WO2002018764A1 - Method for heating up catalysts in the exhaust gas of internal combustion engines - Google Patents

Abstract

The invention relates to a method for heating up a catalyst (15) in the exhaust gas of an internal combustion engine (1) which can be operated in different operating modes and which allows selection of at least one of a plurality of heating measures. According to the inventive method, first an estimation is made for the plurality of heating measures whether an individual heating measure can provide the desired heating effect. In a subsequent estimation step it is estimated whether an individual heating measure can be carried out in the actual operating mode regarding the exhaust gas values and the operating mode of the internal combustion engine required for said heating measure. The operating mode is requested in which these requirements can be best met. Depending on the actual operating mode at least one possible heating measure is then activated. The invention further relates to an electronic control device for carrying out said method.

Description

Process for heating catalysts in the exhaust gas of internal combustion engines

Catalysts in the exhaust gas of internal combustion engines require a certain minimum temperature (light off temperature) in order to develop their pollutant-converting effect. This should be achieved as quickly as possible after a cold start. In engines that are operated with a lean fuel / air mixture, for example in engines with gasoline direct injection and nitrogen oxide storage catalytic converter, further and sometimes changing demands on the catalytic converter temperature result, for example, from a necessary desulfurization of the storage catalytic converter during driving operation. Desulfation, for example, temporarily requires a higher catalyst temperature than is required for storing the nitrogen oxides in normal operation.

Various measures for heating catalysts are already known. For example, engine combustion can take place with a mixture that is so rich that the exhaust gas still contains unburned fuel. The supply of secondary air to the exhaust gas creates a reactive mixture that heats up the catalyst through an exothermic reaction. Furthermore, engine combustion can take place with a lean mixture such that the exhaust gas still contains unused oxygen. In this case, a reactive mixture can be generated by supplying fuel to the exhaust gas.

It is also known to heat up the catalytic converter as a result of a deterioration in the efficiency of engine combustion. A. Efficiency deterioration of engine combustion can be brought about, for example, by a deviation of the ignition timing from the optimal timing, the optimal timing being defined by the maximum efficiency. Due to the loss in efficiency, the exhaust gas is hotter in comparison to operation without loss of efficiency. It therefore has an increased heating effect in the catalytic converter.

In engines with gasoline direct injection, different operating modes of the engine allow different measures for heating the catalytic converter.

From DE 198 50 586 an engine control program is known which controls the switchover between shift operation and homogeneous operation.

In shift operation, the engine is operated with a strongly stratified cylinder charge and a large excess of air in order to achieve the lowest possible fuel consumption. The stratified charge is achieved by means of a late fuel injection, which ideally leads to the combustion chamber being divided into two zones: the first zone contains a combustible air-fuel mixture cloud on the spark plug. It is surrounded by the second zone, the consists of an insulating layer of air and residual gas. The potential for optimizing consumption results from the possibility of operating the engine largely unthrottled while avoiding gas exchange losses. Shift operation is preferred at a comparatively low load.

At higher loads, when the focus is on performance optimization, the engine is operated with a homogeneous cylinder charge. The homogeneous cylinder charge results from early fuel injection during the intake process. As a result, there is more time available for mixture formation until combustion. The potential of this operating mode for performance optimization results, for example, from the use of the entire combustion chamber volume for filling with a combustible mixture.

To heat up a NOx storage catalytic converter in homogeneous operation, an exhaust gas composition can be set that deviates from the stoichiometric exhaust gas composition.

In gasoline direct injection engines, there is also the possibility of specifically injecting fuel into the cylinder after engine combustion in the expansion cycle when operating with excess air, that is to say preferably in shift operation. Here, the post-injected fuel reacts with the excess air from engine combustion, partly in the combustion chamber and partly in the exhaust system. The heat released in the exothermic reaction heats up the catalyst.

The object of the invention is to choose an optimal heating strategy in every operating state. This object is achieved with the features of claim 1.

Specifically, the catalyst according to the invention is heated in the exhaust gas of an internal combustion engine, which can be operated in different operating modes and in which at least one of several heating measures can be selected, so that it is initially estimated for several heating measures whether an individual heating measure provides the desired heating effect can and that it is further estimated whether an individual heating measure with regard to the exhaust gas values and the mode of operation of the internal combustion engine required to carry out the heating measure can be carried out in the current operating state and that the mode of operation in which the requirements can best be met is requested and that at least one possible heating measure is activated depending on the current operating mode. The current operating state is characterized, for example, by values for the catalytic converter temperature, the vehicle speed and the current load.

