Patent ID: 12209519

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

A method and a device100are described in the following, using as example an internal combustion engine102shown inFIG.1, in which an air/fuel mixture is burned and its exhaust gas is expelled through a three-way catalytic converter104, which is located in an exhaust pipe106. Device100in the example is a control unit. Internal combustion engine102in the example is a gasoline engine.

The method and the device are not limited to this example, but rather are usable with other types of catalytic converters, as well.

In the event of an incomplete combustion of the air/fuel mixture in a gasoline engine, in addition to air and unburned fuel, nitrogen (N2), carbon dioxide (CO2) and water (H2O) as well as a variety of combustion products are expelled in an exhaust gas, of which hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (HOx) are limited by law.

According to today's state of technological development, the current exhaust-gas limit values for motor vehicles can only be adhered to using a catalytic exhaust-gas treatment. The pollutant components named are able to be converted by the use of a three-way catalytic converter104.

Using three-way catalytic converter104, a simultaneously high conversion rate for HC, CO and NOx is achieved in a narrow range about a stochiometric operating point at λ=1. The stochiometric operating point represents an example for a reference ratio of air and fuel. The range about the stochiometric operating point is referred to as three-way catalytic-converter window. For example, the three-way catalytic-converter window is defined as a range of λ±0.001 about the stochiometric operating point.

In order to operate three-way catalytic converter104in the three-way catalytic-converter window, in device100, an engine management is provided with a lambda control which is based on the signals of lambda probes upstream and downstream of three-way catalytic converter104.

In the example, a first sensor108is disposed as lambda probe upstream of three-way catalytic converter104and is connected via a first signal line110to device100. A second sensor112in the example is disposed as lambda probe downstream of three-way catalytic converter104and is connected via a second signal line114to device100.

First sensor108in the example is designed to determine a residual oxygen content of the exhaust gas and to transmit a value of the residual oxygen content in the form of an electric voltage to device100. Device100is designed to identify a composition of the exhaust gas upstream of three-way catalytic converter104on the basis of this voltage.

Second sensor112in the example is designed to determine a residual oxygen content of the exhaust gas and to transmit a value of the residual oxygen content in the form of an electric voltage to device100. Device100is designed to identify a composition of the exhaust gas downstream of three-way catalytic converter104on the basis of this voltage.

In the example, the lambda control determines at least one manipulated variable, with which a fuel quantity is supplied to internal combustion engine102for the operation. Internal combustion engine102is controlled with the manipulated variable via at least one control line116. The manipulated variable is determined as a function of a reference variable of a lambda controller, the reference variable being corrected with a pre-control variable.

For the lambda control of a composition of the exhaust gas in the exhaust upstream of three-way catalytic converter104, a residual oxygen content of the exhaust gas upstream of three-way catalytic converter104is measured by first sensor108. The pre-control variable is determined as a function of this.

For determining the reference variable, in addition the exhaust gas downstream of three-way catalytic converter104is analyzed using second sensor112. In the example, a residual oxygen content is determined in the exhaust gas downstream of three-way catalytic converter104. The reference variable is determined as a function of this.

As second sensor112downstream of three-way catalytic converter104, a two-step lambda probe is used in the example, which at λ=1, has a very steep characteristic curve, and therefore is able to indicate λ=1 very precisely.

The lambda controller is designed to correct small deviations from the reference ratio, e.g., λ=1, using the reference variable. The pre-control variable is determined in the example by a lambda pre-control. The lambda controller in the example is designed to be slow in comparison to the lambda precontrol.

The lambda pre-control is designed to rectify large deviations from λ=1 using the pre-control variable. As a result, the three-way catalytic-converter window is reached quickly again, for example, after phases with overrun fuel cutoff.

A departure from the three-way catalytic-converter window is not detectable until late on the basis of a voltage of the two-step lambda probe downstream of three-way catalytic converter104.

An alternative for the control of three-way catalytic inverter104on the basis of the signal of second sensor112downstream of three-way catalytic converter104is a control of at least one average oxygen filling level of three-way catalytic converter104.

This at least one average filling level is not measurable directly. In the example, the at least one average filling level is modeled using a filling-level model. In one example, device100is designed to carry out a model-based control of the at least one average filling level of three-way catalytic converter104as described, for instance, in German Patent Application No. DE 10 2016 222 418 A1.

