EP1422410A2 - Method for operating a multi-cylinder internal combustion engine with NOx-catalyst - Google Patents

Abstract

The invention relates to a method for operating a multi-cylinder internal combustion engine (1), in whose exhaust gas stream a nitrogen oxide (NOx) storage catalytic converter (12) is arranged. In the method, if necessary, a transition from normal operation to heating operation for heating the NOx storage catalytic converter (12). In the heating operation, a part of the cylinders (2, 5) of the internal combustion engine (1) with a lean or stoichiometric air-fuel ratio (Lambda_1) and another part of the cylinders (3, 4) with a rich air-fuel ratio (Lambda_2) operated (so-called split lambda operation). In order to reduce disturbing torque fluctuations during a transition between the normal operation and the heating operation, it is proposed that a lambda efficiency (η (lambda_1)) of the cylinder operated with a lean or stoichiometric air-fuel ratio (lambda_1) (2, 5) the internal combustion engine (1) during the transition from the normal operation in the heating operation or from the heating operation in the normal operation does not exceed a predetermined gradient. This can be achieved, for example, by the lambda efficiency (η (lambda_1)) or an air-fuel ratio (lambda_1) of the lean-operated cylinders (2, 5) of the internal combustion engine (1) being time-dependent or depending on a rotational speed (n) or a desired torque (Mw) of the internal combustion engine (1) can be specified during the transition.

Description

The present invention relates to a method for Operating a multi-cylinder internal combustion engine, in whose exhaust stream is a nitrogen oxide (NOx) storage catalyst is arranged. In the process, if necessary, from a Normal operation in a heating operation for heating the NOx storage catalyst passed. During heating operation is at least temporarily part of the cylinder Internal combustion engine with a lean or stoichiometric Air-fuel ratio and another part of the Cylinders with a rich air-fuel ratio operated.

The invention also relates to a control device for controlling and / or rules of a multi-cylinder internal combustion engine, the one in an exhaust stream of the internal combustion engine arranged nitrogen oxide (NOx) storage catalytic converter has. The control unit transfers the internal combustion engine as needed from a normal operation to a heating operation for heating of the NOx storage catalyst. In the heating operation operates the control unit is a part of the cylinder Internal combustion engine with a lean or stoichiometric Air-fuel ratio and another part of the Cylinders with a rich air-fuel ratio.

Finally, the present invention also relates to a Computer program running on a computing device, in particular on a microprocessor, a controller for controlling and / or rules of a multi-cylinder internal combustion engine is executable.

State of the art

From DE 195 22 165 A1, for example, a multi-cylinder internal combustion engine with a in the Exhaust stream of the internal combustion engine arranged exhaust system known for emission control. The exhaust system includes a Three-way catalyst and a nitrogen oxide (NOx) storage catalyst. In a lean operation is always a Oxygen surplus present in the exhaust system. Out For this reason, nitrogen oxides (NOx) that are present in the exhaust gases contained during lean operation of the Internal combustion engine not in the three-way catalyst be reduced. To the delivery of such nitrogen oxides It is, therefore, to prevent the ambient atmosphere provided, these in the NOx storage catalyst at temperatures between 200 ° C and 500 ° C to reduce periodically with reducing agent. This Operation is called regeneration phase of the NOx storage catalyst designated. Such a thing Exhaust system in which a close-coupled three-way catalyst and downstream of a NOx storage catalyst used is in itself known from the prior art.

In addition, NOx storage catalysts must be periodically desulfurized. The active centers of the NOx storage catalysts have, in addition to their affinity for NOx, a high affinity for sulfur oxides (SOx). These also occur during combustion of the fuel and occupy primarily the active centers of the storage catalyst. The resulting sulfates are thermally stable so that they can not be released at normal operating temperature of the NOx storage catalyst. As a result, the storage capacity of the catalyst for nitrogen oxides decreases with increasing sulfur loading. Only at an elevated temperature in the catalyst above 600 ° C and at the same time reducing conditions (lambda <1), these sulfates are no longer thermodynamically stable and are released as hydrogen sulfide (H ₂ S) and sulfur dioxide (SO ₂ ). In order to maintain or restore the NOx storage capacity and to regenerate the catalyst, the storage catalytic converter has to be briefly operated at elevated temperatures (heating mode) at certain intervals. To do this, the storage catalytic converter must be heated to around 650 ° C while driving. The process of desulfurization is described in detail in EP 0 911 499 A2, for example. This document is expressly incorporated by reference.

For heating the NOx storage catalytic converter during the Heating operation is the internal combustion engine in one operated so-called split-lambda operation. That means, that a part of the cylinder of the internal combustion engine with a lean or at least stoichiometric air-fuel ratio (Lambda) (so-called lean cylinder bank) and the remaining part of the cylinder with a rich air-fuel ratio is operated (so-called fat Cylinder bank). Through the split lambda operation enter the internal combustion engine unburned fuel components and unburned oxygen in the NOx storage catalyst and come there to the reaction. The Heating of the NOx storage catalyst is then carried out by means the exothermic reaction of fuel and oxygen in the catalyst. In split lambda mode, the Temperature of the NOx storage catalyst to over 620 ° C. heated. At about 750 ° C, however, with a thermal Destruction of the catalyst to be expected. In split lambda mode So are the split requirements constantly varies so that the temperature of the catalyst in moved to the specified temperature window. The split lambda mode an internal combustion engine is described in detail in the DE 195 22 165 A1. Regarding the process and how the split lambda operation works expressly referred to this document. The Total exhaust lambda, that is the sum or the Arithmetic means of exhaust lambras with fat Mixture operated cylinder and the lean mixture operated cylinder, is stoichiometric or light skinny. The exhaust gas mixture then reacts in the front part of the NOx storage catalyst.

If the NOx storage catalytic converter is in the indicated temperature window, the stored sulfur can be discharged from the catalytic converter under reducing conditions (exhaust lambda <1). However, if the NOx storage catalytic converter is supplied with constant rich exhaust gas mixture as a reducing agent during desulfurization, sulfur-hydrogen (H ₂ S) is formed. This has an unpleasant odor, and for persons who are exposed to the exhaust gas, in particular inhale the exhaust gas, there is a risk of sulfur poisoning. To avoid this, the total air-fuel ratio may be subject to continuous fluctuations to periodically store unburned oxygen into the catalyst (so-called wobbly exhaust lambda).