One embodiment is characterized in that, as a measure, the efficiency of the engine combustion deteriorates by changing the ignition angle.

Another measure provides that, as a further measure, a post-injection fuel is carried out in an engine with direct gasoline injection after combustion.

Another measure provides that the post-injection is combined with shift operation. Another measure provides that the amount of air drawn in by the internal combustion engine is throttled to such an extent that the required heat flow is reached at a required temperature.

Another measure provides that an exhaust gas composition that differs from the stoichiometric exhaust gas composition is set for heating a NOx storage catalytic converter in homogeneous operation.

The invention also relates to an electronic control device for carrying out the measures and method steps.

The different operating modes of the internal combustion engine with gasoline direct injection allow different measures for heating the catalytic converter. The assignment of heating measures and operating modes according to the invention enables the heating strategy to be optimized with a view to the operating state of the vehicle, which is determined, for example, by parameters such as catalytic converter temperature, vehicle speed and torque requirement.

The possible heating effects of different catalyst heating measures are advantageously estimated and compared with the heating effect requirement. The heating effect requirement for heating a catalytic converter leads, for example, to physical requirements for the quantity and the temperature of the exhaust gas flow which must be provided by the heating measure.

The operating limits for the individual operating modes are also taken into account. This makes it possible to choose an optimal heating strategy in every operating state. All requirements for active heating measures are formulated as heat flow and temperature requirements in the exhaust gas. This means that all requirements can be treated consistently.

An exemplary embodiment of the invention is explained below with reference to the figure. 1 shows the technical environment of the invention. Fig. 2 shows an embodiment of the invention in the form of a flow chart.

The 1 in FIG. 1 represents the combustion chamber of a ^• cylinder of an internal combustion engine. The inflow of air to the combustion chamber is controlled via an inlet valve 2. The air is sucked in via a suction pipe 3. The amount of intake air can be varied via a throttle valve 4, which is controlled by a control unit 5. The control unit receives signals about the driver's torque request, for example about the position of an accelerator pedal 6, a signal about the engine speed n from a speed sensor 7, a signal about the amount ml of the intake air from an air flow meter 8 and a signal Us about the exhaust gas composition and / or exhaust gas temperature supplied by an exhaust gas sensor 12. Exhaust gas sensor 12 can be, for example, a lambda probe whose Nernst voltage indicates the oxygen content in the exhaust gas and whose internal resistance is used as a measure of the probe, exhaust gas and / or catalyst temperature. The exhaust gas is passed through at least one catalytic converter 15 as part of an exhaust gas system 16, in which pollutants from the exhaust gas are converted and / or temporarily stored.

From these and possibly other input signals via further parameters of the internal combustion engine such as intake air and Coolant temperature and so on, the control unit 5 forms output signals for setting the throttle valve angle alpha by an actuator 9 and for controlling a fuel injection valve 10, through which fuel is metered into the combustion chamber of the engine. The control unit also controls the triggering of the ignition via an ignition device 11.

The throttle valve angle alpha and the injection pulse width ti are essential, coordinated manipulated variables. to realize the desired torque, the exhaust gas composition and the exhaust gas temperature and thus the catalyst temperature. Another essential control variable for influencing these variables is the angular position of the ignition relative to the piston movement. The determination of the manipulated variables for setting the torque is the subject of DE 1 98 51 990, which is to be included in the disclosure to this extent.

Furthermore, the control unit controls further functions to achieve efficient combustion of the fuel / air mixture in the combustion chamber, for example exhaust gas recirculation and / or tank ventilation, not shown. The gas force resulting from the combustion is converted into a torque by pistons 13 and crank mechanism 14.

In this technical environment, the catalytic converter temperature can be measured or modeled from the operating parameters of the engine. The modeling of temperatures in the exhaust tract of internal combustion engines is known, for example, from US Pat. No. 5,590,521.

For heating by means of post-injection, the engine control according to the invention requires minimum temperatures in the Exhaust system. Until these are reached, homogeneous operation with late ignition, for example, is required and stopped as a first measure. Once the necessary temperatures have been reached, post-injection is permitted as a possible alternative. There is a switch to shift operation with post-injection to generate a higher heat flow. The air flow is throttled so far that the required heat flow is achieved at a required temperature.

