.

 For this purpose, in accordance with the invention, use is made of the fact that the fuel pressure in the pressure accumulator is known in a determined operating state of the injection device even without measuring the fuel pressure. This is why, in the context of the process according to the invention, the injection device is first brought into the operating state in which the theoretical value of the fuel pressure is known. In addition, in this operating state, a reference measurement value is detected by the pressure sensor. The difference between the reference measurement value of the fuel pressure and the theoretical value of the fuel pressure is then determined. In the case of a pressure sensor operating without error, this deviation must be approximately equal to zero, insofar as it can be assumed that the value

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 The theoretical fuel pressure is correct. In the subsequent operation of the injection device, the measurement values detected by the pressure sensor are then corrected as a function of the previously determined difference between the reference measurement value and the theoretical measurement value.

 Preferably, the first operating state of the injection device is the state in which the injection device finds a pre-set dead time after switching on a low pressure fuel pump, the dead time being time interval required for the low pressure fuel pump to build pressure.

In fact, in this operating state, the pressure prevailing in the low pressure zone situated between the low pressure fuel pump and the high pressure fuel pump has already reached the set value of the fuel pressure, whereas, on the other hand , the high-pressure pump has a longer delay in establishing pressure and therefore does not yet make a significant contribution to the fuel pressure in the pressure accumulator. As a result, the fuel pressure in the accumulator pressure is essentially equal to the known pressure in the low pressure zone, minus the losses due to the high pressure pump which is arranged between the low pressure fuel pump and the pressure accumulator.

 However, in the preferred embodiment of the invention, before the reference measurement value is detected by the pressure sensor, a control measurement value is first detected by the pressure sensor and a procedure is carried out. a control by means of prefixed conditions, the subsequent detection of the reference measurement value being authorized only when the control of the control measurement value has been carried out in a positive manner. Thus, for example, it can be checked whether or not the control measurement value is situated within the limits of the theoretical possible error bandwidth, this bandwidth being obtained from the initial pressure. (ambient pressure) and the maximum error tolerance of the pressure sensor. If the check of the check measured value results in it not falling within the theoretical possible error bandwidth, the measured value is also not measured reference, since such a measurement would be vitiated by error with a probability approaching certainty and it is therefore not possible to use it for the correction of the measured values and for the calibration pressure sensor.

 Furthermore, in the preferred embodiment, the minimum duration prefixed between the last shutdown of the internal combustion engine and the subsequent switching on of the low pressure fuel pump is large enough so that the

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 fuel pressure in a pressure accumulator has decreased approximately to ambient pressure.

 Thus, before detecting the reference measurement value, it is first checked whether the internal combustion engine has been idle long enough before switching on the low pressure fuel pump, so that the fuel pressure in the pressure accumulator may have dropped approximately to ambient pressure. This is important since, otherwise, the residual pressure remaining in the pressure accumulator would cause a faulty calibration of the pressure sensor. This is why the calibration according to the invention of the pressure sensor preferably takes place only when, between the last engine shutdown and the subsequent switching on of the low pressure fuel pump, it at least one preset time interval has elapsed which ensures a sufficient decrease in the fuel pressure in the pressure accumulator.

 As already indicated above, the correction of the measurement values is carried out as a function of the difference between the reference measurement value and the theoretical value of the fuel pressure in the first defined operating state.

 In the preferred embodiment, the subsequent measurement value is in each case corrected by a correction value, the correction value depending on the theoretical value of the fuel pressure in the first operating state, of the measurement value of reference of the fuel pressure in the first operating state and / or of the subsequent measurement value. This can of course relate to subsequent measurement values. Furthermore, in the preferred embodiment of the invention, the correction value decreases as the fuel pressure increases, until, finally, in normal operation of the injection device, there is no longer any correction of the measured values of the pressure sensor. On the one hand, this is only possible because the absolute error of the pressure sensor has only minor effects in the normal operation of the injection device due to the relatively large fuel pressures then prevailing. On the other hand, after starting the internal combustion engine, the fuel temperature increases, so that the calibration no longer leads to completely satisfactory results in the normal operation of the injection device due to the relatively high fuel temperatures. reigning then.

