EP0388412B1 - System for regulating the air/fuel ratio of an internal combustion engine - Google Patents

Abstract

A system for regulating the air/fuel ratio in an internal combustion engine (10), an oxygen probe (Lambda probe) (13) being arranged in the exhaust fumes of said motor, comprises a regulating device (12) permitting continuous regulation. The actual value of the air index Lambda is determined by means of the measured output voltage of the probe in connection with a correlation (16), characteristic of the probe and predefined at least approximately, between the value of the output voltage of the probe and the value of the air index Lambda coupled with the latter. The difference between the theoretical value and the actual value of the air index Lambda is calculated and the air/fuel ratio is regulated on the basis of this difference. This type of regulating system is used essentially to reduce the overall emission of the major hazardous constituents of the exhaust fumes of an engine. In particular in an engine (10) fitted with a catalyser arranged in the exhaust fumes, this system ensures that the value of the air index Lambda (Lambda = 1) required for optimal performance of the catalyser is strictly maintained.

Description

The invention relates to a control system for regulating the air / fuel ratio in an internal combustion engine to an air ratio lambda to be maintained, the control system having an oxygen probe (lambda probe) exposed to the exhaust gas of the internal combustion engine, the output voltage of which represents a measure of the air ratio lambda , changes essentially abruptly in the range of lambda = 1.

If a three-way catalytic converter is used to reduce the main pollutant components (NOx, HC, CO) of an internal combustion engine, it is necessary for its optimal effectiveness, ie to achieve a maximum conversion rate, that a stoichiometric air / fuel mixture (lambda = 1 ) at least an air ratio lambda, which moves in a certain range around lambda = 1 (lambda window), is observed. In the known control systems, the abrupt behavior of the output voltage of the lambda probe during the transition from the rich (λ <1) to the lean range (λ> 1) or during the transition from the lean (λ> 1) to. Fat range (λ <1) for mixture control, i.e. not the value of lambda itself, is evaluated. Using a two-point control, the values for the injection time, which are stored in a characteristic diagram as a function of the speed and load (throttle valve position) of the internal combustion engine, are corrected multiplicatively by means of a correction factor. A two-point controller with PI behavior is usually used for the ongoing correction of the correction factor. Due to the jump characteristic of the output voltage in the range of lambda = 1 and due to the presence of dead times (transport time of the mixture from the injection valves through the internal combustion engine to the lambda probe, reaction time of the probe), a control oscillation for the correction factor occurs. The required lambda air ratio can therefore only be met on average. The amplitude and frequency of this control oscillation have a significant influence on the exhaust gas emission. An increase in the amplitude of the control oscillation leads to the air number lambda temporarily moving outside the lambda window and a drastic increase in the harmful components of the exhaust gases.

From DE-OS 32 31 122 a control system is known in which a control device with a constant control behavior is arranged for control in the lean range (preferably around lambda = 1, 2). Since the probe output signal has a relatively small slope in this area, greater control accuracy is achieved with the continuous control device than with a conventional two-point control. In the aforementioned publication, it is further stated that this continuous control device cannot be used for lambda = 1 control, since with lambda = 1 the lambda probe has a steep voltage jump and the control device would therefore always be at a lean or fat stop.

From US-A-4 641 276 a method and a device are known which regulate the air / fuel mixture of an internal combustion engine by means of oxygen probes, which are each arranged in the combustion chambers. This has the advantage that the direct measurement of the combustion result also enables the overarching control to react very quickly. The detection duration of the proportions of the fuel / air mixture supplied to the internal combustion engine can thus be reduced considerably. Furthermore, extreme conditions such as too rich or too lean can be recognized after a few cycles of the cylinder in question and a reaction can be made accordingly. In addition to monitoring the entire engine, the measurement results being evaluated serially from cylinder to cylinder in the firing order, the invention in US Pat. No. 4,601,276 also allows the simultaneous monitoring of each individual cylinder. This has the advantage that scattering from cylinder to cylinder in the mixture composition can be eliminated. Since signals from oxygen probes, which are arranged directly in combustion chambers of an internal combustion engine, are evaluated for the method and the device of US Pat. No. 4,601,276, the probe signals are evaluated in a different way than with an oxygen probe, which is arranged in the exhaust gas of the internal combustion engine , d. H. the operation of the internal combustion engine is constantly exposed to the same.

