Method and device for the dynamic monitoring of a lambda probe

A method for the dynamic monitoring of a first lambda probe arranged in an exhaust-gas duct of an internal combustion engine upstream of an exhaust-gas purification system. A period of an output signal of the first lambda probe is determined in a controller of the internal combustion engine, and a lambda regulating signal is determined from an output signal of a second lambda probe connected downstream of the exhaust-gas purification system. A first threshold value for a lengthening of the period of the output signal of the first lambda probe is predefined, in that a characteristic signal (46) is derived from the lambda regulating signal, in that a second threshold value for an inadmissible deviation of the characteristic signal (46) is predefined, and in that an inadmissible asymmetric delay of the first lambda probe is inferred if the lengthening of the period exceeds the first threshold value and the characteristic signal (46) deviates from the second threshold value outside predetermined limits.

BACKGROUND OF THE INVENTION

The invention relates to a method for the dynamic monitoring of a first lambda probe arranged in an exhaust-gas duct of an internal combustion engine upstream of an exhaust-gas purification system, a period of an output signal of the first lambda probe being determined in a controller of the internal combustion engine, and a lambda correction being determined from an output signal of a second lambda probe connected downstream of the exhaust-gas purification system.

The invention also relates to a method for detecting a defect upstream of a second lambda probe arranged in an exhaust-gas duct of an internal combustion engine and connected downstream of an exhaust system, a lambda correction being determined in a controller of the internal combustion engine from an output signal of the second lambda probe.

The invention also relates to a device for the dynamic monitoring of a first lambda probe arranged in an exhaust-gas duct of an internal combustion engine upstream of an exhaust-gas purification system, a circuit arrangement or a program sequence being provided in a controller of the internal combustion engine, by means of which circuit arrangement or program sequence a period of an output signal of the first lambda probe can be determined, and a second lambda probe for determining a lambda correction being connected downstream of the exhaust-gas purification system.

Lambda probes are used in the exhaust tract of internal combustion engines to measure the oxygen content of the exhaust gas in order to control the composition of the air/fuel mixture of the internal combustion engine. Lambda probes may be designed as step probes, the output signal of which falls abruptly from 0.9 V to 0.1 V in the event of a change of the lambda value from 0.995 to 1.005. The output signal of the lambda probe is supplied to an engine controller which controls the metering of the fuel in such a way that, temporally on average, a lambda value of lambda=1 is adhered to, at which the catalytic converters arranged in the exhaust tract provide their optimum purification action. If a lambda probe ages, this can lead to a delayed reaction of the output signal to lambda changes, a so-called impairment of dynamics. In this way, the composition of the exhaust gas may intermittently deviate from a value suitable for an optimum purification action of the catalytic converters. Legal regulations therefore stipulate that the aging of the lambda probe must be monitored with regard to an impairment of its dynamics. A slowing of the reaction of the lambda probe can be detected from a lengthening of the period of the lambda regulation, which can therefore be taken into consideration as a criterion for aging.

If the delay of the reaction of the lambda probe is asymmetrical with regard to rich-lean and lean-rich lambda changes, this can lead to a change in the mean lambda value controlled by the engine controller, as a result of which the purification action of the catalytic converters is particularly disadvantageously reduced. This may be observable even in the case of a delay which cannot be detected from period-based monitoring.

If a leak occurs in the exhaust tract upstream of the second lambda probe, air can be sucked into the exhaust-gas duct and, by means of its oxygen content, increase the lambda value determined by the second lambda probe. As a result, an undesirably rich mixture is supplied to the internal combustion engine.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a method which can detect an asymmetrical delay of the reaction of the output signal of a lambda probe.

It is therefore also an object of the invention to provide a method which can detect a leak in the exhaust-gas duct upstream of the second lambda probe.

It is also an object of the invention to provide a device for monitoring an asymmetrical delay of a lambda probe.

The object of the invention relating to the method for detecting an asymmetrical delay of a lambda probe is achieved in that a first threshold value for a lengthening of the period of the output signal of the first lambda probe is predefined, in that a characteristic signal is derived from the lambda regulating signal, in that a second threshold value for an inadmissible deviation of the characteristic signal is predefined, and in that an inadmissible asymmetric delay of the first lambda probe is inferred if both the lengthening of the period exceeds the first threshold value and also the characteristic signal deviates from the second threshold value outside predetermined limits. The derivation of a characteristic signal from the lambda regulating signal, determined by means of the second lambda probe connected downstream of the exhaust-gas purification system, or from a signal derived from said lambda regulating signal, using the proportional and integral components of the output signal of the second lambda probe permits a faster detection of an asymmetrical delay than is possible from the period of the lambda signal of the first lambda probe and from the integral component, as is conventionally used in the prior art, of the lambda signal of the second lambda probe. Instead of the absolute period, a lengthening of the period of the output signal of the first lambda probe may also serve as a measure, and a threshold value may be predefined for this. By means of the method according to the invention, it is possible, as prescribed, for an asymmetrically delayed reaction of the first lambda probe to be detected within three driving cycles.