The signal of first sensor108is converted by an input-emissions model into one or more input variables for the filling-level model. Using the variables calculated by the input-emissions model and possibly additional input variables, e.g., exhaust-gas temperature or catalytic-converter temperature, exhaust-gas mass flow and instantaneous maximum oxygen storage capacity of three-way catalytic converter104, first of all, at least one filling level of three-way catalytic converter104is modeled with the aid of a catalytic-converter model. This is described hereinafter using an average oxygen filling level as example. Secondly, with the aid of the catalytic-converter model, concentrations of the individual exhaust-gas components at the output of three-way catalytic converter104are calculated. They are converted for the adaptation into a signal, which is able to be compared to the signal of second sensor112.

The average oxygen filling level is adjusted in the example to a setpoint value which minimizes the likelihood of a departure from the catalytic-converter window. This leads to the lowest emissions possible, at best, to minimal emissions. The setpoint value is preferably prefiltered. The prefiltered setpoint value for the oxygen filling level is used as reference variable. The output signals of the pre-control and of the lambda controller are added up. The sum signal represents a setpoint ratio upstream of three-way catalytic converter104. Manipulated variable116is determined as a function of the setpoint ratio.

A multistage compensation of measuring and model uncertainties for such a control is described in German Patent Application No. DE 10 2018 251 725 A1

Rapid elimination of a relatively large deviation in terms of amount between a ratio of air and fuel upstream of three-way catalytic converter104and the ratio of air and fuel downstream of three-way catalytic converter104is, of course, possible in principle. For that, however, it is assumed that the ratio of air and fuel upstream of three-way catalytic converter104is determined correctly by first sensor108and the ratio of air and fuel downstream of three-way catalytic converter104is determined correctly by second sensor112at least under steady-state operating conditions.

Measurements with different second sensors112, e.g., two-step lambda probes and broadband lambda probes, downstream of three-way catalytic converter104show that this assumption is not necessarily fulfilled. In particular, deviations larger in amount are often not sensed correctly in practice.

In exemplary measurements, a two-step lambda probe used as second sensor112downstream of three-way catalytic converter104indicates only 1% for an actual deviation of 5%.

In other exemplary measurements, a two-step lambda probe used as second sensor112downstream of three-way catalytic converter104overestimates an actual deviation.

Possible reasons for this are, for example, a cross sensitivity of the two-step lambda probe for certain exhaust-gas components such as hydrogen or a temperature dependency of the two-step lambda probe or dependencies of the two-step lambda probe on an engine operating point, which are not taken sufficiently into account in a probe characteristic curve. One possible cause is a flat characteristic of the voltage-lambda characteristic curve away from λ=1.

In the example, it is provided to model the at least one filling level of three-way catalytic converter104using a model.

Device100is designed to correct an offset in a signal of first sensor108.

Device100is designed to predetermine the reference ratio of air and fuel in the exhaust gas at an output of catalytic converter104.

Device100is designed to receive a signal that characterizes a first residual oxygen content in the exhaust gas upstream of catalytic converter104, which is measured by first sensor108.

Device100is designed to determine a first actual ratio of air and fuel as a function of the first residual oxygen content.

Device100is designed to receive a signal that characterizes a second residual oxygen content in the exhaust gas downstream of catalytic converter104, which is measured by a second sensor112.

Device100is designed to determine a second actual ratio of air and fuel as a function of the second residual oxygen content.

For a first actual ratio in the case where the second actual ratio is greater than the reference ratio, device100is designed to determine a first offset of this first actual ratio relative to the reference ratio.

For a first actual ratio in the case where the second actual ratio is smaller than the reference ratio, device100is designed to determine a second offset of this first actual ratio relative to the reference ratio.

Device100is designed to correct the signal of first sensor108or the first actual ratio if a deviation is detected between the first offset and the second offset. In one example, device100is designed to detect that the deviation is smaller in amount than a limit value, particularly zero, a correction being omitted if it is detected that the deviation is smaller in amount than a limit value, particularly zero.

In one example, device100is designed to correct the signal of first sensor108with a correction value. In one example, device100is designed to correct the first actual ratio with a correction value.

The device is designed to determine the correction value as a function of the deviation. In the example, the device is designed to determine the correction value as a function of the deviation in terms of amount, that is, the difference in terms of amount between the first and second offset.

Device100is designed, for example, to determine the correction value as a function of a factor, and to multiply the deviation by the factor. For instance, the factor may be established in the application. As an example, the factor amounts to 75% or 80%. The factor may also be smaller or larger, for example, may be in the range of 50% to 90%.

In one example, device100is designed to predetermine a time characteristic of manipulated variable116for adjusting the air/fuel mixture. Manipulated variable116in the example is a setpoint value, deviating from the reference ratio, for the second actual ratio.