Furthermore, the method and the device of US Pat. No. 4,641,276 assume that an internal combustion engine with multiple cylinders also uses a plurality of oxygen probes, which is a not inconsiderable cost.

US Pat. No. 4,200,064 describes a mixture metering system with a probe arranged in the exhaust tract of an internal combustion engine. In an area comprising lambda = 1, the output signal, which changes essentially abruptly there, is used in so-called calibration phases for continuous control. For this purpose, it is checked whether the use of certain pilot control values for the fuel metering results in the probe voltage of 0.5 volt associated with the lambda value 1. This is the case, for example, when the injection time is extended by 1 ms. If other lambda values are to be set in control mode after the calibration phase has ended, the control values provided for this are also extended by 1 ms.

The invention has for its object to improve a control system for controlling the air / fuel ratio in an internal combustion engine by means of an oxygen probe arranged in the exhaust system, which is constantly exposed to the exhaust gas during operation of the internal combustion engine with regard to reducing the total emission of the main pollutant components.

Advantages of the invention

The invention is given by the features of claim 1 and the features of the independent claim 2.

The solution according to claim 1 is characterized in that the control system according to the invention has a control device for continuous control, the jump behavior of the output signal of the lambda probe (two-point control) for mixture control not being evaluated as in the prior art, but the actual deviation the air ratio lambda is used as the control deviation from the air ratio lambda to be maintained. In this case, the respective actual value of the air ratio lambda is determined via the respectively measured probe output voltage in connection with an at least approximately predetermined characteristic relationship between the size of the probe output voltage and the size of the air ratio lambda coupled therewith. The target value of the air ratio lambda corresponding to the air ratio lambda to be observed is subtracted from the actual value of the air ratio lambda and the difference is used to regulate the air / fuel ratio.

In the control system according to the invention, deviations from the predetermined air ratio lambda = 1 are corrected faster than in a conventional two-point control system, as a result of which the emission of harmful exhaust gas components is reduced. According to previous attempts, there was an increase in the control frequency by a factor of 1.5 to 3 compared to the conventional two-point control, which both contributes to a reduction in pollutant emissions and improves the smooth running of the internal combustion engine, particularly at low speeds and high loads. Another advantage of the control system according to the invention over the long-standing two-point control for lambda = 1 is that the control system according to the invention is significantly less sensitive to interference with the probe signal in the event of strong cylinder scattering (chemical. noise) reacts as the usual two-point control. The strong cylinder scatter has the result that the two-point control jumps from rich to lean or lean to rich with increased frequency between the extreme values lean and rich when passing through the control threshold, which has an unfavorable effect on the exhaust gas and driving behavior of the internal combustion engine. By using a control device according to the invention with a constant control behavior, this switching between two extreme values, which is operated at an increased frequency, is avoided.

The control system according to the independent claim 2 is characterized by a control device for continuous control, a probe voltage being used as the target value, which is assigned to the air ratio lambda to be maintained according to the respective probe characteristic, and via the difference between the respectively measured actual values the probe voltage with the target value of the probe voltage in conjunction with an at least approximately predefined probe characteristic relationship between the size of the probe voltage difference and the associated size of the air ratio difference, the air ratio difference is determined and the air / fuel ratio is regulated with the air ratio difference. This control system achieves the same advantages over the prior art in controlling the air / fuel ratio as in the control system according to claim 1.

The mentioned advantages of the control system according to the invention according to the independent claims 1 and 2 can, however, only be achieved for a control to lambda = 1 if the output voltage of the lambda probe changes in the range of lambda = 1 only essentially, ie not mathematically ideally leaps and bounds , d. H. a function linking the air ratio lambda and the probe output voltage has a finite slope in the range from lambda = 1.

