EP1581729B1 - Method for controlling an internal combustion engine and lean operating internal combustion engine - Google Patents

Abstract

The invention relates to a method for controlling an internal combustion engine (10) consisting of at least one catalyst arranged in the exhaust channel (14) of the internal combustion engine (10), and a lean operating internal combustion engine (10). According to the invention, when a substochiometric lambda value is required directly after a thrust disconnection phase of the internal combustion engine(10) and/or directly after an operating phase in which oxygen is stored in the catalyst, an approximately stochiometric or less substochiometric lambda value is initially adjusted in a transition phase and is subsequently brought down to the required substochiometric lambda value.

Description

The invention relates to a method for controlling an internal combustion engine and a lean-running internal combustion engine with the features mentioned in the preamble of claim 1 and 18 respectively.

For the aftertreatment of exhaust gases from internal combustion engines, this is usually purified by catalytic means. For this purpose, the exhaust gas is passed over at least one catalyst, which performs a conversion of one or more pollutant components of the exhaust gas. Different types of catalysts are known. Oxidation catalysts promote the oxidation of unburned hydrocarbons (HC) and carbon monoxide (CO), while reduction catalysts promote the reduction of nitrogen oxides (NO _x ) of the exhaust gas. Furthermore, 3-way catalysts are used to simultaneously catalyze the conversion of the three aforementioned components (HC, CO, NO _x ). In addition, storage catalysts, for example NO _x storage catalysts, are known. These are used in the exhaust gas purification of internal combustion engines, which are operated for reasons of consumption optimization at least temporarily in a lean operating mode, ie with an oxygen-rich exhaust gas with λ,> 1. The resulting nitrogen oxides NO _x can be implemented in a catalytic oxidative conversion of unburned hydrocarbons HC and carbon monoxide CO in only a very small extent to environmentally neutral nitrogen. To remedy the aforementioned NO _x storage catalytic converters are arranged in the exhaust ducts of internal combustion engines, which store in lean operating phases NO _x as nitrate. At intervals, the NO _x storage catalytic converter must be regenerated.

The NO _x storage catalytic converters used in lean-running gasoline engines for exhaust gas purification have lower high-temperature stability in the current state of development compared to conventional 3-way catalysts. The use of this catalyst technology therefore requires special efforts to limit the Temperature load of these catalysts. For this purpose, on the one hand measures come into consideration, which lead to the reduction of the stationary temperature levels, such as the exhaust gas cooling or the reduction of the residual oxygen content of the exhaust gases by optimizing the combustion process. On the other hand, measures are appropriate here as well as for 3-way systems, which lead to a reduction of load during transient engine operation, such as the optimization of the application in terms of HC peaks.

Particularly critical with regard to catalyst aging are high-temperature cycles with fuel cut-off phases, that is to say with interposed, unfired engine operation (see, for example, US Pat EP 093521A2 ). In addition to the increased raw HC emissions, which result from wall film effects or inaccuracies in the fuel metering in dynamic operation, the main causes are the high oxygen concentrations, which can lead to a reduction in the conversion performance of the catalysts due to oxidation and sintering processes. It may be necessary to prohibit overrun fuel cutoff at very high catalyst temperatures.

Another problem with thermal catalyst loading occurs in the transition from a fuel cut phase to fired engine operation when a stoichiometric mixture composition is immediately adjusted. During the fuel cut-off phase, the oxygen storage of the catalyst is completely filled, that is, oxygen is temporarily stored in the catalyst coating or the washcoat. If now the catalyst during re-insertion - for example, in a subsequent full-load acceleration - applied with very low-oxygen exhaust gas, the partial pressure difference leads to a very rapid dissolution of the stored oxygen. Since the exhaust gas contains substoichiometric engine operation high concentrations of combustible components (HC, CO, H ₂ ), it comes to violent oxidation reactions that can lead to exceeding the maximum allowable temperature of the coating at sufficiently high base temperature level of the catalyst at least locally.

The invention is therefore an object of the invention to provide a method for controlling an internal combustion engine with at least one disposed in an exhaust passage of the internal combustion engine catalyst and a corresponding internal combustion engine, in which or during the transition phase after a fuel cut-off phase, thermal load peaks of at least one catalyst by exothermic Processes are safely avoided.

This object is achieved by a method and an internal combustion engine with the features mentioned in claim 1 or 19.