In one example, device100is designed to predetermine the time characteristic of manipulated variable116in such a way that the time characteristic of manipulated variable116includes a jump of manipulated variable116. In one example, device100is designed to predetermine a time characteristic of manipulated variable116in such a way that the time characteristic of manipulated variable116includes a combination of a jump of manipulated variable116with a subsequent ramp of manipulated variable116.

In one example, device100is designed to correct the signal of first sensor108or the first actual ratio in iterations.

In one example, device100is designed to determine in one iteration, a first correction value for the signal of the first sensor. In one example, device100is designed, in an iteration following this iteration, to determine a second correction value as a function of the signal of first sensor108corrected with the first correction value. In one example, device100is designed to determine in one iteration, a first correction value for the first actual ratio. In one example, device100is designed, in an iteration following this iteration, to determine a second correction value as a function of the first actual ratio corrected with the first correction value.

A method for operating internal combustion engine102is described in the following with reference toFIG.2.

The method includes a step200.

In step200, a reference ratio of air and fuel in the exhaust gas at an output of catalytic converter104is predetermined.

A step202is then carried out.

In step202, the time characteristic of manipulated variable116for adjusting the air/fuel mixture is predetermined.

In one example, the time characteristic of manipulated variable116includes the jump of manipulated variable116. In one example, the time characteristic of manipulated variable116includes the ramp of manipulated variable116. In one example, the time characteristic of manipulated variable116includes the combination of the jump of manipulated variable116with the subsequent ramp of manipulated variable116.

A step204is then carried out.

In step204, the first residual oxygen content is measured by first sensor108.

A step206is then carried out.

In step206, the first actual ratio of air and fuel is determined as a function of the first residual oxygen content.

A step208is then carried out.

In step208, the second residual oxygen content is measured by second sensor112.

A step210is then carried out.

In step210, the second actual ratio of air and fuel is determined as a function of the second residual oxygen content.

A step212is then carried out.

In step212, for the first actual ratio in the case where the second actual ratio is greater than the reference ratio, the first offset of the first actual ratio relative to the reference ratio is determined.

A step214is then carried out.

In step214, for the first actual ratio in the case where the second actual ratio is smaller than the reference ratio, the second offset of the first actual ratio relative to the reference ratio is determined.

A step216is then carried out.

In step216, it is checked whether or not the deviation between the first offset and the second offset is detected.

If the deviation is detected, a step218is carried out. Otherwise, step200is carried out.

In one example, the correction is omitted if it is detected that the deviation is smaller in amount than a limit value, particularly zero. In other words, in this case, after step216, step200is carried out.

In step218, the signal of first sensor108or the first actual ratio is corrected.

In one example, the signal of first sensor108is corrected with the correction value. In one example, the first actual ratio is corrected with the correction value.

The correction value is determined as a function of the deviation. In the example, the correction value is determined as a function of the deviation in terms of amount between the first offset and the second offset.

The correction value may be determined as a function of a factor, the deviation being multiplied by the factor.

Step200is then carried out.

In other words, in this iteration, the signal of first sensor108or the first actual ratio is corrected with the first correction value.

In the iteration following this iteration, the second correction value is determined as a function of the signal of first sensor108corrected with the first correction value or as a function of the first actual ratio corrected with the first correction value.

FIG.3shows a time characteristic of variables which ensues upon execution of the method by the use of manipulated variable116described for adjusting the air/fuel mixture.

The time characteristic is subdivided into a first phase302, a second phase304and a third phase306. Top graph308shows a time characteristic of an oxygen filling level in three-way catalytic converter104. Center graph310shows a voltage characteristic of second sensor112. Bottom graph312shows a characteristic of the first actual ratio with values between 0 and 1, the value 1 representing the reference ratio. In addition, in bottom graph312, a characteristic of a presumed correction value is represented with a dashed line, and a characteristic of an already learned and plausibilized correction value is represented with a solid line, starting from 0. In the example, the characteristic of the first actual ratio initially changes by Δλ1, the presumed correction value changing by x*Δλ1and the learned and plausibilized correction value not changing. In the example, the characteristic of the first actual ratio then changes by Δλ2, the presumed correction value changing by x*Δλ2and the learned and plausibilized correction value changing by x*Δλ1.

In first phase302, internal combustion engine102is operated in a normal state. In second phase304, there is suspicion of an error, that is, the suspicion that the deviation exists. In third phase306, internal combustion engine102is operated in a manner adapted to the deviation. In other words, the signal of second sensor112is corrected with the correction value.

