Method for heating a catalyst used in internal combustion engine with direct fuel injection

Abstract

A method for heating a catalytic converter in internal combustion engines is introduced wherein: an index for the temperature of the catalytic converter is formed; below a predetermined temperature of the catalytic converter, a first heating measure takes place wherein the temperature of the exhaust gas is increased; and, above the predetermined temperature, alternatively or supplementary to the first heating measure, a second heating measure takes place wherein a reaction-capable mixture is supplied to the catalytic converter in addition to the exhaust gas and the reaction of this mixture releases heat in the catalytic converter.

Description

[0001] It is already known to heat up the catalytic converter because of the consequences of a deterioration of the efficiency of the engine combustion. A deterioration of efficiency of the engine combustion can, for example, be caused by a deviation of the ignition time point from the optimal time point. The optimal time point is defined by the maximum efficiency. With a loss of efficiency, the exhaust gas is hotter in comparison to the operation without a loss of efficiency. Accordingly, the exhaust gas develops an increased heating operation in the catalytic converter.

[0002] In gasoline-direct injection engines, the possibility is present to inject fuel in a targeted manner into the cylinder after the engine combustion in the expansion stroke when operating with air excess. Here, the subsequently injected fuel reacts with the excess air of the engine combustion partially in the combustion chamber and partially in the exhaust-gas system. The heat, which is released in the exothermal reaction, heats the catalytic converter.

[0003] An efficiency can be assigned also to the catalytic converter heating measures. The efficiency of the after-injection is at a maximum when the chemical energy, which is introduced additionally into the catalytic converter as air/fuel mixture, is completely converted into heat in the catalytic converter. Up to now, both of the above-mentioned measures were utilized alternatively. Here, losses in the heat effect were observed. The losses are referred to the heating action which is achieved with the maximum efficiency of the catalytic converter heating measure.

[0004] The invention is directed to improving the efficiency of the catalytic converter heating measures.

[0005] This improvement is achieved with the features of claim 1.

[0006] In detail, the method of the invention relates to the heating of a catalytic converter in internal combustion engines with the steps:

[0007] forming an index for the temperature in the exhaust-gas system;

[0008] triggering a first heating measure wherein the temperature of the exhaust gas is increased below a predetermined temperature of the exhaust-gas system;

[0009] triggering a second heating measure above the predetermined temperature wherein a reactionable mixture is supplied to the catalytic converter in addition to the exhaust gas and the reaction in the exhaust gas and catalytic converter releases heat there and this takes place alternatively or as a supplement to the first heating measure.

[0010] A further embodiment of the invention provides that, as a first measure, a deterioration of the efficiency of the engine combustion takes place via a change of the ignition angle.

[0011] Another embodiment provides that, as a second measure, in an engine having gasoline-direct injection, a fuel after-injection takes place after the combustion.

[0012] A further embodiment provides that the after-injection is combined with stratified operation.

[0013] A further embodiment provides that the air quantity, which is inducted by the internal combustion engine, is throttled to the extent that the needed heat flow is achieved for a requested higher temperature. A throttling of this kind improves the after-reaction in the catalytic converter (only slightly lean lambda improves the conversion in the catalytic converter), the exhaust-gas temperature is higher and the spatial velocity of the exhaust gas is less which leads to a longer dwell time of the exhaust gas in the catalytic converter.

[0014] A further embodiment provides that an exhaust-gas composition is adjusted which departs from the stoichiometric exhaust-gas composition for heating an NOx-storage catalytic converter in homogeneous operation.

[0015] The invention also relates to an electronic control arrangement for carrying out the method.

[0016] In the context of one embodiment, a deterioration of the efficiency of the engine combustion via a change of the ignition angle takes place as a first measure.

[0017] As a second measure, a fuel after-injection can take place after the combustion in an engine having gasoline-direct injection.

[0018] The above-mentioned after-injection can, for example, be especially combined with stratified operation.

[0019] An engine control program is known from DE 198 50 586 which controls the switchover between stratified operation and homogeneous operation.

[0020] In stratified operation, the engine is operated with an intensely stratified cylinder charge and high air excess in order to achieve the lowest possible fuel consumption. The stratified charge is achieved via a late fuel injection which, in the ideal case, leads to a partitioning of the combustion chamber into two zones: the first zone contains a combustible air/fuel mixture cloud at the spark plug. The first zone is surrounded by the second zone and this second zone comprises an insulating layer of air and residual gas. The potential for optimizing consumption results from the possibility of operating the engine substantially unthrottled while avoiding charge exchange losses. The stratified operation is preferred at comparatively low loads.

[0021] At higher load, when the power optimization is primary, the engine is operated with a homogeneous cylinder charge. The homogeneous cylinder charge results from an early fuel injection during the induction operation. As a consequence, a longer time is available for mixture formation up to the combustion. The potential of this operating mode for power optimization results, for example, from utilizing the entire combustion chamber volume for filling with a combustible mixture.