During normal operation of an internal combustion engine is usually in all cylinders of the internal combustion engine same air-fuel ratio is set. The means that the air-fuel ratio of both Cylinder banks is the same size, for example lambda = 1, and lambda split mode is disabled. in the Heating mode or split lambda operation will be the Internal combustion engine, however, with different air-fuel ratios for the two cylinder banks the Internal combustion engine operated. The lambda efficiency of a Cylinder has its maximum in a range between easily rich and stoichiometric air-fuel ratio (Lambda ≈ 1). By operating the cylinder banks with a fat or a lean air-fuel ratio, can Lambda efficiency can be reduced up to 60%. Due to the different lambda efficiencies in the Normal operation and in heating mode, it can when switching between the two operating modes to clear Variations of a given by the internal combustion engine Torque come that is both subjective and also objectively extremely disturbing on the drivability of a with the Internal combustion engine equipped vehicle. The Torque fluctuations are as disturbing jerking the Internal combustion engine noticeable. The torque fluctuations occur especially with changes in the lean air-fuel ratio on.

The present invention is therefore the task Basically, when operating an internal combustion engine with a NOx storage catalyst when switching from one Normal operation in a heating mode and vice versa, in particular with dynamic variation of the cylinder lambda, unwanted fluctuations of one of the To reduce internal combustion engine output torque.

To solve this problem, the present invention proposes starting from the method of the type mentioned, that a lambda efficiency of having a lean or operated stoichiometric air-fuel ratio Cylinder of the internal combustion engine during the transition from Normal operation in the heating mode or by the Heating operation in normal operation a predetermined Gradient does not exceed.

Advantages of the invention

According to the invention, it has been recognized that Torque fluctuations in a transition of a Internal combustion engine from a normal operation in a split-lambda operation or, conversely, their cause is in themselves have fast-changing lambda efficiencies of the cylinders. These changes are caused by the changing ones Air-fuel ratios in the individual cylinders, there the total air-fuel ratio of all cylinders should remain as constant as possible. In particular, a change the air-fuel ratio of lean Air-fuel ratio operated cylinder affects strong on the lambda efficiency and thus on the Output from the internal combustion engine torque. By a limitation on the maximum change in Lambda efficiency the one with a lean air-fuel ratio operated cylinders can Torque fluctuations on subjectively and objectively acceptable values are limited, increasing driveability of a motor vehicle during the transition clearly is improved. The gradient to which the lambda efficiency is limited, is preferably such chosen that there is no disturbing jerking movements of the Internal combustion engine comes.

To limit the maximum allowable change in the Lambda efficiency, the course of the lambda efficiency at least for the duration of the transition between normal operation and split lambda operation be specified. But it is also conceivable that the Course of at least one size, which is indirect or directly affects lambda efficiency, is given. Such a size is for example the Air-fuel ratio or the air-filling of the operated a lean air-fuel ratio Cylinder. The gradients are chosen so that the Changes in lambda efficiency within specifiable Move boundaries. But it is also conceivable, the courses of the Lambda efficiency easy on predefinable thresholds to limit.

The air-fuel ratio of having a lean or stoichiometric air-fuel ratio (lambda ≥ 1) operated cylinder of the internal combustion engine can for example, depending on a load and speed-dependent operating state of the internal combustion engine be specified. It can be operating states in which the Air-fuel ratio leaner and others Operating conditions are defined in which the air-fuel ratio is a bit fatter, but still in the lean or at least in the stoichiometric range.

Depending on the operating condition in which Internal combustion engine is operated, will be different Values for the air-fuel ratio of the with a lean or stoichiometric air-fuel ratio operated cylinder specified. The air-fuel ratio the lean-burn cylinder can for example, based on a model or a model Map of the engine speed and load requirements be determined by the driver. The default air-fuel ratio can for example a lambda control supplied as a setpoint, which then a Lambda actual value for the lean-operated cylinder on the regulated setpoint.

The maximum lean exhaust lambda will depend on the current speed and that given by the driver Desired moment determined. Depending on a measured or modeled temperature, this lean exhaust lambda can on to the stoichiometric air-fuel ratio be moved. This makes it possible to get one desired temperature range for the NOx storage catalyst observed.

The air-fuel ratio of having a rich air-fuel ratio operated cylinder the Internal combustion engine is dependent on the given lean air-fuel ratio operated cylinder and of a predetermined total air-fuel ratio all cylinders of the internal combustion engine (so-called total lambda) given.

The air filling of all cylinders is through the Throttle valve adjusted so that the moment of all Cylinder on average to the desired torque given by the driver equivalent.

For example, if during operation of the Internal combustion engine the driver a new desire torque this leads to a corresponding change in the Air-fuel ratio of lean operated Cylinder of the internal combustion engine. That pulls one again Change in the air-fuel ratio with the fat Mixture operated cylinder after, wherein the total exhaust lambda remains substantially constant.

If the driver's desired torque changes only slightly or with a low gradient (stationary or quasi-stationary case), results for those operated with a lean air-fuel mixture Cylinder a steady transition from the original Operating state corresponding lean air-fuel ratio to the new operating state corresponding lean air-fuel ratio. If that is from the Driver given desired torque with a large Gradients, for example abruptly, change (more dynamically Case), for those with a lean air-fuel mixture operated cylinder also a sudden transition from that corresponding to the original operating state lean air-fuel ratio to that of the new one Operating state corresponding lean air-fuel ratio. It may at least be necessary for a short time the lean air-fuel ratio is stoichiometric (Lambda = 1), which is a deactivation of the Split lambda operation corresponds. The rich air-fuel ratio must be correspondingly leaps and bounds change, so the resulting total lambda on average remains equal to 1.

The split lambda operation is not just for heating the NOx storage catalyst before desulfurization, but also for keeping the NOx storage catalytic converter warm during desulfurization can be used. That's it required that the total exhaust lambda in time Medium is slightly fat.

