Method and device for exhaust gas aftertreatment in an internal combustion engine

The invention relates to a method for exhaust gas aftertreatment in an internal combustion engine. For purposes of the exhaust gas aftertreatment in the internal combustion engine, an exhaust gas system is provided in which a first three-way catalytic converter is arranged, as seen in the direction in which the exhaust gas of the internal combustion engine flows through the exhaust gas system, while at least another three-way catalytic converter is arranged downstream from the first three-way catalytic converter. Here, at least one lambda probe is arranged in an exhaust gas channel of the exhaust gas system upstream from the appertaining three-way catalytic converters. In the proposed method, a component temperature of the three-way catalytic converters is determined and compared to a light-OFF temperature. In this process, the lambda control of the internal combustion engine is carried out by means of the lambda probe upstream from the last three-way catalytic converter that has reached its light-OFF temperature.Moreover, according to the invention, an exhaust gas aftertreatment system for carrying out such a method is being proposed.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority from German Patent Application No. 10 2018 112 263.2, filed May 22, 2018, which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The invention relates to a method for exhaust gas aftertreatment in an internal combustion engine as well as to an exhaust gas aftertreatment system for carrying out such a method according to the generic parts of the independent claims.

BACKGROUND OF THE INVENTION

Future emissions legislation will make high demands with respect to engine raw exhaust gas emissions and to the exhaust gas aftertreatment in internal combustion engines. With the introduction of the emissions legislation Euro 6d-Temp, motor vehicles will have to meet the emissions limits during real-world operation (real driving emissions). The use of multi-stage exhaust gas aftertreatment concepts in the realm of gasoline engines raises questions for new emissions regulations in terms of the ability to regulate the various exhaust gas aftertreatment components as well as in terms of the quality of the regulation and the operational readiness of the exhaust gas aftertreatment components. In this context, it has been found that a lambda control concept with a rigidly configured control system only yields suboptimal emission results.

German patent application DE 10 2010 002 586 A1 discloses a method for operating an internal combustion engine for a motor vehicle having an exhaust gas system in which at least one catalytic converter and at least one lambda probe are installed. After a cold start, the internal combustion engine is operated alternately with a lean and a rich air-fuel ratio in order to warm up the catalytic converter. After the cold start, the lambda probe is warmed up in such a way that it is ready for operation after ten seconds at the maximum and the internal combustion engine is operated with a two-point regulation based on a signal from the lambda probe. In this process, a switchover between operation with a lean air-fuel ratio and operation with a rich air-fuel ratio is triggered in each case by the signal from the lambda probe.

German patent application DE 10 2013 210 734 A1 discloses a method for operating a lambda probe array in the exhaust gas system of an internal combustion engine having at least a first lambda probe upstream from a catalytic converter and with at least a second lambda probe downstream from the catalytic converter. In this context, the second lambda probe is configured as a step change sensor. Here, a diagnosis of a characteristic line offset of the first lambda probe and, if applicable, an adaptation of a characteristic line offset error are carried out. Here, for purposes of the diagnosis in the case of an active lambda adjustment, a value representing the oxygen storage capacity of the catalytic converter as well as another value representing the oxygen discharge capacity from the catalytic converter are ascertained. A characteristic line offset of the first lambda probe is calculated on the basis of the ratio of the oxygen storage capacity to the oxygen discharge capacity.

European patent application EP 2 884 066 A1 discloses a method for diagnosing an object such as a catalytic converter or a filter. For purposes of obtaining very precise information about the function of the catalytic converter, it is provided here that, in order to measure catalytic reactivity, a device is used to impinge a test medium having a defined composition, such as propane gas or carbon monoxide, onto an end face of the catalytic converter through an opening, and the concentration of at least one reduced or oxidized constituent of the test medium is measured at a downstream position after passage through the catalytic converter. Such an impingement with an (exhaust) gas having a defined composition, however, is only possible in the laboratory. Consequently, such a method is not suitable for optimizing the emissions under real-world driving conditions.

A drawback of the prior-art methods, however, is that they do not sufficiently take into account the temperature-dependent conversion behavior of the catalytic converters and thus they only lead to a suboptimal exhaust gas aftertreatment, especially during a starting phase or after an operating phase in which the exhaust gas system has cooled off considerably.