In the method according to the invention for controlling an internal combustion engine with at least one catalytic converter arranged in an exhaust duct of the internal combustion engine, it is provided that, during a (operational) requirement of a substoichiometric lambda value immediately following a fuel cut-off phase of the internal combustion engine, initially in a transitional phase, an approximately stoichiometric or less sub-stoichiometric lambda value is set as requested and then to the requested lambda value, preferably to a predetermined value of an engine control of the internal combustion engine, whereby an oxygen storage of at least one catalyst, preferably at least one NO _x storage catalyst, which may comprise at least one precatalyst, gradually is emptied. In this case, the exhaust gas temperatures may be briefly above predetermined limit temperatures and cause the higher residual oxygen content of the exhaust gases increased Grundexothermie. Overall, however, this advantageously prevents too rapid emptying of the oxygen storage and thus also a sudden release of energy, which would lead to an excessive temperature peak or load on the catalyst.

The requirement of a substoichiometric lambda value can be based on a performance specification for the internal combustion engine or a specification of the engine control, for example due to the exceeding of a permissible exhaust gas temperature.

The transition phase is preferably subdivided into at least two phases, wherein in the transition phase as a whole or else only in at least one phase with different speeds or different steepness can be controlled to the required substoichiometric lambda value.

Preferably, upon request of a substoichiometric lambda value by the engine control of the internal combustion engine following a fuel cut-off phase initially at the beginning of the transition phase or in the first phase of the transition phase, a lambda value between 0.9 and 1.05, preferably between 0.93 and 1.02, and more preferably set between 0.97 and 1.0 and then gradually or continuously diverted to the originally requested lambda value, wherein the stepwise or continuous Absteuerung can take place in the transition phase as a whole or only in at least one phase.

In this case, preferably the measure for controlling the lambda value in the transition phase or in at least one of the phases can be made dependent on exceeding a temperature threshold for at least one of the catalysts.

The transition phase or preferably at least one of the phases should preferably take place with a duration of at least ten working cycles, in particular of at least thirty working cycles of the internal combustion engine.

The change or the ablation of the lambda value in the transition phase or in at least one phase of the transition phase should preferably take place with an average enrichment rate of -0.01 to 0.3 s ^-1 , in particular of about -0.1 S ^-1 .

A particularly preferred embodiment of the method according to the invention is given when using an oxygen-sensitive measuring device, which is arranged downstream of at least one catalyst. As an oxygen-sensitive measuring device, for example, a lambda probe or a NO _x sensor can be used with appropriate measurement function. In this case, at the beginning of the re-start of the fired operation of the internal combustion engine, the lambda before the at least one catalyst to a lambda value, preferably <1.00, optimally between 0.92 and 0.99, ideally between 0.94 and 0.96 is, set. By means of at least one oxygen-sensitive measuring device located downstream of the internal combustion engine, but upstream of an at least first catalytic converter, the actual lambda can be regulated to the desired specification in a known manner.

The operation with the lambda value predetermined in this way is maintained until a lambda threshold value is reached below the oxygen-sensitive measuring device downstream of an at least first catalytic converter or downstream of the most stress-critical catalytic converter. This threshold is close to 1.00, preferably in the range 0.95 to 1.03, more preferably 0.97 to 1.01 and ideally 0.98 to 0.9995.

After falling below the threshold value, the lambda value can be lowered as described to the originally requested lambda value determined in a known manner. It is also possible according to the invention, after falling below the measured threshold value, to delay the lowering of the lambda value for a short period of time from 0 to 4000 ms, ideally 100 to 1000 ms and optimally 200 to 500 ms, in order to eliminate at least almost complete emptying of the stored oxygen to ensure that at least one catalyst.

Both the lambda value at the beginning of the transition phase or at the beginning of the first phase and the enrichment rate are preferably at least one depending on the engine speed, a temperature of the at least one catalyst and / or a precatalyst, an exhaust gas temperature, an exhaust gas mass flow, the operating point, the oxygen storage capacity the catalysts of an elapsed since the beginning of the transition phase time and / or a lambdaabezogenen exhaust gas composition.