According to an advantageous embodiment of the present Invention is proposed that the lambda efficiency that with a lean or stoichiometric air-fuel ratio operated cylinder the Internal combustion engine during the transition from the Normal operation in the heating mode or from the heating mode in normal operation falls linearly over time or increases. By such a ramp-shaped course The gradient of lambda efficiency can be simple Be limited.

Alternatively, it is proposed that the lambda efficiency of the cylinder of the internal combustion engine operated with a lean or stoichiometric air-fuel ratio during the transition from the normal operation to the heating operation or from the heating operation to the normal operation decreases or increases in a time-dependent manner along a sigmoid function , A sigmoid function has the equation (1 + e ^-cx ) ^-1 , whereby the factor c can be used to specify the steepness of the sigmoid function. When changing the lambda efficiency along a sigmoid function, care should be taken to ensure that the sigmoid function does not rise too steeply in its inflection point in order to avoid undesirable torque fluctuations in the transition from normal operation to heating operation. In addition to the ramp-like and sigmoid-like course, a large number of further variations of the lambda efficiency during the transition of the internal combustion engine between normal operation and split lambda operation are conceivable.

According to a preferred embodiment of the present invention Invention is proposed that the lambda efficiency that with a lean or stoichiometric air-fuel ratio operated cylinder the Internal combustion engine during the transition from the Normal operation in the heating mode or from the heating mode in normal operation as a function of a speed of Internal combustion engine and / or of a predetermined desired torque linear decreases or increases.

Alternatively, it is suggested that the lambda efficiency that with a lean or stoichiometric air-fuel ratio operated cylinder the Internal combustion engine during the transition from the Normal operation in the heating mode or from the heating mode in normal operation as a function of a speed of Internal combustion engine and / or of a predetermined desired torque along a sigmoid function drops or increases.

The embodiments in which the lambda efficiency speed-dependent and / or torque-dependent, are intended for those cases where during the Transition of the internal combustion engine and a speed change and / or a change in the predetermined desired torque he follows. Also a combination of these embodiments with Other methods are conceivable. That's the way it would be conceivable, if the heating operation of the NOx storage catalytic converter at constant speed and / or constant desired torque is activated, the lambda efficiency Specify time-dependent.

According to a further advantageous embodiment of The present invention proposes that the air-fuel ratio the one with a lean or operated stoichiometric air-fuel ratio Cylinder of the internal combustion engine time-dependent during the Transition from normal operation to heating mode initially rises steeply from an initial value and then slowly approaches a final value or during the Transition from heating to normal operation in reverse direction from the final value slowly and towards the end of the transition drops steeply to the initial value. Although the air-fuel ratio approaches the final value slow, but it reaches very well after one finite time. The final value is in the lean range (Lambda> 1) and corresponds to the air-fuel ratio after or before the transition.

According to another preferred embodiment of the Invention is proposed that the air-fuel ratio the one with a lean or stoichiometric Air-fuel ratio operated cylinder the Internal combustion engine as a function of a speed of Internal combustion engine and / or from a predefinable desired torque during the transition from normal operation to the heating operation starting from an initial value first rises steeply and then slowly approaches a final value or during the transition from the heating operation in the Normal operation in the opposite direction from the End value slowly and towards the end of the transition steeply on the Initial value drops. This embodiment, in which the Air-fuel ratio speed-dependent and / or is determined depending on the torque, is for such cases thought that during the transition of the Internal combustion engine also a speed change and / or Desired torque change takes place. Also a combination This embodiment with other methods is conceivable. So it would be conceivable, for example, if the heating operation of the NOx storage catalyst at a constant speed and / or constant desired torque is activated, the Specify air-fuel ratio time-dependent.

According to yet another advantageous embodiment of The present invention proposes that the inventive method at constant or slow varying predetermined desired torque of Internal combustion engine (stationary or quasi-stationary Case) is executed. Advantageously, the inventive method with rapidly varying predetermined desired torque of the internal combustion engine (dynamic case) disabled and instead the lambda efficiency or the air-fuel ratio of the a lean or stoichiometric air-fuel ratio operated cylinder of the internal combustion engine during the transition from normal operation to the Heating mode or from the heating mode to normal operation is suddenly raised or lowered.

According to a further preferred embodiment of the The present invention proposes that the air-fuel ratio the one with a rich air-fuel ratio operated cylinder of the internal combustion engine in Dependence on the given air-fuel ratio the one with a lean or stoichiometric Air-fuel ratio operated cylinder and one Total air-fuel ratio of all cylinders determined becomes.

Advantageously, for the withdrawal of sulfur oxides from the NOx storage catalyst, a regeneration operation activated, in which a total air-fuel ratio all cylinders between rich and lean back and forth is switched, wherein the time average of the total air-fuel ratio is fat. The total lambda the cylinder of the internal combustion engine can, for example be changed periodically.

Preferably, the total air-fuel ratio all cylinders through a variation of the air-fuel ratio the one with a rich air-fuel ratio operated cylinder of the internal combustion engine between slightly fat and slightly lean back and forth is switched. A variation of the air-fuel ratio the rich cylinder during lambda split operation an internal combustion engine affects much less on that of the internal combustion engine output torque as a variation of the lean ones Cylinder. In this way, the total lambda can be between bold and lean switched back and forth, the resulting torque fluctuations to a minimum reduced and with little effort by appropriate measures can be reduced or even compensated.

As a further solution to the problem of the present Invention is based on the control unit of the above mentioned type proposed that the control unit the Internal combustion engine during the transition from the Normal operation in the heating mode or from the heating mode in normal operation so controls and / or regulates that a Lambda efficiency of having a lean or operated stoichiometric air-fuel ratio Cylinder of the internal combustion engine a predefinable Gradient does not exceed.

According to an advantageous embodiment of the invention is suggested that the control device means of execution of the method according to the invention.