SUMMARY OF THE INVENTION

Before this backdrop, the invention is based on the objective of improving the conversion behavior of the catalytic converters arranged in the exhaust gas system of the internal combustion engine, thereby further reducing the emission of pollutants under real-world driving conditions.

According to the invention, this objective is achieved by a method for exhaust gas aftertreatment in an internal combustion engine whose outlet is connected to an exhaust gas system, whereby—as seen in the direction in which an exhaust gas of the internal combustion engine flows through an exhaust gas channel of the exhaust gas system—a first three-way catalytic converter is arranged in the exhaust gas system and at least another three-way catalytic converter is arranged downstream from the first three-way catalytic converter. Here, a first lambda probe is arranged in the exhaust gas channel upstream from the first three-way catalytic converter, while another lambda probe is arranged downstream from the first three-way catalytic converter and upstream from the other three-way catalytic converter. The method comprises at least the following steps:determination of the component temperatures of the first and second three-way catalytic converters,comparison of the component temperatures of the first and second three-way catalytic converters to the appertaining light-OFF temperatures of the three-way catalytic converters,lambda control of the internal combustion engine by means of the lambda probe upstream from the last three-way catalytic converter—as seen in the flow direction—that has reached its light-OFF temperature.

With a method according to the invention, the component temperatures momentarily prevailing on the three-way catalytic converters can be taken into account in order to expand the lambda control to the largest possible controlled segment and thus to variably adapt it to the conditions that are momentarily prevailing in the exhaust gas system.

Thanks to the features cited in the dependent claims, advantageous refinements and improvements of the method put forward in the independent claim are made possible.

In a preferred embodiment of the invention, it is provided that the lambda control is carried out on the basis of the principle of natural frequency control. Within the scope of this patent application, the term natural frequency lambda control refers to a control system in which the gas flows through the entire catalytic converter volume that is being used until the lean or rich breakthrough of each pilot-controlled component occurs. As a result, the “breakthrough” of the component can be detected at the lambda probe downstream from the catalytic converter volume through which the gas is flowing, and only then is a switchover made to the other pilot control. This approach means that it is indispensible to have a subsequent converter volume through another catalytic converter, especially another three-way catalytic converter. If the lambda control is carried out according to the principle of natural frequency control, then the entire converter volume of the catalytic converter can be used to convert the emissions. Moreover, in this process, ageing hot spots in the catalytic converter can be avoided by means of the alternative balanced switchover prior to a breakthrough, thereby increasing the service life of the catalytic converter. Furthermore, a rapid adjustment of the lambda middle position is possible over the entire exhaust gas system, whereby an offset can be adapted. As a result, it is possible to achieve a very effective exhaust gas aftertreatment with minimal emissions.

In a preferred embodiment of the method, it is provided that, after a cold start of the internal combustion engine, the lambda control is carried out by the first lambda probe upstream from the first three-way catalytic converter. Once the internal combustion engine has been started, first of all, the first catalytic converter near the engine warms up and, as a rule, it is the first component of the exhaust gas aftertreatment to reach its light-OFF temperature TLOK1. Therefore, during the starting phase, it is expedient to strive towards an optimal conversion of the exhaust gases through the first three-way catalytic converter until the exhaust gas aftertreatment components located further downstream—as seen in the direction in which the exhaust gas flows through the exhaust gas channel—have likewise reached their light-OFF temperature.

Here, when a three-way catalytic converter located further downstream in the exhaust gas system has reached its light-OFF temperature, it is preferable for the lambda control by the lambda probe to be expanded upstream from this additional three-way catalytic converter. By expanding the lambda control once the light-OFF temperature TLOK2of the second three-way catalytic converter has been reached, both three-way catalytic converters can be operated under the best possible operating conditions in order to convert pollutants, so that an emission optimum is reached for the exhaust gas aftertreatment.

In a preferred embodiment of the invention, it is provided for the exhaust gas aftertreatment system to also comprise a particulate filter, whereby, in addition to the component temperatures of the three-way catalytic converters, the component temperature of the particulate filter is likewise determined. Seeing that a limit value for the particle emissions has also been prescribed for gasoline engines since the emissions legislation Euro 6 went into force, it might be necessary to use a particulate filter for the exhaust gas aftertreatment in a large number of motor vehicles that have gasoline engines. The particulate filter can have a coating that has a three-way catalytic effect. Such a particulate filter is referred to as a four-way catalytic converter. Within the scope of this patent application, such a four-way catalytic converter is also to be understood as a three-way catalytic converter since it fulfills the function of a three-way catalytic converter.