[0022] In a further embodiment of the invention, an after-injection takes place after a combustion with air excess in combination with a throttling of the air quantity inducted by the internal combustion engine. The after-injected fuel quantity determines the heat amount which is to be released. The throttling of the air supply effects a metering of the air quantity for this purpose. This air quantity can, for example, be designed so that a stoichiometric air/fuel ratio results in the exhaust gas from the sum of the regularly injected fuel and the after-injected fuel. This makes possible an exhaust-gas decontamination with a three-way catalytic converter. The air supply is thereby throttled to the extent that the necessary heat flow is reached for a requested temperature.

[0023] An exhaust-gas composition can be adjusted for heating an NOx-storage catalytic converter during homogeneous operation which deviates from the stoichiometric exhaust-gas composition.

[0024] This invention is based upon the fact that the reaction of the after-injected fuel requires an air excess in the combustion chamber. This takes place substantially during stratified operation for internal combustion engines having gasoline-direct injection. Increased raw emissions occur because the additionally injected fuel is not completely combusted in the combustion chamber. Furthermore, the exhaust-gas temperatures are rather colder in stratified operation. For this reason, the increased emissions cannot be converted exothermally in the catalytic converters which are too cold. In this way, the heat energy of the fuel, which is not combusted in the combustion chamber, is lost.

[0025] In the procedure of the invention, first hot exhaust gas is generated during homogeneous operation with retarded ignition. In this way, the temperature of the catalytic converters is raised with clearly less raw emissions. If minimum temperatures for the catalytic converter are reached, then the increased raw emissions which occur for after-injection, can be better converted in the catalytic converters. In this way, the advantage of a significant reduction of the emissions and the advantage of an increased efficiency of the release of heat in the catalytic converter is achieved.

[0026] An exhaust-gas composition close to lambda equals one is achieved with throttling during the injection of additional fuel. Additionally, the exhaust gas has a higher temperature and a lower spatial velocity for the same fuel conversion. In this way, the emissions are further reduced because the conversion of the catalytic converters is again improved.

[0027] An exhaust-gas composition is adjusted in the case of a heating of an NOx-storage catalytic converter with this exhaust-gas composition departing from the stoichiometric exhaust-gas composition.

[0028] In this way, the stored NOx can be reduced when there is a rich exhaust gas.

[0029] For lean exhaust gas, the NOx discharge is prevented which would occur for exhaust gas having lambda equal one and increased temperature. In this way, an NOx peak in the exhaust gas is advantageously avoided.

[0030] In the following, an embodiment of the invention is explained with reference to the figures.

[0031] FIG. 1 shows the technical background of the invention; and,

[0032] FIG. 2 shows a flow diagram as an embodiment of the method of the invention.

[0033] In FIG. 1, 1 represents the combustion chamber of a cylinder of an internal combustion engine. The flow of air to the combustion chamber is controlled via an inlet valve 2. The air is drawn in by suction via an intake manifold 3. The intake air quantity can be varied via a throttle flap 4 which is driven by a control apparatus 5. The control apparatus 5 defines an embodiment of the electronic control arrangement of the invention in combination with the given method. The following are supplied to the control apparatus: signals as to the torque command of the driver (for example, via the position of an accelerator pedal 6); a signal as to the engine rpm n from an rpm transducer 7; a signal as to the quantity ml of the inducted air by an air quantity sensor 8; and, a signal US as to the exhaust-gas composition and/or exhaust-gas temperature from an exhaust-gas sensor 12. Additionally, a separate exhaust-gas temperature sensor or catalytic converter temperature sensor can be provided. The exhaust-gas temperature and/or catalytic converter temperature can, however, also be computed from the remaining operating parameters. This is known, for example, from U.S. Pat. No. 5,590,521. The exhaust-gas sensor 12 can, for example, be a lambda probe whose Nernst voltage indicates the oxygen content in the exhaust gas and whose internal resistance can be applied as an index for the probe temperature, exhaust-gas temperature and/or catalytic converter temperature. The exhaust gas is conducted through at least one catalytic converter 15 wherein toxic substances of the exhaust gas are converted (for example, a three-way catalytic converter) and/or are temporarily stored (NOx-storage catalytic converter).

[0034] In this technical background, the catalytic converter temperature can be measured (sensors 16 and 17) or can be modeled from operating variables of the engine. The modeling of temperatures in the exhaust-gas system of internal combustion engines is known, for example, from U.S. Pat. No. 5,590,521. Compared to the position in or ahead of the catalytic converter, the position after a pre-catalytic converter but ahead of an NOx-storage catalytic converter is to be preferred for BDE systems. The position of the temperature sensors is therefore not limited to the illustrated positions in or ahead of a catalytic converter. Also, a position after the catalytic converter can be considered.