Of particular importance is the realization of the inventive method in the form of a Computer program. The computer program is on one Computing device, in particular on a microprocessor, executable and for the execution of the invention Suitable method. In this case, so the Invention realized by the computer program, so that this computer program in the same way the invention represents how the method to perform the Computer program is suitable. The computer program is preferably stored on a memory element. When Memory element may in particular an electrical Storage medium are used, for example, a Random access memory (RAM), a read only memory (ROM) or a flash memory.

drawings

Figur 1

eine erfindungsgemäße Brennkraftmaschine gemäß einer bevorzugten Ausführungsform;

Figur 2

ein Drehzahl-Drehmoment-Kennfeld zur Ermittlung eines Luft-Kraftstoff-Verhältnisses von mit einem mageren Kraftstoff-Luft-Verhältnis betriebenen Zylindern der Brennkraftmaschine aus Figur 1;

Figur 3

einen Schnitt durch das Drehzahl-Drehmoment-Kennfeld aus Figur 2 entlang der Linie A-A;

Figur 4

einen Schnitt durch das Drehzahl-Drehmoment-Kennfeld aus Figur 2 entlang der Linie B-B;

Figur 5

zeitliche Verläufe von Lambda-Wirkungsgraden bei einem Betrieb der Brennkraftmaschine aus Figur 1 gemäß einem erfindungsgemäßen Verfahren;

Figur 6

zeitliche Verläufe von Luft-Kraftstoff-Verhältnissen bei einem Betrieb der Brennkraftmaschine aus Figur 1 gemäß einem erfindungsgemäßen Verfahren;

Figur 7

einen zeitlichen Verlauf der Luft-Füllung bei einem Betrieb der Brennkraftmaschine aus Figur 1 gemäß einem erfindungsgemäßen Verfahren;

Figur 8

einen Verlauf des Lambda-Wirkungsgrades in Abhängigkeit von dem Luft-Kraftstoff-Verhältnis;

Figur 9

ein Drehzahl-Drehmoment-Kennfeld zur Ermittlung eines Lambda-Wirkungsgrades von mit einem mageren Kraftstoff-Luft-Verhältnis betriebenen Zylindern der Brennkraftmaschine aus Figur 1;

Figur 10

einen Schnitt durch das Drehzahl-Drehmoment-Kennfeld aus Figur 8 entlang der Linie C-C; und

Figur 11

einen Schnitt durch das Drehzahl-Drehmoment-Kennfeld aus Figur 8 entlang der Linie D-D.

Other features, applications and advantages of the invention will become apparent from the following description of embodiments of the invention, which are illustrated in the drawings. All described or illustrated features, alone or in any combination form the subject of the invention, regardless of their combination in the claims or their dependency and regardless of their formulation or representation in the description or in the drawings. Show it:

FIG. 1

an internal combustion engine according to the invention according to a preferred embodiment;

FIG. 2

a speed-torque map for determining an air-fuel ratio of lean air-fuel ratio cylinders of the internal combustion engine of FIG. 1;

FIG. 3

a section through the speed-torque map of Figure 2 along the line AA;

FIG. 4

a section through the speed-torque map of Figure 2 along the line BB;

FIG. 5

temporal profiles of lambda efficiencies in an operation of the internal combustion engine of Figure 1 according to a method according to the invention;

FIG. 6

time profiles of air-fuel ratios during operation of the internal combustion engine of Figure 1 according to a method of the invention;

FIG. 7

a time course of the air filling in an operation of the internal combustion engine of Figure 1 according to a method of the invention;

FIG. 8

a profile of the lambda efficiency as a function of the air-fuel ratio;

FIG. 9

a speed-torque map for determining a lambda efficiency of lean fuel-air ratio operated cylinders of the internal combustion engine of Figure 1;

FIG. 10

a section through the speed-torque map of Figure 8 along the line CC; and

FIG. 11

a section through the speed-torque map of Figure 8 along the line DD.

Description of the embodiments

FIG. 1 shows an internal combustion engine according to the invention of a motor vehicle in its entirety with the Reference numeral 1 denotes. The internal combustion engine 1 has four cylinders 2, 3, 4, 5 on. Of course, the Internal combustion engine 1 also a different number of cylinders, for example, two, three, five, six, eight, ten or twelve. The internal combustion engine 1 can be used as a be formed direct injection internal combustion engine, at the fuel via injectors (not shown) directly into combustion chambers of the cylinders 2, 3, 4, 5 is injected. In the combustion chambers, the mixes injected fuel with air that is over (not shown) inlet channels into the combustion chambers of the cylinder 2, 3, 4, 5 passes. The internal combustion engine 1 can also as an internal combustion engine with intake manifold injection be formed at the fuel in inlet channels of the Cylinder 2, 3, 4, 5 is injected. The fuel-air mixture then passes from the inlet channels in the Combustion chambers of cylinders 2, 3, 4, 5.

The total cylinder of the internal combustion engine 1 are at two split so-called cylinder banks. In the present Embodiment are two cylinders 2, 4 respectively 3, 5 summarized to a cylinder bank. From the Combustion chambers of the cylinders of a cylinder bank 2, 4; 3, 5 become the exhaust gases through outlet channels 7 each to a common pre-catalyst 8, 9 passed. The Pre-catalysts 8, 9 are, for example, as three-way catalysts educated. In the flow direction behind the Both pre-catalysts 8, 9 are each a lambda probe 10, 11 arranged in the exhaust stream to the air-fuel ratio (Lambda) in the exhaust behind the To detect pre-catalysts 8, 9. Alternatively or In addition, it is conceivable, before the two precatalysts 8, 9 lambda probes to arrange. Behind the two Pre-catalysts 8, 9, the exhaust gases are combined and passed together through a main catalyst 12, the For example, formed as a NOx storage catalyst is. Such exhaust system is due to the Line arrangement also referred to as Y-arrangement.

In the cylinders 2, 3, 4, 5 are piston back and forth movably guided. By burning the fuel-air mixture in the combustion chambers of the cylinders 2, 3, 4, 5 the pistons are set in motion. The linear Movement of the piston turns into a rotary motion Crankshaft 6 of the internal combustion engine 1 implemented.