The above-described process variant with a gradual re-start of the fired operation of the internal combustion engine offers the additional advantage that the time period of the first stage can be adapted to the current catalyst state and thus the phase of short-term thermal overload is kept as short as possible and as long as necessary ,

The lean-burn internal combustion engine with at least one arranged in an exhaust passage of the internal combustion engine catalyst according to the invention comprises means with which immediately after a fuel cut-off phase of the internal combustion engine and / or immediately after an operating phase in which oxygen is stored in the catalyst, first in one Transition phase is adjustable in approximately stoichiometric or less stoichiometric lambda, and with which the lambda value can then be deducted with a predeterminable course to an originally, preferably requested by an engine control of the internal combustion engine lambda value.

According to a particularly preferred embodiment, the at least one catalyst is at least one NO _x storage catalyst which may have at least one precatalyst.

Depending on the embodiment of the internal combustion engine, the means may comprise at least one oxygen-sensitive measuring device arranged downstream of the at least one catalytic converter. A further oxygen-sensitive measuring device can be arranged downstream of the internal combustion engine, but upstream of an at least first catalytic converter.

These means also include a control unit, which is preferably integrated in an engine control unit, in which models and algorithms for the coordinated control of exhaust gas and performance-relevant measures are stored in digitized form.

The control and coordination of the aforementioned means and other conventional means via the control unit or the engine control unit.

In the internal combustion engine according to the invention is a gasoline engine, in particular a direct injection gasoline engine, or a diesel engine.

Advantageously, the at least one arranged in the exhaust passage of the internal combustion engine catalyst has a relation to the prior art reduced precious metal content.

Vehicles with lean-burn internal combustion engines, which in the New European Driving Cycle NEDC with thermally undamaged catalysts with a stored sulfur mass <0.2 g / l catalyst volume and a timed fired lean operating component without fuel cut phases with a lambda> 1.15 of at least 250 s, in particular at least 350 s , reach an HC emission of <0.07 g / km and an NO _x emission of <0.05 g / km, are today equipped in the prior art with catalysts, the noble metal contents of ≧ 3.59 g / dm ³ (100 g / ft ³ ).

Advantageously, the downstream of a preferred embodiment of the internal combustion engine according to the invention catalyst system consisting of at least one NO _x storage catalyst and possibly at least one upstream precatalyst, a noble metal content ≦ 3.59 g / dm ³ (100 g / ft ³ ), in particular ≦ 2.87 g / dm ³ (80 g / ft ³ ) and preferably ≦ 2.15 g / dm ³ (60 g / ft ³ ). Advantageously, the catalyst system with inventively lowered noble metal content with increasing vehicle mileage in the NEDC compared to the original design with higher noble metal content and without the inventive method no higher emissions.

Further preferred embodiments of the invention will become apparent from the remaining, mentioned in the dependent claims characteristics.

Figur 1

eine Prinzipdarstellung einer Verbrennungskraftmaschine mit einer Abgasanlage;

Figur 2

zeitliche Verläufe verschiedener Parameter beim Wechsel aus einer Schubabschaltungsphase nach dem erfindungsgemäßen Verfahren und

Figur 3

zeitliche Verläufe von Lambda vor und nach dem Katalysator beim Wechsel aus einer Schubabschaltungsphase nach einer zweiten Variante des erfindungsgemäßen Verfahrens.

The invention will be explained in more detail in embodiments with reference to the accompanying drawings. Show it:

FIG. 1

a schematic diagram of an internal combustion engine with an exhaust system;

FIG. 2

temporal courses of various parameters when changing from a fuel cut-off phase according to the inventive method and

FIG. 3

Timing of Lambda before and after the catalyst when changing from a fuel cut-off phase according to a second variant of the method according to the invention.

The in FIG. 1 shown internal combustion engine 10 is an exhaust system 12 downstream. The exhaust system 12 has an exhaust gas channel 14, in which a pre-catalyst arranged close to the engine 16 and a large-volume NO _x storage 18 are located. In addition to the primary catalytic converter 16 and the NO _x storage catalytic converter 18, the exhaust gas duct 14 usually has various gas and / or temperature sensors (not shown here) for controlling the internal combustion engine 10. Shown in FIG. 1 For example, only two oxygen-sensitive measuring devices 20, 22, upstream or downstream of the NO _x storage catalytic converter 18, which provide a signal for the respective lambda value in the exhaust gas, and a temperature sensor 24, by means of which the temperature of the NO _x storage 18 is determined. The signals are transmitted to a control unit 26, in which they are used to control the operating modes of the internal combustion engine 10. The control unit 26 is also integrated in an engine control unit 28. By means of the control unit 26 and the engine control unit 28, at least one operating parameter of the internal combustion engine 10, in particular an air-fuel mixture (combustion lambda) to be supplied, is influenced as a function of the signals.