In a particularly advantageous embodiment, the characteristic signal is determined from the lambda regulating signal by virtue of the lambda regulating signal being limited to in each case a predefinable minimum value and maximum value and being filtered with a time constant of between 5 seconds and 50 seconds, preferably with a time constant of 10 seconds. This permits a particularly fast settling time of the characteristic signal, and therefore a fast detection of an asymmetrical delay of the first lambda probe.

An undesirably early response of the detection of an asymmetrical delay may be avoided by virtue of an inadmissible asymmetrical delay of the first lambda probe being inferred if, beyond a predefined time span, both the lengthening of the period exceeds the first threshold value and also the characteristic signal deviates from the second threshold value outside predetermined limits.

The object of the invention relating to the method for detecting a leak upstream of the second lambda probe is achieved in that, from the lambda regulating signal, a leakage signal is determined by virtue of the lambda regulating signal being limited to in each case a predefinable minimum value and maximum value and being filtered with a time constant of between 5 seconds and 50 seconds, preferably with a time constant of 10 seconds, and in that a leak is inferred if, in a load-speed range with high pulsation of the lambda value, the leakage signal lies further in the lean direction than a predefinable threshold value. In the event of a leak, oxygen enters from the ambient air, which oxygen is compensated by the lambda regulation based on the output signal of the second lambda probe. A leak has the effect that, in a selected load-speed range with high pulsation, for example around zero load at 2000 revolutions per minute, the leak signal is higher to an inadmissible extent than the steady-state value of the rest of the operating characteristic map. Since a leak acts only in the “lean” direction, it is necessary to monitor only a threshold in the “rich” direction.

The object of the invention relating to the device is achieved in that the controller of the internal combustion engine comprises a circuit arrangement or a program sequence by means of which an inadmissible asymmetrical delay of the first lambda probe can be inferred from a comparison of the period of the output signal of the first lambda probe with a threshold value and a comparison of a characteristic signal determined from the lambda regulating signal with predefined threshold values. By means of the device, it is possible to realize a passive diagnostic method for an asymmetrically delayed lambda probe. An active intervention of the lambda regulator based on the output signal of the first lambda probe is therefore not necessary.

DETAILED DESCRIPTION

FIG. 1shows a first lambda signal diagram10in which a lambda signal13of a first lambda probe arranged in an exhaust-gas duct of an internal combustion engine upstream of an exhaust-gas purification system is shown along a first signal axis11and a first time axis12. If such a lambda probe ages asymmetrically, such that the output signal of the lambda probe reacts with a delay in the case of a lambda value varying in the “lean” direction, a delayed lambda signal15is generated which is delayed in relation to the lambda signal13by a probe delay14. In this asymmetrically delayed lambda signal15, voltage increases are slower than increases of the lambda signal13, whereas the voltage decreases take place at the same speed. The period of the delayed lambda signal15is longer than the period of the lambda signal13by a period lengthening16.

FIG. 2shows a regulation signal diagram20of a signal profile of a two-position regulator based on the output signal of the first lambda probe. The signal diagram20shows a regulating signal23along a second signal axis21and a second time axis22. The regulating signal23has a rising ramp24, a first delay time25and a normal falling ramp31. A lean-rich signal28has a lean-rich step29occurring at the same time as the end of the rising ramp24. At the end of the normal falling ramp31, the lean-rich signal28has a rich-lean step30. The described signal profile with the normal delay time25has a normal centroidal axis33with which the normally conventional lambda shift is attained, which takes place here in the direction of slight enrichment.

As a result of the unidirectional delay of the lambda probe, a lengthening of the rising ramp24by a rise lengthening26takes place until, after the probe delay14, the probe signal steps from “lean” to “rich”. This is followed by a second delay time27which is of the same length as the first delay time25. Over the further course of the signal profile, via a lengthened falling ramp32, the signal returns to the non-delayed level and then continues in the same way as the normally falling ramp31. The period lengthens by more than the delay time of the lambda probe, the period lengthening16takes effect. The period lengthening16is dependent on the ratio of step and ramp component and may, for example in the case of a dominant ramp component, amount to twice the value of the delay time of the probe, as is the case in the example illustrated inFIG. 2. This results in a longer residence time on the rich side than on the lean side, as a result of which the trimmed centroidal axis34lies further into the rich range than the normal centroidal axis33which should be set for an optimum purification action of the exhaust-gas purification system.

FIG. 3shows a signal analysis diagram40in which a proportional component43and an integral component44of a lambda regulating signal of a post-cat regulation arrangement of the internal combustion engine are shown along a third signal axis41and a third time axis42. The post-cat regulation arrangement serves for the correction of lambda deviations on the basis of the output signal of a second lambda probe connected downstream of the exhaust-gas purification system in the exhaust-gas duct of the internal combustion engine. Here, the integral component44is used for the correction of the lambda deviations. According to the invention, a characteristic signal46is attained from the integral component44taking into consideration the proportional component43. For this purpose, the sum of the integral component44and proportional component43is limited to predefinable threshold values, and thus form the limited lambda regulating signal45. From the limited lambda regulating signal45, the characteristic signal is attained by means of a time filter with a filter constant of between 5 and 50 seconds, typically a filter constant in the region of 10 seconds. From the signal analysis diagram40, it can be clearly seen that the characteristic signal46settles more quickly than the integral component44, and a faster detection of an asymmetrical delay of the first lambda probe is therefore made possible.