Throttling takes place in a first exemplary embodiment by a controlled closing of the throttle valve by a predetermined angle or to a predetermined opening angle. In other words: the throttling is unregulated in this example. The mixture composition should be close to 1 for maximum heat release near lambda. Dynamic driving with changing torque requirements can lead to temporary mixture enrichments with lambda values less than one. This can undesirably worsen the exhaust gas emissions.

To avoid deterioration of the exhaust gas, the post-injection is advantageously regulated with the aid of the existing exhaust gas probe. This can prevent a breakthrough of rich exhaust gas. Breakthrough denotes the occurrence of HC emissions behind the catalyst. As a further advantage, the exothermic energy release is used at a maximum of 1.

In detail, a fuel quantity required for post-injection with the maximum possible throttling is determined on the basis of the heating requirement. In addition to the heating requirement, the air requirement for post-injection and the temperature increase caused by throttling is taken into account. The latter is particularly important in order to prevent overheating of components in the exhaust system.

As an alternative to regulating the amount of fuel injected via the measured exhaust gas lambda, throttling can be controlled via the measured exhaust gas lambda.

Fig. 2 shows an embodiment in the form of a flow chart.

In step 2.1 it is checked whether there is a request for a catalyst heating measure. If this is the case, in step 2.2 there is an estimate for at least one heating measure X, whether it can provide the desired heating effect and whether this heating measure with regard to the exhaust gas values and the mode of operation of the internal combustion engine required to carry out the heating measure in the current operating state (exhaust system and /or

Catalyst temperature, vehicle speed, current load) is allowed.

The various heating measures are calculated and evaluated based on physical requirements (heat flow and temperature). The operating limits for the individual operating modes are also taken into account. This makes it possible to choose an optimal heating strategy in every operating state. For each heating measure, it is also checked whether it is possible in view of the exhaust gas values and the required operating mode in the current driving state, ie at the current catalytic converter temperature, vehicle speed and load. With this information, the individual heating measures can be evaluated depending on the operating state and a decision made for the best measure. The assessment of a heating measure takes place in a slow time grid for each heating measure when there is a demand. The necessary interventions are only calculated for the activated heating measure in a fast time grid in order to save computing time.

If necessary, the required optimal operating mode is requested in step 2.3. For example, if the exhaust system is still cold, post-injection in shift operation may not be optimal, because the exhaust gas system that is too cold cannot have the necessary after-reaction of the resulting mixture in the exhaust gas. In particular, too cold catalysts in the exhaust system cannot support the after-reaction. In this case it would be one

Efficiency deterioration due to late ignition is appropriate to increase the exhaust gas temperature. This can preferably be carried out in the operating mode with homogeneous operation. Accordingly, a switchover to homogeneous operation can take place in step 2.3.

The selected heating measure is then activated in step 2.4.

If, on the other hand, the query in step 2.1 is answered in the negative, no heating measure is requested and any activated heating measures are deactivated.

Claims

Expectations 1. Method for heating a catalyst in the exhaust gas of an internal combustion engine, - which can be operated in different operating modes and - In which at least one of several heating measures can be selected - It is estimated for several heating measures whether an individual heating measure can provide the desired heating effect - And it is estimated whether an individual heating measure with a view to the exhaust gas values and the operating mode of the internal combustion engine necessary to carry out the heating measure in the current operating state (Temperatures in the exhaust system, vehicle speed, current load) can be carried out - And where the operating mode is requested in which the requirements can best be met - And wherein at least one possible heating measure is activated depending on the current operating mode. 2. The method according to claim 1, characterized in that as a measure a deterioration of Efficiency of engine combustion takes place by changing the ignition angle. 3. The method according to claim 1, characterized in that as a further measure in an engine with gasoline direct injection after the fuel is carried out a post-injection. 4. The method according to claim 3, characterized in that the post-injection is combined with stratified operation. 5. The method according to claim 4, characterized in that the amount of air sucked in by the internal combustion engine is throttled to such an extent that the required heat flow is reached at a required temperature. 6. The method according to claim 1, characterized in that an exhaust gas composition is set for the heating of a NOx storage catalyst in homogeneous operation, which differs from the stoichiometric exhaust gas composition. 7. Electronic control device for performing the method according to at least one of claims 1-6.