A first reference voltage314, e.g., 710 mV, a second reference voltage316, e.g., 650 mV and a third reference voltage318, e.g., 350 mV are indicated for the signal of second sensor112. Second reference voltage316corresponds to a lower limit of the catalytic-converter window for the first actual ratio of, e.g., λ=0.999. Third reference voltage318corresponds to an upper limit of the catalytic-converter window for the first actual ratio of, e.g., λ=1.001. Other limits of the catalytic-converter window are handled correspondingly.

In the example, the signal of second sensor112begins at first reference voltage314and then falls below third reference voltage318. The signal of second sensor112subsequently rises again above second reference voltage316. In the example, this repeats once in first phase302and three times in second phase304. At the same time, the signal of second sensor112in the example remains below first reference voltage314up to the end of second phase304. In third phase306, the signal of second sensor112rises from a value below third reference voltage318up to the value of first reference voltage314and then bounces around the value of first reference voltage314. In the state thereby achieved, the deviation is corrected by the correction value.

First phase302includes a first reinitialization phase320of three-way catalytic converter104, in which the oxygen filling level, which in the example may assume values between 0 and 1, is increased from a first value322greater than 0 abruptly to a second value324less than 1, and then is decreased in ramp-like fashion to first value322. In this first reinitialization phase320, an intervention is carried out into the lambda control.

The time characteristic of the actual ratio begins in first phase302with a value of 1 and drops abruptly with the jump of the oxygen filling level to a value, lower in contrast, of greater than 0. While the oxygen filling level decreases in ramp-like fashion, the actual ratio at first remains unchanged in the example and then increases in ramp-like fashion to the value 1.

In the example, second phase304begins in a first iteration with a ramp-like decrease of the actual ratio beginning at a value of 1, followed by a ramp-like increase of the actual ratio up to a value lower than 1. During the ramp-like decrease, the signal of second sensor112rises with increasing gradient up to a value between first reference voltage314and second reference voltage316. When the signal of second sensor112reaches second reference voltage316, the ramp-like increase of the actual ratio takes place until the signal of second sensor112reaches third reference voltage318. At this point in time, the deviation and the correction value are determined. InFIG.3, two further iterations are depicted in second phase304, which proceed in corresponding manner. At the end of the third iteration, the correction value is determined iteratively and is used in third phase306for the correction.

Third phase306includes a second reinitialization phase326of three-way catalytic converter104, in which the oxygen filling level, which in the example may assume values between 0 and 1, is increased abruptly from a first value322greater than 0 to a second value324less than 1, and then is decreased in ramp-like fashion to first value322. In this second reinitialization phase326, an intervention is carried out into the lambda control.

The time characteristic of the actual ratio begins in third phase306with a value of 1 and drops abruptly with the jump of the oxygen filling level to a value, lower in contrast, of greater than 0. While the oxygen filling level decreases in ramp-like fashion, the actual ratio at first remains unchanged in the example and then increases in ramp-like fashion to the value 1.

When second sensor112unambiguously indicates a high or a low voltage, its voltage signal correlates with an instantaneous filling level of three-way catalytic converter104. This is especially the case when this voltage signal does not correspond to an actual ratio in the catalytic-converter window. In this case, three-way catalytic converter104is either freed of oxygen to the extent that rich exhaust gas is flowing out of three-way catalytic converter104, or is filled with oxygen to the extent that lean exhaust gas is flowing out of three-way catalytic converter104. This is utilized to reinitialize one or more modeled filling levels. In the reinitialization in the example, the modeled oxygen filling levels are reinitialized in several axial areas of three-way catalytic converter104, if an unambiguously high or low voltage of the lambda probe occurs downstream of three-way catalytic converter104. Owing to this reinitialization, the modeled filling levels of three-way catalytic converter104are brought into a defined state in which they agree at least approximately well with the corresponding filling levels of real three-way catalytic converter104.

Such a discontinuous reinitialization of the modeled filling levels leads in first phase302to a deviation of the average modeled filling level from the setpoint value, the deviation being corrected.

The reinitialization leads to an adjustment of the air/fuel mixture in the direction of the setpoint value of the filling-level control and brings three-way catalytic converter104very quickly in the direction of the catalytic-converter window. It thus leads directly to an improvement of emissions and simultaneously brings three-way catalytic converter104into a defined state in which, as expected, the reference ratio of λ=1, for example, appears downstream of three-way catalytic converter104.

However, the catalytic-converter window and the reference ratio of, e.g., λ=1 are actually only reached when the signal of first sensor108, on which the modeling of the adjusted oxygen filling level is based, exhibits no deviation. But if this is the case, then λ=1 does not appear downstream of three-way catalytic converter104, but rather a second actual ratio differing by this deviation from the reference value of, e.g., λ=1.