Furthermore, a control unit 13 is provided, which a control and / or regulation of the internal combustion engine. 1 allows. The control unit 13 receives via input signals 14 Information about the operating status of the Internal combustion engine 1 or other components of Motor vehicle. The input signals 14 are from suitable sensors measured, for example, the lambda probes 10, 11, or are from other available sizes modeled. Input signals 14 include signals about the torque request of the driver Mw, the example. About the position of an accelerator pedal is recorded, signals about the engine speed n, which via a example. At Crankshaft 6 is arranged arranged speed sensor, or signals over the sucked air, over one Air mass meter are recorded. In the control unit 13 an electrical storage element 15 is provided which for example, is designed as a flash memory. On the memory element 15 is a computer program stored, which the control and / or regulating function of Controller 13, when it is on a computing device 16, which is designed in particular as a microprocessor, expires. For processing the computer program this is either by command or by section from the Memory element 15 via a data link 17 to the Calculator 16 transfer. Likewise, in reverse Direction results of calculations made in the context of Processing the computer program on the computing device 16 or received inputs 14 over the Data connection 17 transmitted to the memory element 15 and be stored there. The data connection 17 is for example, formed as a data bus. As part of the Processing of the computer program are output signals 18 generates, for driving the internal combustion engine 1 at suitable actuators, for example, to a throttle valve in the intake ports for varying the intake air amount, on Intake valves or exhaust valves of the combustion chambers or on Injectors are guided.

In a lean operation of the internal combustion engine 1 is always an excess of oxygen is present in the exhaust system. Out For this reason, nitrogen oxides (NOx) that are present in the exhaust gases contained during lean operation of the Internal combustion engine 1 not in the three-way catalyst 8, 9 are reduced. To the delivery of such nitrogen oxides It is, therefore, to prevent the ambient atmosphere provided, this in the NOx storage catalyst 12th trap and periodically with reducing agent to reduce. This process is described below Regeneration of the NOx storage catalyst 12 referred to.

In addition, the NOx storage catalyst 12 must be periodically desulfurized. The active centers of the NOx storage catalyst 12 have, in addition to their affinity for NOx, a high affinity for sulfur oxides (SOx). These also arise during the combustion of the fuel and occupy primarily the active centers of the storage catalyst 12. The resulting sulfates are thermally stable so that they can not be released at normal operating temperature of the NOx storage catalyst 12. As a result, the storage capacity of the catalyst 12 for nitrogen oxides decreases with increasing sulfur loading. Only at an elevated temperature in the catalyst 12 in the range of over 600 ° C and at the same time reducing conditions (lambda <1), these sulfates are no longer thermodynamically stable and are released as hydrogen sulfide (H ₂ S) and sulfur dioxide (SO ₂ ). In order to maintain or restore the NOx storage capacity and to regenerate the catalyst 12, the storage catalytic converter 12 has to be briefly operated at elevated temperatures for a short time. For this, the storage catalytic converter 12 has to be heated while driving to approximately 650.degree. During operation of the internal combustion engine 1 is temporarily switched from time to time of the Einspeicherphase to the regeneration phase. The process of desulfurization is described in detail in EP 0 911 499 A2, for example. This document is expressly incorporated by reference.

For heating the NOx storage catalytic converter 12 is the Internal combustion engine 1 in a so-called split lambda mode operated. That means' that part of the cylinder 2, 5 of the internal combustion engine 1 with a lean or at least stoichiometric air-fuel ratio (Lambda) and the remaining part of the cylinder 3, 4 with a rich air-fuel ratio is operated. By the Split lambda operation reaches the internal combustion engine 1 unburnt fuel and not burnt Oxygen in the NOx storage catalyst 12 and is there burned. The heating of the NOx storage catalytic converter 12 then takes place by means of the exothermic reaction of fuel and oxygen in the catalyst 12. In split lambda mode the temperature of the NOx storage catalyst 12 heated to over 620 ° C. At about 750 ° C, however, with a thermal destruction of the catalyst 12 to expected. In split lambda mode so are constantly the Split requirements about the lean air-fuel ratio Lambda_1 and the rich air-fuel ratio Lambda_2 varies, so the temperature of the catalyst 12 in the indicated temperature window emotional. The split lambda operation of an internal combustion engine is described in detail in DE 195 22 165 A1. Regarding the operation and operation of split lambda operation is expressly to this document Referenced.

The total exhaust lambda (2 / Lambda_ges = 1 / Lambda_1 + 1 / Lambda_2; Lambda_ges = 2.Lambda_1.Lambda_2 / (Lambda_1 + Lambda_2)), that is the mixing lambda of the exhaust lambras Lambda_1 of the lean-burn cylinder 2, 5 and the exhaust lambda Lambda_2 with the rich mixture operated cylinder 3, 4, is stoichiometric or light skinny. The exhaust gas mixture then reacts in the front part of the NOx storage catalyst 12.

If, however, rich exhaust gas mixture (Lambda_ges <1) is supplied as reducing agent to the NOx storage catalytic converter 12 during the desulfurization, sulfur-hydrogen (H ₂ S) can form. Apart from the unpleasant smell of sulfur-hydrogen, for persons who are exposed to the exhaust gas, in particular the exhaust gas inhale, the risk of sulfur poisoning. To avoid this, the total air-fuel ratio (Lambda_ges) may be subject to continuous, preferably periodic, fluctuations during operation of the internal combustion engine 1 (so-called wobbling exhaust lambda).

The changes in the air-fuel ratio in the Cylinder banks during operation of the internal combustion engine 1, especially with changes in the lean air-fuel ratio Lambda_1, too strong changes the resulting efficiency and thus fluctuations of the output from the internal combustion engine 1 torque to lead. The torque fluctuations are disturbing jerking the internal combustion engine 1 clearly noticeable and affect the driveability of the motor vehicle both subjective as well as objective.

By the present invention, the Torque fluctuations during the transition of Internal combustion engine 1 from a normal operation in the split-lambda operation and in the reverse direction of the split lambda mode significantly reduced to normal operation be, so that the drivability of the motor vehicle decisively improved.

FIG. 5 shows a profile of a lambda efficiency _η over time t. The course of the lambda efficiency _η (lambda = 1) for a stoichiometric air-fuel ratio is denoted by reference numeral 20. Reference numeral 21 denotes the profile of the lambda efficiency _η (lambda_2 <1) for a rich air-fuel ratio. The course of the lambda efficiency _η (lambda_1> 1) for a lean air-fuel ratio is designated by reference numeral 22. Reference numeral 23 denotes the profile of a mean lambda efficiency _η (lambda) _mittel, which results from the arithmetic mean of the two lambda efficiencies η (lambda_1) and η (lambda_2). The output from the internal combustion engine 1 torque is dependent on the lambda efficiency η. In a first region A, the split lambda mode is active, ie, the NOx storage catalytic converter 12 is heated. In a second region B, split lambda is inactive, ie the internal combustion engine is in normal operation. In between there is a transition area C, in which a transition is made between normal operation and heating operation.