[0035] The control apparatus 5 forms output signals for adjusting the throttle flap angle (&agr;) via an actuating member 9 and for driving a fuel injection valve 10 via which the fuel is metered into the combustion chamber. The control apparatus 5 forms these output signals from the above, and, if required, additional input signals as to other parameters of the internal combustion engine such as intake air temperature and coolant temperature, et cetera. Furthermore, a triggering of the ignition via an ignition device 11 is controlled by the control apparatus.

[0036] The throttle flap angle (&agr;) and the injection pulse-width (ti) are essential actuating variables, which are to be matched to each other, for realizing the desired torque, the exhaust-gas composition and the exhaust-gas temperature. A further significant actuating variable for influencing these variables is the angular position of the ignition relative to the piston movement. The determination of the actuating variables for adjusting the torque is the subject matter of DE 198 51 990 which is to be incorporated into the disclosure.

[0037] Furthermore, the control apparatus controls additional functions for achieving an efficient combustion of the air/fuel mixture in the combustion chamber, for example, an exhaust-gas recirculation (not shown) and/or tank venting. The gas force, which results from the combustion, is converted into a torque by the piston 13 and the crankshaft 14.

[0038] FIG. 2 shows a flow diagram of an embodiment of the method of the invention. For heating by means of after-injection, the engine control according to the invention requires minimum temperatures in the exhaust-gas system. Until these temperatures are reached, homogeneous operation with retarded ignition is required and adjusted, for example, as a first measure. This is realized via the step sequence 2.1, 2.2 and 2.3 in FIG. 2 and these steps are reached from a higher order engine control program. When the necessary temperatures are reached, the after-injection is permitted as a possible alternative (or as a second measure). A switchover to stratified operation takes place with after-injection in order to generate a higher heat flow. This is realized by the sequence of steps 2.1, 2.2 and 2.4 in FIG. 2. Here, the air flow is throttled to the extent that the necessary heat flow is achieved with a required temperature.

[0039] The throttling takes place in a first embodiment via a controlled closure of the throttle flap by a predetermined angle or to a predetermined opening angle. Stated otherwise, the throttling takes place uncontrolled in this example. The mixture composition should be close to lambda equal one for a maximum release of heat. Temporary mixture enrichment toward lambda values less than one can occur because of a dynamic driving operation with changing torque requests. In this way, the exhaust-gas emissions are deteriorated in an unwanted manner.

[0040] The after-injection is advantageously controlled with the aid of an available exhaust-gas probe to avoid a deterioration of the exhaust gas. In this way, a breakthrough of rich exhaust gas can be prevented. The term “breakthrough” is the occurrence of HC emissions rearward of the catalytic converter. As a further advantage, the exothermal energy released is maximally utilized at lambda equals one.

[0041] Because of the heat request, a necessary fuel quantity for the after-injection is determined for maximum possible throttling. In addition to the heat request, also the air requirement of the after-injection and the temperature increase because of the throttling must be considered. The latter is especially important in order to prevent overheating of components in the exhaust-gas system.

[0042] Alternatively to the control of the after-injected fuel quantity via the measured exhaust-gas lambda, the throttling can be controlled via the measured exhaust-gas lambda.

[0043] For reasons of safety, the control is so designed that the control intervention operates only to reduce but not to increase the after-injected fuel quantity. For exhaust gas which is continuously too lean, the throttling can be increased in lieu of an increase of the after-injected fuel quantity. A minimum value may not be exceeded in order to protect components.

Claims

1. Method for heating up a catalytic converter in internal combustion engines, characterized in that:

an index for the temperature of the exhaust-gas system is formed;

a first heating measure takes place below a predetermined temperature of the exhaust-gas system at which the temperature of the exhaust gas is increased; and,

above a predetermined temperature, alternatively or supplementary to the first heating measure, a second heating measure takes place for which a reaction-capable mixture is supplied to the catalytic converter in addition to the exhaust gas and the reaction of this mixture releases heat in the catalytic converter.

2. Method of claim 1, characterized in that, as a first measure, a deterioration of the efficiency of the engine combustion takes place via a change of the ignition angle.

3. Method of claim 1, characterized in that, as a second measure for an engine having gasoline-direct injection, a fuel after-injection takes place after the combustion.

4. Method of claim 3, characterized in that the after-injection is combined with stratified operation.

5. Method of claim 4, characterized in that the air quantity, which is inducted by the internal combustion engine, is throttled to the extent that the needed heat flow is achieved for a requested higher temperature.

6. Method of claim 1, characterized in that an exhaust-gas composition is adjusted for a heating of an NOx-storage catalytic converter in homogeneous operation, the exhaust-gas composition deviating from the stoichiometric exhaust-gas composition.

7. Electronic control arrangement for carrying out the method of the claims 1 to 6.