In the example, an emissions minimum is assumed at the reference value of, e.g., λ=1. The method is also usable if the setpoint value for the oxygen filling level to attain minimal emissions is a ratio differing slightly from λ=1.

If within a defined period of time, for example, in the sense of an exhaust-gas mass pushed through, following an alignment of the modeled oxygen filling levels of the catalytic converter with the signal of second sensor112, e.g., by the reinitialization of the modeled filling levels, a marked deviation is ascertained between the measured second actual ratio and the reference ratio, in second phase304, a deviation is assumed in the signal of first sensor108. The recognition of this deviation is possible in the example because the signal of second sensor112is regarded as trustworthy and second sensor112is operational. Optionally, a waiting time may be provided which must be observed before the deviation is detected or evaluated. Optionally, a minimum quantity of exhaust gas may be designated, which must be put through at the least, before the deviation is detected or evaluated.

If the deviation is detected, or if the deviation exceeds the limit value, at the beginning of second phase304, the model-based control of the filling level in three-way catalytic converter104is interrupted or reduced for a time. Preferably, the filling level continues to be observed or modeled with the aid of the filling-level model. Manipulated variable116thus resulting takes into account its output variable, though only reduced, or inhibits its influence completely.

In second phase304, the method described above is employed, which facilitates a rapid correction of the signal of first sensor104.

The method brings about an adjustment of the air/fuel mixture and thus of the first actual ratio. As a consequence, a reaction takes place in three-way catalytic converter104, which alters the signal of second sensor112.

Various time characteristics of the adjustment of the air/fuel mixture are possible, of which the combination of the jump with the subsequent ramp is illustrated inFIG.3. For example, at first the air/fuel mixture is adjusted in the direction of a rich mixture, until such time as the signal of second sensor112as shown inFIG.3, coming out of a range of a lean air/fuel mixture, reaches the reference ratio of λ=1 in the example, and then indicates a slightly rich air/fuel mixture, for example, λ=0,999. The method may also provide an adjustment up to the reference ratio of λ=1 in the example, without a slightly rich air/fuel mixture being set.

If this is the case, the direction of the adjustment of the air/fuel mixture is changed, here, for example, in the direction of a lean air/fuel mixture, until the signal of second sensor112as shown inFIG.3, coming out of a range of a rich air/fuel mixture, reaches the reference ratio of λ=1 in the example, and then indicates a slightly lean air/fuel mixture, for example, λ=0.001. The method may also provide an adjustment up to the reference ratio of λ=1 in the example, without a slightly lean air/fuel mixture being set.

In order to achieve the greatest possible accuracy of the ascertained deviation, it may be provided to determine the type and the time characteristic of the change of manipulated variable116as a function of the amount of the deviation.

A reaction of second sensor112—delayed as a result of finite system dynamics—to the adjustment of the air/fuel mixture is thus taken into account. For example, it is especially advantageous if, in response to a deviation which is great in terms of amount, manipulated variable116initially proceeds in stepped fashion and then in ramp-like fashion in order to achieve the reference ratio or a ratio in the catalytic-converter window as quickly as possible downstream of three-way catalytic converter104. On the other hand, in the case of a small deviation in terms of amount, it is advantageous, for example, if manipulated variable116proceeds in a ramp shape from the beginning.

Since second sensor112usually has very high accuracy in the catalytic-converter window, a difference of the change of manipulated variable116toward rich or lean necessary for the adjustments of the air/fuel mixture until second sensor112indicates a slightly rich or lean air/fuel mixture corresponds with relatively high accuracy to an actual deviation, even if the lambda indicated by second sensor112at the beginning of the adjustment should be falsified because of a defective vision of second sensor112away from the reference ratio. The accuracy of the ascertainment of the deviation is further increased owing to the fact that an approach toward the reference ratio or a slight exceeding of the reference ratio at second sensor112with respect to the air/fuel mixture takes place both from the lean and from the rich side. Thus, influences of defective vision are ruled out almost completely in the area of the reference ratio, as well.

The method offers several advantages. First of all, the speed of the correction is increased considerably, since because the reference ratio is exceeded repeatedly at second sensor112, the deviation is quantified very quickly and precisely. Because the adjustments are repeated several times, higher factors are possible for the deviation than when using other methods, without adversely affecting the robustness of the correction.

Due to the rich adjustments and lean adjustments following each other repeatedly, a deviation in the signal of first sensor108is compensated for stepwise very quickly, and at the same time, an effect of the respective previous compensation step is plausibilized. The method therefore leads directly to a compensation of a deviation of the second actual ratio from the reference ratio, caused by a deviation of the signal, so that lower emissions result more quickly than when using previous methods.