FIG. 6 shows the course of the air-fuel ratio (lambda) over time t. The reference numeral 24 denotes the stoichiometric curve (lambda = 1). The time profile of the lean air-fuel ratio (lambda> 1) is designated by the reference numeral 25. The reference numeral 26 denotes the time course of the rich air-fuel ratio (lambda <1). FIG. 7 shows the course of the filling of the combustion chambers of the internal combustion engine 1 with air over the time t. FIG. 8 shows the relationship between the lambda efficiency _η (lambda) and the air-fuel ratio lambda. It can be clearly seen that the maximum actual torque Md_max is achieved at a slightly rich air-fuel ratio (lambda <1). An efficiency _η (lambda) of 100% is achieved at a stoichiometric air-fuel ratio (lambda = 1).

During normal operation in the area B, all the cylinders 2, 3, 4, 5 of the internal combustion engine 1 are operated with a stoichiometric air-fuel ratio (lambda = 1). The lambda efficiency _η (lambda) is about 100%. If now comes a request for heating the NOx storage catalytic converter 12, the internal combustion engine 1 goes into the heating mode in the area A. The time axis t is traversed from right to left. According to the invention, during the transition from normal operation (region B) to heating operation (region A), the lambda efficiency η (lambda) of the cylinders 2, 5 of the internal combustion engine 1 operated with a lean or stoichiometric air-fuel ratio lambda_1 is prevented exceeds a predetermined gradient.

According to the exemplary embodiment from FIG. 5, the profile 22 of the lambda efficiency η (lambda_1) of the cylinders 2, 5 operated with a lean or stoichiometric air-fuel ratio lambda_1 drops in the transitional region C in a time-dependent continuous and substantially ramp-shaped linear manner. The curve 22 of the lambda efficiency _η (lambda_1> 1) of the lean air-fuel ratio lambda_1 is specified, because changes in the air-fuel ratio there have a particularly pronounced effect on the lambda efficiency _η and thus on the output torque , The resulting curve 25 of the air-fuel ratio Lambda_1 is shown in FIG. For a given curve 22 of the lambda efficiency η (lambda_1), the profile 25 of the resulting air-fuel ratio can be determined on the basis of the relationships from FIG. 8 or using a characteristic diagram. The curve 26 of the cylinders 3, 4 of the internal combustion engine 1 operated with a rich air-fuel ratio Lambda_2, depending on the temperature of the NOx storage catalytic converter 12, becomes the lean air-fuel ratio Lambda_1 and the total air-fuel ratio Lambda_ges all cylinders 2, 3, 4, 5 of the internal combustion engine 1 determined. Again, with reference to the relationships of Figure 8 or based on a map is based on the curve 26 of the rich air-fuel ratio Lambda_2 the corresponding curve 21 of the lambda efficiency _η (Lambda_2) determined. From the curves 21 and 22 of the lambda efficiencies _η (lambda_1) and _η (lambda_2) results in the curve 23 of the mean lambda efficiency η (lambda) _mittel. The resulting filling curve is shown in FIG.

If in the reverse direction, for example, due to a termination request of the lambda split operation, to be transferred from the heating operation in the area A in the normal operation in area B, the time axis t is traversed from left to right. During active split lambda operation in the region A, the lambda efficiencies η (lambda_1> 1), η (lambda_2 <1) and thus also η (lambda) _mittel have a substantially horizontal course, that is to say they are essentially constant. The internal combustion engine 1 is in a stationary or quasi-stationary state. Under given boundary conditions (Lambda_ges ≈ 1), the lambda efficiency η (lambda_1) on the lean cylinder bank 2, 5 is continuously driven to 100% after aborting the operating mode lambda split. The aim is to limit the change in efficiency for the lean-operated cylinder 2, 5 to a specific gradient and to obtain a substantially constant desired torque. According to the above statements, the curve 25 of the lean air-fuel ratio lambda_1, the curve 26 of the rich air-fuel ratio lambda_2, the curve 21 of the lambda efficiency _η (lambda_2), the profile 23 of the average lambda efficiency η (lambda) _mittel and the filling curve.

Instead of a ramp-like curve 22 of the lambda efficiency _η (lambda_1) of the lean cylinders 2, 5, the profile 22 in the transition region C can also be made sigmoidal. A sigmoid function has the equation (1 + e ^-cx ) ^-1 , whereby the factor c can be used to specify the steepness of the sigmoid function. The sigmoid function is shown in dashed lines in FIG. 5 and designated by the reference numeral 22 '. Apart from the ramp-like and the sigmoid-like course 22 in the area C, a large number of other courses are conceivable. Decisive for the choice of the course 22 in the transition region C is that a predeterminable gradient is not exceeded.

According to a further embodiment, which in the figures 9 to 11 is shown, the curve 22 of the lambda efficiency η (lambda_1) with a lean air-fuel ratio operated cylinder 2, 5 not time-dependent, but depending on a rotational speed n and a desired torque given by a driver Mw of the internal combustion engine 1 predetermined. The course 22 of Lambda efficiency η (Lambda_1) becomes a characteristic map taken as in Figure 9 and in Figures 10 and 11 is shown in section. FIG. 9 can be seen that the lean air-fuel ratio lambda_1 at low speeds n, for example, at idle, and at a high desired torque Mw stoichiometric (Lambda_1 = 1). This will ensure that when idle and at extremely high load requirements Stoichiometric operation is driven and the operating mode Split lambda is suppressed. The lambda efficiency η (lambda_1) is then 100%. At medium speeds n and at a mean desired torque Mw is the Lambda efficiency η (lambda_1), however, particularly low (η (lambda_1) «100%). The resulting air-fuel ratio Lambda_1 is very lean (Lambda_1 >> 1). Depending on the measured or modeled temperature of the NOx storage catalyst 12 is the lambda efficiency continue towards 100% lambda efficiency shifted to the catalyst 12 in the to maintain the desired temperature window.