At the in FIG. 2 progressions of parameters shown are the graph 100a and 100b lambda, graph 102a and 102b, the temperature of the NO _x storing catalyst 18 and Graph 104a and 104b of the oxygen level of the NO _x storing catalyst 18 at the time During the transition from overrun to fired operation of internal combustion engine 10, graphs 100a, 102a and 104a are shown in phantom and show the histories of the prior art while graphs 100b, 102b and 104b trace the graphs represent the inventive method. According to the invention, the lambda value or the mixture composition is initially set stoichiometrically immediately after the fuel cut-off phase at time t ₁ . The lambda value originally requested by the engine control unit 28 is only reached at a time t ₂ , the course of the lowering of the lambda value being predetermined. By this measure, the oxygen storage of the NO _x storage catalyst 18 is gradually emptied. As a result, the temperature at the NO _x storage catalyst 18 increases between the two times t ₁ and t ₂ , but this increase is less than the temperature peak that occurs in the prior art process. As a result, too high a temperature load and a concomitant damage to the NO _x storage catalytic converter 18 are avoided.

A variant of the method according to the invention is in FIG. 3 shown. Here, the graph 106 with the dashed line shows the lambda after the NO _x storage 18 and the graph 108 the lambda before the NO _x storage 18. It is at time t _1, the lambda before NO _x storage 18 to a less substoichiometric Lambda value set as originally requested. Via at least one oxygen-sensitive measuring device 22 located downstream of the internal combustion engine 10, but upstream of the primary catalytic converter 16, the actual lambda is regulated to the lambda value in a known manner. The operation with the lambda thus given is maintained until downstream of the NO _x storage 18 at the oxygen-sensitive measuring device 20 at time t _3, a Lambda threshold is exceeded. After falling below the lambda threshold value, the lambda value is lowered to the lambda value required for protecting the catalytic converter, wherein the lowering of the lambda value is still delayed for a short period of time and thus only takes place at time t _{4 in} order to ensure at least almost complete removal of the stored oxygen.

LIST OF REFERENCE NUMBERS

10

Internal combustion engine

12

exhaust system

14

exhaust duct

16

precatalyzer

18

NO _x storage catalyst

20, 22

oxygen-sensitive measuring device

24

temperature sensor

26

control unit

28

Engine control unit

100a, 100b

lambda curve

102a, 102b

Temperature profile of the NO _x storage catalytic converter

104a, 104b

Course of the oxygen level of the NO _x storage catalytic converter

106

Lambda curve after NO _x storage catalytic converter

108

lambda curve

t ₁

Time / end of the fuel cut-off phase

t ₂

Time / Reaching the originally requested lambda value

t ₃

Time / falling below a threshold value

t ₄

Time / end of delay

Claims (25)