Here, it is preferable if the possibility to regenerate the particulate filter above a threshold temperature of the particulate filter is detected. An oxygen excess in the particulate filter and, at the same time, a minimum temperature of 550° C. are both needed in order to oxidize the soot particles that have been retained in the particulate filter and in order to regenerate the particulate filter. When such a temperature is detected, it is very simple to carry out a (partial) regeneration of the particulate filter by adjusting the air-fuel ratio in the “lean” direction, that is to say, in the direction of a superstoichiometric air-fuel ratio.

It is especially preferable for the internal combustion engine to be operated at a superstoichiometric air-fuel ratio when it is detected that the particulate filter needs to be regenerated and, at the same time, when it is detected that the temperature of a component of the particulate filter is above the threshold temperature is detected. The superstoichiometric operation oxidizes the soot that has been retained in the particulate filter.

In another improvement of the method, it is provided that a superstoichiometric amplitude is selected by means of the control concept in such a way that a continuous regeneration of the soot that has been retained in the particulate filter is carried out within the relevant temperature range. Through the selection of a suitable amplitude, it is possible that no lean breakthrough occurs through the second three-way catalytic converter while the particulate filter is being regenerated and consequently, that no increase in the nitrogen oxide emissions occurs. Thus, an essentially emission-neutral regeneration of the particulate filter can be carried out.

Here, it is particularly preferred if a correspondingly larger quantity of oxygen in the exhaust gas is provided exclusively for the particulate filter and if an essentially stoichiometric exhaust gas flows through the three-way catalytic converter within the regulating oscillations. In this context, the amplitude of the lambda control can be selected in such a way that the oxygen storage units of the three-way catalytic converters, especially of the second three-way catalytic converter, are filled or emptied, without a lean or rich breakthrough occurring through the appertaining three-way catalytic converter. As a result, an increase in secondary emissions during the regeneration of the particulate filter can be avoided.

According to the invention, an exhaust gas aftertreatment system is being put forward for an internal combustion engine, having an exhaust gas system in which a first three-way catalytic converter is arranged—as seen in the direction in which an exhaust gas flows through an exhaust gas channel of the exhaust gas system—and at least another three-way catalytic converter is arranged downstream from the first three-way catalytic converter, whereby a first lambda probe is arranged upstream from the first three-way catalytic converter while another lambda probe is arranged downstream from the first three-way catalytic converter and upstream from the second three-way catalytic converter, and also having a control unit that is configured to carry out a method according to the invention when a machine-readable program code is being executed by the control unit. An exhaust gas aftertreatment system according to the invention makes it possible to take into account the appertaining operational readiness and its conversion capacity as a function of the component temperature, and thus to ensure an emission-optimal exhaust gas aftertreatment. Moreover, special operating situations such as, for example, the regeneration of a particulate filter, can be taken into account during the exhaust gas aftertreatment in order to further improve the result of the exhaust gas aftertreatment and in order to avoid secondary emissions.

In a preferred embodiment of the exhaust gas aftertreatment system, it is provided for a particulate filter to be arranged downstream from the first three-way catalytic converter and upstream from the second three-way catalytic converter. Seeing that a limit value for the particle emissions has also been prescribed for gasoline engines since the emissions legislation Euro 6 went into force, it might be necessary to use a particulate filter for the exhaust gas aftertreatment in a large number of motor vehicles that have gasoline engines. Thus, not only the gaseous exhaust gas constituents but also the particles can be removed from the exhaust gas.

The particulate filter can have a catalytically active coating and can be configured as a four-way catalytic converter. Thanks to the catalytically active coating on the particulate filter, the latter additionally fulfills the function of a three-way catalytic converter. Thus, the total catalytic converter volume that is available for converting pollutants can be increased, especially so as to make additional catalytic converter volume available at high loads and to avoid an increase in the emissions under real-world driving conditions.

In this context, it is preferable if a second lambda probe is arranged downstream from the first three-way catalytic converter and upstream from the particulate filter, and if a third lambda probe is arranged in the exhaust gas channel downstream from the particulate filter and upstream from the second three-way catalytic converter. As a result, a lambda control is possible for every component that is three-way catalytically active, so that an optimal result can be achieved in terms of the conversion capacity.