The speed and torque dependent transition between the Heating and normal operation can be linear (solid line in Figures 10 and 11). But it is also possible that the transition along a Sigmoid function (dashed line in FIGS. 10 and 11) or any other function.

It is quite conceivable, this embodiment, in which the Lambda efficiency η (lambda_1) as a function of Speed n and / or a desired torque Mw predetermined is combined with the embodiment in which the Lambda efficiency η (lambda_1) given as a function of time becomes. For example, from this embodiment the time-dependent default will be switched if the Transition at constant speed n and / or constant Desired torque Mw occurs.

According to yet another embodiment, incorporated in the Figures 2 to 4 is shown, the course 25 of the Air-fuel ratio Lambda_1 with a lean air-fuel ratio operated cylinder 2, 5 not time-dependent, but depending on a Speed n and a desired torque given by the driver Mw of the internal combustion engine 1 predetermined. Of the History 25 of the air-fuel ratio Lambda_1 is taken from a map, as shown in Figure 2 and in the Figures 3 and 4 is shown in section. Figure 2 can be taken that lean air-fuel ratio Lambda_1 at low speeds n, for example, at idle, and at a high desired torque Mw is stoichiometric (lambda_1 = 1). Thereby Ensures that at idle and at extremely high Load requirements a stoichiometric operation driven and the operating mode split lambda is suppressed. at mean speeds n and at a mean desired torque Mw is the air-fuel ratio Lambda_1 very lean (Lambda_1 >> 1). Depending on the measured or modeled temperature of the NOx storage catalyst 12, the air-fuel ratio Lambda_1 becomes lean Cylinders 2, 5 continue toward stoichiometric air-fuel ratio moved to the catalyst 12 in to hold the desired temperature window.

The speed and torque dependent transition between the Heating operation and normal operation can be done along in the Figures 3 and 4 drawn by solid line. But it is also conceivable that the transition along a steeper or flatter line (dashed lines in the FIGS. 3 and 4). The maximum value for the particularly lean air-fuel ratio (Lambda_1 >> 1) is preferably in the range of the burning limit of Fuel-air mixture in which the mixture just barely can be safely inflamed and completely burned.

It is quite conceivable, this embodiment, in which the Air-fuel ratio Lambda_1 as a function of Speed n and / or a desired torque Mw predetermined is combined with the embodiment in which the Air-fuel ratio Lambda_1 given time-dependent becomes. For example, from this embodiment the time-dependent default will be switched if the Transition at constant speed n and / or constant Desired torque Mw occurs.

The air-fuel ratio Lambda_2 with a rich air-fuel ratio operated cylinder 3, 4 the internal combustion engine 1 is a function of the Temperature of the NOx storage catalyst 12 and the given air-fuel ratio Lambda_1 with the a lean or stoichiometric air-fuel ratio operated cylinder 2, 5 and a predetermined Total air-fuel ratio (lambda_ges) of all Cylinders 2, 3, 4, 5 determined.

In FIGS. 2 and 9, two different cases are one Transition from an active split lambda mode into an inactive split lambda mode (normal operation) shown. In case 1, this changes from the driver specified desired torque Mw only slightly (stationary or quasi-stationary case). It takes place steady transition from the initial lean air-fuel ratio Lambda_1 to the richer air-fuel ratio Lambda_1 of the stoichiometric Operation. In case 2, this changes from the driver specified desired torque Mw erratic (more dynamic Case). In such a case may well be a jump in the output from the internal combustion engine 1 torque in Purchase, as this dynamics of the driver is wanted. It takes place instead of the one described above Transition (see transition region C in Figure 5) sudden transition between the heating mode and the Normal operation and consequently an abort of the operating mode Split-lambda.

The present invention provides in the form of a Feedforward control setpoint values for a downstream lambda control. With the invention, the transition to and realized from the mode lambda split such that the desired driver request torque implemented as a target torque can be and thus undesirable torque changes and Consequently, a jerking of the internal combustion engine 1 prevented can be. In the present invention, despite one (in the transition region C) of one Combustion / ignition to other changing air-fuel ratio (Lambda) a constant torque be set.

If the NOx storage catalytic converter 12 is in the desired temperature window between about 620 ° C. and 750 ° C. during the heating operation, the stored sulfur can be discharged from the catalytic converter 12 under reducing conditions (Lambda_ges on average over time <1). However, if rich NOx exhaust gas mixture is fed as reducing agent to the NOx storage catalytic converter 12 during desulfurization, sulfur-hydrogen (H ₂ S) is formed. This has an unpleasant odor, and for persons who are exposed to the exhaust gas, in particular inhale the exhaust gas, there is a risk of sulfur poisoning. To avoid this, the total air-fuel ratio Lambda_ges may be subject to continuous fluctuations to periodically store unburned oxygen into the catalytic converter 12 (so-called wobbly exhaust lambda). In previous proposals, the wobble of the total exhaust lambda (Lambda_ges) was generated by periodically changing all cylinder lambda. Advantageously, however, a lambda change will be pre-controlled only for the cylinders 3, 4 operated with a rich air-fuel ratio Lambda_2. Since the efficiency curve for a rich air-fuel ratio Lambda_2 has a significantly lower gradient, this can limit the gradient of the efficiency change.

If an additional heating power for the Storage catalytic converter 12 is required, this can by a delayed ignition of the cylinders 2, 3, 4, 5 provided become. The temperature of the lean-burned cylinders 2, 5 downstream three-way catalysts 8, 9 can either measured or modeled. At the Exceeding a limit temperature becomes split lambda Tempered to a destruction of three-way catalysts 8, 9 to avoid. Reach at least one of the three-way catalysts 8, 9 a critical temperature, that can Air-fuel ratio enriched for component protection become.