The characteristic relationship between the probes is advantageous between sensor voltage and air ratio lambda or sensor voltage difference and air ratio difference stored in a map. According to a further advantageous embodiment of the invention, on the one hand the probe voltage or probe voltage difference and, to take into account the temperature-dependent relationship between probe voltage or probe voltage difference and temperature, a temperature-dependent internal probe resistance or the probe temperature itself are used as input parameters of this characteristic diagram.

To save storage space and computing time, it has proven to be advantageous to reduce this characteristic map to a characteristic curve which is designed for a medium or particularly frequently occurring probe temperature.

To save storage space, it proves to be advantageous to map the probe-specific relationship by using mathematical functions, whereby it has been found to be particularly advantageous, based on the usual probe characteristics of the lambda probe, to use a third-order parabola as the mathematical function.

According to a further advantageous embodiment of the invention, when regulating to lambda = 1 up to a control deviation of preferably 3% (ie lambda = 0.97 to lambda = 1.03), a control device is used which has constant behavior and is larger in the case of a control deviation as a 3% switch from continuous control to two-point control. The limitation to a narrow lambda band used for evaluation by the value lambda = 1 has the advantage that the influence of errors in the assumed probe characteristic due to temperature changes of the probe is relatively small, since the probe characteristic in the range of lambda = 1 is quite is temperature stable. Therefore, the accuracy of a zero point offset correction to be carried out for the probe voltage can be reduced, since in the temperature-sensitive range outside the lambda band Two-point regulation is applied.

adaptiert, wobei K ein konstanter Faktor ist, der anhand der Sondencharakteristik bestimmt wird. Die Korrektur des Regel-Sollwertes erfolgt zusätzlich über einen Tiefpaß. Weiterhin werden die gemessenen Sondenspannungsextremwerte gespeichert und für den Fall, daß keine neuen Extremwerte der Sondenspannung gemessen werden, langsam abgeregelt. Mit dieser Adaption ist es möglich, das Verschieben des Regel-Sollwerts der Sondenspannung infolge Alterung der Sonde oder Temperaturänderung der Sonde zu berücksichtigen.In a further advantageous development, the setpoint value of the probe voltage U _S becomes dependent on the measured maximum and minimum probe voltage according to the formula. $${\text{U}}_{\text{S}}{\text{= (U}}_{\text{S (max)}}{\text{- U}}_{\text{S (min)}}{\text{) x K + U}}_{\text{S (min)}}$$

adapted, where K is a constant factor that is determined based on the probe characteristics. The control setpoint is also corrected using a low-pass filter. Furthermore, the measured probe voltage extreme values are stored and slowly reduced in the event that no new extreme values of the probe voltage are measured. With this adaptation it is possible to take into account the shifting of the control setpoint of the probe voltage due to aging of the probe or temperature change of the probe.

drawing

Embodiments of the invention are shown in the drawing and explained in more detail in the following description. Fig. 1 shows a simplified block diagram of a control arrangement with a control system for controlling the air / fuel ratio in an internal combustion engine according to claim 1. Fig. 2 shows a control arrangement with a control system according to the invention for controlling the air / fuel ratio in an internal combustion engine according to the independent claim 2, but not the entire control arrangement is shown, but only components are shown in which the control arrangement according to claim 2 differs from that according to claim 1.

Description of the embodiments

1 has an internal combustion engine (BKM) 10 as a controlled system with injection valves (EV) 11 as actuators, a control device 12, (dashed border) a lambda probe 13 arranged in the exhaust gas of the internal combustion engine and a basic map 14. The basic map 14 is preferably designed as a read-only memory (ROM), which is addressed by supplied operating variables (here: speed n and throttle valve position α). Depending on these addresses, a corresponding injection time t _L for the injection valves 11 of the internal combustion engine 10 is read from the basic field 14. The lambda probe 13 emits an output signal (output voltage U _S ) which is fed to the control device 12. The control device 12 outputs a correction factor KF as a manipulated variable, which multiplicatively corrects the injection time t _L output from the basic characteristic map 14, which results in the corrected injection time t _LK . Furthermore, the control device 12 is supplied with a control target value 15 of the air ratio lambda, which in turn can depend on the throttle valve position α and the speed n of the internal combustion engine 10. When using a three-way catalytic converter, this target value = 1 is set, since the presence of a stoichiometric mixture (lambda = 1) ensures optimal conversion behavior of the catalytic converter.