In an advantageous embodiment of the invention, a secondary air system is provided with which secondary air can be blown into the outlet of the internal combustion engine or into the exhaust gas system downstream from the outlet and upstream from the first three-way catalytic converter, especially downstream from the outlet and upstream from a turbine of an exhaust gas turbocharger, and into the exhaust gas channel. A secondary air system can accelerate the heating up of the three-way catalytic converters following a cold start of the internal combustion engine. Moreover, the oxygen needed to regenerate the particulate filter can be supplied without the internal combustion engine having to be operated at a superstoichiometric air-fuel ratio and without increasing the raw emissions of the internal combustion engine, especially the nitrogen oxide emissions.

Unless otherwise indicated in specific cases, the various embodiments of the invention put forward in this application can be advantageously combined with each other.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1shows an internal combustion engine10configured as a gasoline engine that is externally ignited by spark plugs18. The internal combustion engine10has an intake12, a plurality of combustion chambers14and an outlet16. The outlet16of the internal combustion engine10is connected to an exhaust gas system20. The internal combustion engine10is preferably configured as an internal combustion engine10that is charged by means of an exhaust gas turbocharger22. For this purpose, the exhaust gas turbocharger22has a turbine26that is arranged in an exhaust gas channel38of the exhaust gas system20and that drives a compressor24in an air supply system (not shown here) of the internal combustion engine10, thereby improving the filling of the combustion chambers14. In the exhaust gas channel38—as seen in the direction in which an exhaust gas flows through the exhaust gas channel38—a first three-way catalytic converter30is arranged downstream from the turbine26, a particulate filter32is arranged downstream from the first three-way catalytic converter30, and a second three-way catalytic converter36is arranged downstream from the particulate filter32. The particulate filter32can have a three-way catalytically active coating and can be configured as a so-called four-way catalytic converter34. A first lambda probe40is arranged downstream from the turbine26of the exhaust gas turbocharger22and upstream from the first three-way catalytic converter30, said first lambda probe40preferably being configured as a broadband lambda probe. A second lambda probe42is arranged downstream from the first three-way catalytic converter30and upstream from the particulate filter32or from the four-way catalytic converter34. A third lambda probe44is arranged in the exhaust gas channel38downstream from the particulate filter32or from the four-way catalytic converter34and upstream from the second three-way catalytic converter36. A first temperature sensor46is arranged in the exhaust gas channel38upstream from the first three-way catalytic converter30and upstream from the particulate filter32or from the four-way catalytic converter34. A second temperature sensor48is arranged downstream from the particulate filter32or from the four-way catalytic converter34and upstream from the second three-way catalytic converter36. The lambda probes40,42,44and the temperature sensors46,48are connected via signal lines to a control unit50of the internal combustion engine10. The internal combustion engine10can have a secondary air system28,54,56that comprises a secondary air pump28, a secondary air line54and a secondary air valve56. The secondary air line54opens up into the cylinder head on the outlet side of the internal combustion engine10or in a section of the exhaust gas channel38upstream from the first three-way catalytic converter30, especially downstream from the outlet16and upstream from the turbine26of the exhaust gas turbocharger22.

The invention puts forward a lambda control concept that takes into account knowledge about the component temperature (TK1, TK2, TOPF) of each individual exhaust gas aftertreatment component30,32,34,36and that adapts its amplitude of control and trim regulation to the largest possible controlled segment. Moreover, the boundary conditions of the exhaust gas aftertreatment components30,32,34,36are reflected in the amplitude of control and in the parameters of the controlled segment so that an optimal setting is achieved in terms of the best emission point that applies in each case.