Claims (16)

Method for operating a multi-cylinder internal combustion engine (1), in whose exhaust gas stream a nitrogen oxide (NOx) storage catalytic converter (12) is arranged, in which, if necessary, from a normal operation to a heating operation for heating the NOx storage catalytic converter (12), in which a part of the cylinders (2, 5) of the internal combustion engine (1) with a lean or stoichiometric air-fuel ratio (lambda_1) and another part of the cylinders (3, 4) with a rich air-fuel ratio (lambda_2) operated is characterized in that a lambda efficiency (η (lambda_1)) of the lean or stoichiometric air-fuel ratio (Lambda_1) operated cylinder (2, 5) of the internal combustion engine (1) during the transition from the normal operation in the Heating operation or from the heating to normal operation does not exceed a predetermined gradient. A method according to claim 1, characterized in that the lambda efficiency ( _η (lambda_1)) of the lean or stoichiometric air-fuel ratio (Lambda_1) operated cylinder (2, 5) of the internal combustion engine (1) during the transition from the Normal operation in the heating or from the heating to normal operation time-dependent linear decreases or increases. A method according to claim 1, characterized in that the lambda efficiency (η (lambda_1)) of the lean or stoichiometric air-fuel ratio (Lambda_1) operated cylinder (2, 5) of the internal combustion engine (1) during the transition from the Normal operation in the heating or from the heating to normal operation time-dependent along a sigmoid function drops or increases. Method according to one of claims 1 to 3, characterized in that the lambda efficiency ( _η (lambda_1)). the cylinder (2, 5) of the internal combustion engine (1) operated with a lean or stoichiometric air-fuel ratio (lambda_1) during the transition from the normal operation to the heating operation or from the heating operation to the normal operation as a function of a rotational speed (n) the internal combustion engine (1) and / or linearly decreases or increases from a predefinable desired torque (Mw). Method according to one of claims 1 to 3, characterized in that the lambda efficiency (η (lambda_1)) of a lean or stoichiometric air-fuel ratio (Lambda_1) operated cylinder (2, 5) of the internal combustion engine (1) during the transition from the normal operation in the heating operation or from the heating operation in the normal operation in dependence on a speed (n) of the internal combustion engine (1) and / or by a predetermined desired torque (Mw) along a sigmoidal function decreases or increases. Method according to one of claims 1 to 5, characterized in that the air-fuel ratio (Lambda_1) operated with a lean or stoichiometric air-fuel ratio (Lambda_1) cylinder (2, 5) of the internal combustion engine (1) time-dependent during the transition From the normal operation in the heating operation, starting from an initial value first rises sharply and then slowly approaches a final value (lambda_1> 1) or during the transition from the heating operation in the normal operation in the reverse direction from the final value (lambda_1> 1) slowly and against End of the transition drops steeply to the initial value. Method according to one of claims 1 to 6, characterized in that the air-fuel ratio (lambda_1) operated with a lean or stoichiometric air-fuel ratio (lambda_1) cylinder (2, 5) of the internal combustion engine (1) in response to a Speed (s) of the internal combustion engine (1) and / or from a predefinable desired torque (Mw) during the transition from the normal operation in the heating operation initially rises steeply from an initial value and then slowly approaches a final value (lambda_1> 1) or during the transition from the heating mode to the normal mode in the reverse direction, starting from the final value (lambda_1> 1) drops slowly and towards the end of the transition steeply to the initial value. Method according to one of claims 1 to 7, characterized in that the method is carried out according to one of claims 1 to 7 at a constant or only slowly varying predetermined desired torque (Mw) of the internal combustion engine (1). A method according to claim 8, characterized in that the method according to one of claims 1 to 7 deactivated at rapidly varying predetermined desired torque (Mw) of the internal combustion engine (1) and instead the lambda efficiency (η (lambda_1)) or the air Fuel ratio (Lambda_1) of the lean or stoichiometric air-fuel ratio (Lambda_1) operated cylinder (2, 5) of the internal combustion engine (1) during the transition from the normal operation in the heating operation or from the heating operation to the normal operation abruptly raised or is lowered. Method according to one of claims 1 to 9, characterized in that the air-fuel ratio (lambda_2) of the operated with a rich air-fuel ratio cylinder (3, 4) of the internal combustion engine (1) in dependence on the predetermined air Fuel ratio (lambda_1) of the lean or stoichiometric air-fuel ratio operated cylinder (2, 5) and a total air-fuel ratio (Lambda_ges) of all cylinders (2, 3, 4, 5) is determined. Method according to one of claims 1 to 10, characterized in that for the recovery of sulfur oxides (SOx) from the NOx storage catalyst (12) a regeneration mode is activated, in which a total air-fuel ratio (Lambda_ges) of all cylinders (2 , 3, 4, 5) is switched between rich and lean, the time average (lambda_means) of the total air-fuel ratio (lambda_ges) being rich. A method according to claim 11, characterized in that the total air-fuel ratio (Lambda_ges) of all the cylinders (2, 3, 4, 5) by a variation of the air-Kraftstoffve'rverhältnis (Lambda_2) with a rich air-fuel -Ververhältnis (Lambda_2) operated cylinder (3, 4) of the internal combustion engine (1) between slightly rich and slightly lean switched back and forth. Control unit (13) for controlling and / or regulating a multi-cylinder internal combustion engine (1) which has a nitrogen oxide (NOx) storage catalytic converter (12) arranged in an exhaust gas flow of the internal combustion engine (1), wherein the control unit (13) controls the internal combustion engine (1). if necessary from a normal operation in a heating operation for heating the NOx storage catalytic converter (12) and converted in the heating operation, a part of the cylinders (2, 5) of the internal combustion engine (1) with a lean or stoichiometric air-fuel ratio (lambda_1) and operates another part of the cylinders (3, 4) with a rich air-fuel ratio (Lambda_2), characterized in that the control unit (13), the internal combustion engine (1) during the transition from the normal operation in the heating operation or from the heating operation in normal operation so controls and / or regulates that a lambda efficiency ( _η (lambda_1)) with a lean or stoichiometric air-fuel ratio (L Ambda_1) operated cylinder (2, 5) of the internal combustion engine (1) does not exceed a predetermined gradient. Control device (13) according to claim 13, characterized in that the control device (13) comprises means for carrying out a method according to one of claims 2 to 12. Computer program executable on a computing device (16), in particular on a microprocessor, of a control device (13) for controlling and / or regulating a multi-cylinder combustion engine (1), characterized in that the computer program for executing a method according to one of claims 1 to 12 is suitable when it runs on the computing device (16). Computer program according to Claim 15, characterized in that the computer program is stored on a memory element (15), in particular on a random access memory (RAM), a read-only memory (ROM). Only memory) or a flash memory is stored.

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