 The process according to the invention can also have one or more of the following features:

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 the control measurement value is compared with a prefixed lower threshold value and with a prefixed upper threshold value, the detection of the reference measurement value taking place only when the control measurement value is situated between the lower threshold value and upper threshold value, - the minimum prefixed time between the last shutdown of the internal combustion engine and the subsequent switching on of the low pressure fuel pump is large enough so that the pressure of fuel in a pressure accumulator has decreased approximately to ambient pressure, - the subsequent measurement values of the fuel pressure which must be corrected are compared with a preset maximum value, the correction of the subsequent measurement values not being carried out only when the measured value each time considered is less than the maximum prefixed value, - for values in subsequent measurements, the difference with respect to the maximum value is determined in each case, the correction of the measurement values being carried out each time as a function of the difference with respect to the maximum value, - the value correction value decreases to zero with the difference between the measured value each time considered and the maximum value.

 Other advantageous developments of the invention are described below in detail using the description of the preferred embodiment of the invention made with reference to the figures. We see: in Figure 1, in the form of block diagram, an injection device operating according to the principle according to the invention, in Figures 2a to 2c, the method according to the invention in the form of a flowchart and , in Figure 3, a characteristic line graph of the pressure sensor.

 The injection device shown in FIG. 1 has to a large extent a conventional structure and comprises a low pressure fuel pump 1 which sucks fuel from a fuel tank 2 and sends it to a high pressure pump 3 , arranged downstream, which produces the fuel pressure necessary for injecting the fuel into the combustion chambers of the internal combustion engine.

 There is provided, arranged in the low pressure zone between the pump located between the low pressure fuel pump 1 and the high pressure fuel pump 3, a low pressure regulating valve 4 which, when a fuel pressure prefixed in the low pressure zone, diverts part of the fuel flow from the low pressure pump 1 and returns it to the tank 2,

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 so that in normal operation of the injection device, the fuel pressure in the low pressure zone located between the low pressure pump 1 and the high pressure pump 3 remains approximately constant.

 The drive of the low pressure pump 1 is carried out by means of an electric motor not shown, while, on the other hand, the drive of the high pressure pump 3 is carried out by means of a camshaft 5, but a common drive by means of the camshaft 5 is also possible.

 On the outlet side, the high pressure pump 3 is connected via a non-return valve 6 to a pressure accumulator 7 from which the injectors associated with the different combustion chambers of the engine and represented here only in a way schematically, take the fuel for injection into the combustion chambers of the engine.

 In a conventional manner, the pressure accumulator 7 is connected to a pressure sensor 8 which produces a voltage signal reproducing the fuel pressure in the accumulator 7, the voltage signal from the sensor 8 being sent to a control of electronic motor 9 (CPU or central unit).

 The electronic motor control 9 has, among other things, the task of controlling the high pressure fuel pump 3, controlled by volume flow, in such a way that the flow of the pump 3 is adapted to the fuel needs each time, so that the fuel pressure in the pressure accumulator remains approximately constant. The control signal delivered by the electronic motor control 9 to the high pressure pump 3 also determines the flow rate of the pump 3 and therefore the fuel pressure in the accumulator 7.

 It is however also possible to use, in place of the high pressure pump 3 with volume flow control, a high pressure fuel pump delivering continuously in connection with a proportional control valve. In the case of this technique, the proportional regulation valve is subjected to the action of a voltage modulated in pulse width, the voltage creating a current flow which causes a fuel pressure which is proportional to it.

 In this case, the electronic motor control 9 executes the method according to the invention described below, in order to calibrate the pressure sensor 8 and thereby compensate for the inherent error of the sensor 8.

 The way in which the process according to the invention takes place is set out below with reference to the flowchart shown in FIGS. 2a to 2c.

 After a start step 10, the low pressure pump 1 is first switched on in step 11, then, in step 12, it is determined whether the combustion engine

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 internal or not has been at rest for a sufficiently long time interval before switching on the low pressure pump 1, so that the fuel pressure in the pressure accumulator can decrease to ambient pressure. This is important, given that, otherwise, the residual pressure remaining in the accumulator 7 disturbs the calibration in the context of the process according to the invention. If the rest time TAus is not greater than a minimum rest time T MIN prefixed, we go to step 13 and there is no correction of the signal supplied by the pressure sensor 8, since the calibration would be distorted by the residual pressure in the accumulator 7.