1. Method for controlling an internal combustion engine (10) having at least one catalytic converter arranged in an exhaust duct (14) of the internal combustion engine (10), characterized in that, in the event of a demand for a substoichiometric lambda value directly after an overrun cut-off phase of the internal combustion engine (10), initially, in a transition phase, a lambda value is set which is approximately stoichiometric or which is less substoichiometric than that demanded, and subsequently, a downregulation adjustment is made to the demanded substoichiometric lambda value.
2. Method according to Claim 1, characterized in that the demanded lambda value and/or the lambda value initially set in the transition phase is predefined by an engine controller.
3. Method according to Claim 1 or 2, characterized in that the at least one catalytic converter is at least one NO_x storage catalytic converter (18).
4. Method according to one of Claims 1 to 3,
   characterized in that the at least one catalytic converter has a pre-catalytic converter (16).
5. Method according to one of Claims 1 to 4,
   characterized in that the transition phase is divided into at least two phases in which the downregulation to the demanded substoichiometric lambda value is carried out at different speeds.
6. Method according to one of Claims 1 to 5,
   characterized in that the downregulation of the lambda value takes place continuously in at least one of the phases or in the transition phase as a whole.
7. Method according to one of Claims 1 to 6,
   characterized in that the downregulation of the lambda value takes place in steps in at least one of the phases or in the transition phase as a whole.
8. Method according to one of Claims 1 to 7,
   characterized in that the transition phase as a whole, preferably even one of the phases, is executed over a duration of at least ten, in particular at least thirty working cycles of the internal combustion engine (10).
9. Method according to one of Claims 1 to 8,
   characterized in that the downregulation of the lambda value in the transition phase or in at least one of the phases is made with an average enrichment speed of -0.01 to -0.3 s^-1, preferably with an average enrichment speed of approximately -0.1 s^-1.
10. Method according to one of Claims 1 to 9,
    characterized in that the introduction of the downregulation of the lambda value in the transition phase or in at least one of the phases is made as a function of the exceedance of a temperature threshold of at least one catalytic converter.
11. Method according to one of Claims 1 to 10,
    characterized in that, at the start of the transition phase or in the first phase of the transition phase, a lambda value of between 0.9 and 1.05, in particular between 0.93 and 1.02 and preferably between 0.97 and 1.0 is set.
12. Method according to one of Claims 1 to 10,
    characterized in that an oxygen-sensitive measuring device (20) is arranged downstream of the at least one catalytic converter.
13. Method according to Claim 12, characterized in that, at the start of the transition phase or in the first phase of the transition phase, a lambda value of < 1.00, preferably of between 0.92 and 0.99 and particularly preferably of between 0.94 and 0.96 is set.
14. Method according to Claim 12 or 13, characterized in that the approximately stoichiometric or less substoichiometric lambda which is initially set in the transition phase is maintained until a lambda threshold value is undershot downstream of a first catalytic converter or downstream of the load-critical catalytic converter at the oxygen-sensitive measuring device (20).
15. Method according to Claim 14, characterized in that the lambda value lies close to 1.00, preferably in the range from 0.95 to 1.03, particularly preferably in the range from 0.97 to 1.01 and optimally in the range from 0.98 to 0.9995.
16. Method according to Claim 14 or 15, characterized in that after the undershooting of the lambda threshold value, the reduction of the initially-set lambda value is delayed further for a time period of 0...4000 ms, preferably 100...1000 ms, particularly preferably 200...500 ms.
17. Method according to one of Claims 1 to 16,
    characterized in that the lambda value at the start of the transition phase and/or the enrichment speed during the transition phase are defined as a function of the engine speed, a temperature of the at least one catalytic converter and/or of a pre-catalytic converter, an exhaust gas temperature, an exhaust gas mass flow, the operating point, the oxygen storage capacity of at least one of the catalytic converters, a time which has elapsed since the start of the transition phase and/or a lambda-related exhaust-gas composition.
18. Method according to one of Claims 1 to 17,
    characterized in that the demand for the substoichiometric lambda value after the overrun cut-off phase takes place for operational reasons, in particular on account of a power demand of the internal combustion engine or on account of a demand of the engine controller.
19. Internal combustion engine (10) capable of lean running having at least one catalytic converter arranged in an exhaust duct (14) of the internal combustion engine (10), characterized in that the internal combustion engine (10) has means with which, in the event of a demand for a substoichiometric lambda value directly after an overrun cut-off phase of the internal combustion engine (10), initially, in a transition phase, a lambda value is set which is approximately stoichiometric or which is less substoichiometric than that demanded, and with which the lambda value is subsequently downregulated to the originally demanded lambda value.
20. Internal combustion engine according to Claim 19, characterized in that the at least one catalytic converter is at least one NO_x storage catalytic converter (18).
21. Internal combustion engine according to Claim 19 or 20, characterized in that the at least one catalytic converter has at least one pre-catalytic converter (16).
22. Internal combustion engine according to one of Claims 19 to 21, characterized in that the means comprise at least one oxygen-sensitive measuring device (20) which is arranged downstream of the at least one catalytic converter.
23. Internal combustion engine according to one of Claims 19 to 22, characterized in that the means comprise a control unit (26) which is, if appropriate, integrated into an engine control unit (28), in which models and algorithms for the coordinated control of exhaust-gas- and power-relevant measures are stored in digitized form.
24. Internal combustion engine according to one of Claims 19 to 23, characterized in that the internal combustion engine (10) is a spark-ignition engine or a diesel engine.
25. Internal combustion engine according to one of Claims 19 to 24, characterized in that the at least one catalytic converter has a nobel metal content of ≤ 3.59 g/dm³, preferably ≤ 2.87 g/dm³, particularly preferably ≤ 2.15 g/dm³.

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