The control unit 12 has a conversion device 16, by means of which the probe output signals U _{S of} the lambda probe 13 are converted into lambda values in accordance with the characteristic relationship between the lambda value and the probe voltage. Either a mathematical function, a table or a map is used to map the probe characteristic relationship. The probe characteristic is strongly influenced by the probe temperature in the area of larger and smaller lambda = 1. In order to increase the control accuracy, it is therefore advantageous to use the temperature of the probe or the temperature-dependent internal resistance of the probe as an input parameter when determining the lambda value, for example from a map, in addition to the probe voltage U _S.

Inside the control device 12 is the conversion device 16 is followed by a timing element 17 and this a correction device 18 for calculating a correction factor KF. This correction factor KF is fed to a multiplication unit 19, which multiplies the correction factor KF by the injection time t _L output from the basic characteristic diagram 14. The output of the correction factor KF can be interrupted by a switch 20, which is switched via a control release device 21. In certain operating phases of the internal combustion engine (for example starting phase, warm-up phase, unsteady phases), regulation to a predefined lambda air ratio is not desired. In these cases, the control release device 21 interrupts the output of the correction factor KF via the switch 20.

If the control release device 21 has released the control, the output signal of the lambda probe arranged in the exhaust gas of the internal combustion engine 10 is fed to the conversion device 16. Since the correction factor KF is preferably calculated using a computer, the analog probe output signal is converted into a digital signal after amplification via an A / D converter (not shown in FIG. 1). The conversion unit 16 calculates the respectively measured actual value of the air ratio lambda from the output signal of the lambda sensor 13 via a predetermined probe-characteristic relationship between the output voltage of the probe and the air ratio lambda. The subsequent comparison of the actual value and the target value 15 of the air ratio lambda leads to a control deviation .DELTA.-lambda, which is fed to a timing element 17. The timing element then emits a signal to a correction device 18, which carries out the calculation of the correction factor KF.

The correction factor KF is then multiplied by the injection time t _L output from the basic characteristic map 14, which results in the corrected injection time t _LK . By adding the injection time t _LK and an injection time t _S , which takes into account the dead time influence of the injection valves 11, finally leads to the actual injection time t _I. The digitally calculated injection time t _I is given to an output stage, not shown in FIG. 1, and is output to the injection valves 11 as an analog opening time signal.

The control arrangement shown in FIG. 2 essentially has a similar structure to the control arrangement of FIG. 1. The same components have the same reference numerals as in Fig. 1 and are not explained again here. The difference from the control arrangement shown in FIG. 1 is that the control deviation .DELTA.-lambda is determined in another way. A target voltage 22 is used as the control target value, which in turn can depend on the throttle valve position α or the speed n.

Furthermore, the control arrangement according to FIG. 2 has a conversion unit 23 which stores the characteristic curve between the probe voltage difference and the air ratio difference coupled therewith. After the comparison of the actual probe voltage with the target probe voltage 22, this conversion unit 23 is supplied with a control deviation Δ-U _S , from which the control deviation Δ-Lambda is calculated. The further control sequence corresponds to the control sequence of the control arrangement according to FIG. 1, which is why, in order to avoid repetitions, it is not described again.

A continuous controller with PID behavior of the timing element 17 is particularly advantageously used to increase the control speed, the control deviation being multiplied by suitable factors for the respective P, I and D components, which are stored in characteristic diagrams as a function of the speed and load.