 Otherwise, we go to step 14 and the first voltage value U, supplied by the pressure sensor 8 is measured, which value essentially corresponds to the ambient pressure of approximately 1 bar. Then, we pass to step 15 where the error bandwidth in the current operating state of the injection device is determined. For this purpose, the maximum possible theoretical value U1, MAX and the minimum possible theoretical value U1, MIN are calculated using the voltage-pressure characteristic line f (P) prefixed with the pressure sensor 8 and the error UFEHLER of sensor 8 which is also known.

 Then, at a step S16, a plausibility check of the measurement value U1 is carried out, this measurement value being compared respectively with the maximum value and the minimum value calculated in step 15. If the measurement value U, is located outside the possible theoretical error bandwidth, this indicates a malfunction and we go to step 13, so that neither correction nor calibration of the pressure sensor 8 is carried out. Otherwise, it must be assumed that there is no malfunction, so that at a next step 17, first wait during the dead time TTOT of the low pressure fuel pump 1. This is important, since the low pressure pump 1 needs a certain time interval to build up pressure in the low pressure area. This is why the dead time TTOT is dimensioned in such a way that the low pressure pump 1 has enough time to establish the permanent pressure in the low pressure zone, the dead time TTOT being however small enough for the pump to high pressure fuel 3 makes no contribution to the fuel pressure in the pressure accumulator 7 due to its delayed pressure build-up.

 Once the dead time TTOT has elapsed, the fuel pressure in the pressure accumulator 7 is therefore essentially equal to the known permanent PVOR fuel pressure present in the low pressure zone, minus the PVERLUSTE pressure losses due to the high pump. pressure 3.

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 This is why, at a next step 18, once the dead time TTOT has elapsed, the voltage U2 which corresponds to the fuel pressure in the pressure accumulator 7 at this instant is detected.

 At a next step 19, the measured voltage excursion UMESS = Us-Ui is then calculated.

tension est calculée conformément à la formule suivante :

UAD. START = f (PvOR'PvERLUSTE)'Uz.

Cette valeur d'adaptation de début correspond à l'écart entre la valeur de mesure de référence U2 et la valeur théorique f (PVOR-PVERLUSTE) de la pression de carburant après l'établissement de pression au moyen de la pompe de carburant basse pression 1. In addition, at a next step, the theoretical voltage excursion is obtained, which is obtained using the known voltage-pressure characteristic line of the pressure sensor 8 from the initial ambient pressure of approximately 1 bar and subsequent pressure P2 = PVOR-PVERLUSTE-
At a next step 21, an adaptation value at the start of the correction of

voltage is calculated according to the following formula:

DSUs. START = f (PvOR'PvERLUSTE) 'Uz.

This start adaptation value corresponds to the difference between the reference measurement value U2 and the theoretical value f (PVOR-PVERLUSTE) of the fuel pressure after the pressure has been established by means of the low pressure fuel pump 1.

UAD, STOP = UMAX + UAD, START. At a next step 22, a stop value of the voltage adaptation is then calculated according to the following formula:

UAD, STOP = UMAX + UAD, START.

 This is important, since the correction, in accordance with the invention, of the voltage values supplied by the pressure sensor 8 should only take place up to a prefixed voltage value or pressure value, as described below. in detail. In the engine starting operation which follows, subsequent measurement values are, at a step 23, detected by the pressure sensor 8 in a loop, a control being first carried out at a next step on the question of knowing whether or not the measured value each time considered already exceeds the stop value of the voltage adaptation which is calculated in step 22.

 If this is the case, we go to step 25 and no correction of the voltage takes place, since it is obvious that the injection device is already in the normal operating state.

If, on the other hand, the voltage value UMESS supplied by the pressure sensor 8 is still less than the stop value UAD, STOP calculated in step 22, a correction value UAD of the measured voltage is calculated in a next step 26 in accordance with the following formula:

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In next step 27, the measurement value of the voltage UMESS is then corrected by the correction value UAD calculated in step 26, so that in next step 28, the corrected voltage value UKORR can then be used for the control of the entire injection device.