The invention comprises a lambda control according to the principle of natural frequency control for a multi-stage exhaust gas aftertreatment system with more than one catalytic converter. Here, the momentarily prevailing component temperature TK1, TK2, TOPFof the exhaust gas aftertreatment components30,32,34,36, especially of the three-way catalytic converters30,36, is taken into account, either by means of sensors—especially by means of the temperature sensors46,48shown inFIG. 1—or else by means of an exhaust gas temperature model, in order to expand the natural frequency to the largest possible controlled segment. In case of a thoroughly warmed-up first three-way catalytic converter30, the natural frequency is effectuated by means of the first lambda probe40and by means of the parameters known for this controlled segment. As soon as the other exhaust gas aftertreatment components32,34,36have also been warmed up on the basis of the driving profile selected by the customer and as soon as they have reached their light-OFF temperature TLOK2, the lambda control is automatically expanded to these additional exhaust gas aftertreatment components32,34,36, especially to the second three-way catalytic converter36and, in each case, specifically the lambda probe42,44that is before the most recently activated exhaust gas aftertreatment component is used to evaluate control breakthroughs. If the activation conditions of a downstream exhaust gas aftertreatment component are no longer present, especially when the exhaust gas system20cools off or due to a systematic switching off of these exhaust gas aftertreatment components32,34,36, then the lambda control is reduced to the minimal control level, that is to say, exclusively to control by the first lambda probe40. Moreover, the special features of the appertaining exhaust gas aftertreatment component30,32,34,36can be taken into account within the scope of the lambda control. When an HC adsorber—preferably arranged near the engine—is used as the first component of the exhaust gas aftertreatment, the lambda control is configured in such a way that there is more of a tendency for excess unburned hydrocarbons (HC) to be formed during the cold start of the internal combustion engine10, since they can accumulate in the HC adsorber. In this case, a superstoichiometric control strategy is not conducive to achieve the envisaged objective.

When a particulate filter32,34is used, the proposed concept for lambda control can serve to select a superstoichiometric amplitude of control in such a way that a continuous regeneration of the soot retained in the particulate filter32,34takes place in the relevant temperature range. Here, on the basis of the known segment times, a formation of the amplitude of control can be selected in such a way that only for the particulate filter32,34is a correspondingly higher quantity of oxygen provided in the exhaust gas and stoichiometric operation is possible for the three-way catalytic converters30,36within one regulating oscillation. Thus, the method can achieve an optimum in terms of emissions.

FIG. 2shows a flow chart of a method according to the invention for exhaust gas aftertreatment. When the internal combustion engine10is started, a first method step <100> determines the component temperatures TK1, TK2of the three-way catalytic converters30,36and of other exhaust gas aftertreatment components32,34that might be present. In a method step <110>, these temperatures TK1, TK2are then compared to the individual light-OFF temperatures TLOK1, TLOK2. Initially, the natural frequency control is limited to the first three-way catalytic converter30and a lean or rich breakthrough through the first three-way catalytic converter30that has been detected by the lambda probe42is evaluated for the switchover of the amplitude of control, that is to say, for a switchover from a slightly substoichiometric operation to a slightly superstoichiometric operation and vice versa.

In another method step <120>, during continuous operation of the internal combustion engine10, the additional exhaust gas aftertreatment components32,34,36arranged downstream from the first three-way catalytic converter30also warm up and reach their light-OFF temperature TLOK2. Once the light-OFF temperature TLOK2has been reached in the second three-way catalytic converter36, the lambda control is expanded to the third lambda probe44and, if applicable, to additional lambda probes. In case of operation of the natural frequency control over several three-way catalytic converters30,36, special requirements of the exhaust gas aftertreatment can be taken into account. These include especially the warm-up operation, the regeneration of the particulate filter32,34, or a diagnostic function of the exhaust gas aftertreatment components30,32,34,36and/or of the lambda probes40,42,44.

If a method step <130> ascertains a component temperature of 550° C. or more for the particulate filter32,34, then oxidation of the soot retained in the particulate filter is possible. For this purpose, in a method step <140>, additional oxygen is provided by adjusting the air-fuel ratio of the internal combustion engine10to a superstoichiometric ratio or by blowing secondary air into the exhaust gas system20. Owing to the continuous lambda measurement and adaptation of the segment parameters of the controlled segment, the gas travel time through the exhaust gas system20to the particulate filter32,34is known and can be taken into consideration in the pilot control of the amplitude for the superstoichiometric operating section. As soon as a lean breakthrough at the second lambda probe42downstream from the first three-way catalytic converter30is detected, in a method step <150>, a certain additional quantity of oxygen is fed into the exhaust gas system20, thus effectuating a discharge of the soot mass from the particulate filter32,34.

When an HC adsorber is used, the loading of the HC adsorber can likewise be balanced and can be taken into account in the configuration of the superstoichiometric amplitude in order to regenerate the HC adsorber.

LIST OF REFERENCE NUMERALS