A mass offset between the probe mass and the mass of the analog / digital converter, not shown in the figures, would falsify the result of the measurement of the probe voltage. A correction device therefore eliminates this mass offset by measuring the minimum probe voltage that occurs in longer overrun phases (e.g. after 800 msec) and saves the difference to the expected minimum value using a filter as a correction variable for the probe voltages to be measured. To detect a negative ground offset, the probe voltage upstream of the analog / digital converter is increased in hardware by a fixed voltage value. This elimination of the mass offset leads to a higher accuracy in the detection of the probe output voltage and thus to a higher control accuracy of the continuous control device.

On the other hand, this correction device serves to compensate for a drift of the lean characteristic branch (increase) z. B due to aging. The mass offset alone can also be compensated for if necessary by using a differential amplifier.

To monitor the conversion capability of a catalytic converter, a second lambda probe is preferably arranged downstream of the latter, which emits a signal which, with optimal conversion of the exhaust gas pollutants into signal behavior, has a slight ripple around the temperature-stable value lambda = 1. A deviation from this temperature-stable point is advantageously used for offset correction / adaptation of the probe output voltage.

Claims (10)

1. Control system for controlling the air/fuel ratio in an internal-combustion engine (10) to an air ratio lambda to be maintained, the control system having an oxygen probe (lambda probe) (13), which is arranged in the exhaust system of the internal-combustion engine, is exposed continuously to the exhaust of the internal-combustion engine during operation thereof and the output voltage of which, which represents a measure of the air ratio lambda, changes essentially abruptly in the region of lambda equals one, characterised by a control device (12) for continuous-action control in a control region comprising lambda = 1, which determines the respective actual value of the air ratio lambda via the probe output voltage measured in each case in conjunction with an at least approximately predetermined probe-characteristic relationship between the size of the probe output voltage and the associated size of the air ratio lambda, and subtracts the set value of the air ratio lambda corresponding to the air ratio lambda to be maintained from the actual value of the air ratio lambda and controls the air/fuel ratio on the basis of the difference.

2. Control system with the features of the preamble of Claim 1, characterised by a control device (12) for continuous-action control in a control region comprising lambda = 1, which uses as set value a voltage which is assigned to the respective probe characteristic corresponding to the air ratio lambda to be maintained, and determines the air ratio difference via the difference of the actual values of the probe voltage measured in each case with the set value of the probe voltage in conjunction with an at least approximately predetermined relationship between the size of the voltage difference and the associated size of the air ratio difference, and controls the air/fuel ratio on the basis of the air ratio difference.

3. Control system according to Claim 1 or 2, characterised in that the probe-characteristic relationship is stored in a map (16; 23).

4. Control system according to Claim 3, characterised in that the probe voltage or the voltage difference and a variable dependent on the temperature of the probe are used as input parameters of the map (16; 23).

5. Control system according to Claim 1 or 2, characterised in that the probe-specific relationship is mapped by using mathematical functions.

6. Control system according to Claim 5, characterised in that a third order parabola is used.

7. Control system according to one of the preceding claims, characterised in that, when controlling to lambda equals one, the control device (12) has a continuous control action up to a predetermined small system deviation, of for example 3%, and the control device has a control action corresponding to a two-position control with a greater system deviation, of for example 6%, in the case of a greater system deviation.

8. Control system according to Claims 2 to 7, characterised in that the control set value of the voltage U_S(set) is adapted as a function of the measured maximum and minimum probe voltage (U_S(max), U_S(min)) according. to the formula $${\text{U}}_{\text{S(set)}}{\text{= (U}}_{\text{S(max)}}{\text{- U}}_{\text{S(min)}}{\text{x K + U}}_{\text{S(min)}}\text{,}$$

where K is a constant factor which is determined on the basis of the probe characteristic.

9. Control system according to Claims 1 to 8, characterised in that the probe voltage measured in each case is superimposed by an offset correction.

10. Control system according to Claims 1 to 9, characterised in that the control device (12) is designed as a device with continuous PID action.

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