 At a next step 29, it is then checked whether the internal combustion engine has been stopped, which would cause an interruption and the end of the process according to the invention. However, during a subsequent operation of the engine, a loop is passed in step 23 and a subsequent measurement value is detected by the pressure sensor 8.

 The calibration is now explained below using the characteristic line graph shown in Figure 3.

 Thus, FIG. 3 first shows a specific theoretical voltage-pressure characteristic line 30 of the pressure sensor 8.

 In addition, the characteristic line graph represents the reference measurement value U2 which corresponds to the fuel pressure which is established in the pressure accumulator after the low pressure fuel pump has established the permanent fuel pressure in the low pressure zone and before the high pressure pump 3 provides a contribution to the fuel pressure.

In this operating state, the fuel pressure in the pressure accumulator 7 corresponds to the known permanent fuel pressure PVOR present in the low pressure zone, minus the VERLUSTE pressure loss due to the high pressure pump 3. In in addition, the characteristic line graph represents the correction values UAD, so that it can be seen that, when the injection device is activated, the correction values decrease, according to a prefixed envelope curve 31, by a linearly to zero as the fuel pressure increases.

 The invention is not limited to the preferred embodiment previously described. On the contrary, it is possible to imagine multiple variants and modifications which also use the concept of the invention and therefore fall within the scope of protection.

Claims (8)

1. Method for calibrating a pressure sensor (8) of an injection device for an internal combustion engine, comprising the following operations: - detecting a reference measurement value (U2) of the fuel pressure in a first defined operating state of the injection device, - calculating a theoretical value of the fuel pressure in the first defined operating state, using a known prefixed dependence of the fuel pressure vis-à-vis -vis the operating state of the injection device, - to determine the difference (UAD, START) between the reference measurement value and the theoretical value of the fuel pressure in the first defined operating state, - detecting at least one subsequent measurement value of the fuel pressure in a subsequent operating state of the injection device and - correcting the subsequent measurement value as a function of the determined deviation in the first operating state, characterized in that before the detection of the reference measurement value, a control measurement value (U1) of the fuel pressure is first detected and is checked by means of prefixed conditions , the detection of the reference measurement value (U2) taking place only when the control of the control measurement value has taken place in a positive manner, and / or - in that the time interval (TAus) between the last shutdown of the internal combustion engine and the subsequent switching on of the low pressure fuel pump is measured, the correction of the measured values only taking place when this time interval exceeds a minimum duration (T MIN) prefixed.  2. Method according to claim 1, characterized in that the subsequent measurement value is in each case corrected by a correction value, the correction value depending on the theoretical value of the fuel pressure in the first operating state, the reference measurement value of the fuel pressure in the first operating state and / or the subsequent measurement value.  3. Method according to any one of claims 1 and 2, characterized in that the first operating state of the injection device is the state in which the injection device finds a dead time prefixed after switching on of a low pressure fuel pump, the dead time being the time interval necessary for the low pressure fuel pump to build up pressure.  4. Method according to any one of claims 1 to 3, characterized in that the control measurement value (Ui) is compared to a threshold value <Desc / Clms Page number 12>  prefixed lower and to a prefixed upper threshold value, the detection of the reference measurement value taking place only when the control measurement value is situated between the lower threshold value and the upper threshold value.  5. Method according to any one of claims 1 to 4, characterized in that the minimum period prefixed between the last shutdown of the internal combustion engine and the following circuiting of the low pressure fuel pump is large enough that the fuel pressure in a pressure accumulator (8) has decreased approximately to ambient pressure.  6. Method according to any one of claims 1 to 5, characterized in that the subsequent measurement values of the fuel pressure which are to be corrected are compared with a prefixed maximum value, the correction of the subsequent measurement values not being carried out only when the measurement value each time considered is less than the maximum value prefixed.  7. Method according to claim 6, characterized in that, for the subsequent measurement values, the difference with respect to the maximum value is determined in each case, the correction of the measurement values being carried out each time as a function of the difference from the maximum value.  8. Method according to claims 2 and 6, characterized in that the correction value decreases to zero with the difference between the measurement value each time considered and the maximum value.