EP1199457B1 - Method for controlling a lean burning combustion engine - Google Patents

Description

The invention relates to a method for controlling a lean-running internal combustion engine with a catalyst system arranged in an exhaust duct of the internal combustion engine, comprising at least one catalytic converter.

To minimize fuel consumption, internal combustion engines are preferably operated at least temporarily in a lean operating mode, that is to say with a mixture having a low fuel fraction and an excess of air (λ> 1). In addition, under these conditions, emission of unburned hydrocarbons HC and other pollutants can be reduced. In the case of direct injection internal combustion engines, by realizing a so-called stratified operation in which the injected fuel concentrates in the combustion chamber of the engine in the area of a spark plug, the allowable lambda value can be greatly increased toward lean. Since nitrogen oxides NO _x can not be completely converted (due to a small amount of reducing agent) in the lean exhaust gas, it is known to absorb them during lean operation by means of an NO _x storage catalytic converter arranged in an exhaust duct of the internal combustion engine. In order to avoid NO _x breakthroughs, it is also customary to restrict or block approval of the lean operation if a NO _x storage capacity of the storage catalytic converter decreases. A disadvantage of this control of the lean operation in exclusive dependence on the NO _x storage capacity of the storage catalyst, however, is that a slump in HC conversion rate of the catalyst system, in particular a usually the NO _x storage catalytic converter upstream of the primary catalyst is not taken into account. It has been found that, despite the overall low HC raw emissions of the internal combustion engine in lean operation, the HC endemission increases sensitively with already slight drops in the HC conversion rate of the catalyst system. This is partly due to the presence of relatively high proportions difficult to oxidize HC components in lean exhaust gas. As a result, the HC conversion rate in lean operation is generally lower than in a stoichiometric operating mode at λ = 1, where the HC conversion rate is significantly above 99.5%.

US Pat. Nos. 5,947,077 and 5,904,129 each disclose a method for controlling lean-running direct-injection internal combustion engines, in which misfires are detected and when a critical limit for a frequency of misfire is exceeded, certain measures are taken to restore sufficient combustion efficiency. In particular, a reduction of an exhaust gas recirculation rate, a change of an ignition angle, a shifting of an injection time from the compression to the intake stroke and / or a reduction of the air-fuel ratio is performed.

The invention has for its object to improve the known method for controlling a lean-running internal combustion engine to control the lean operation in a simple and cost-effective manner such that the HC conversion rate of the catalyst system is taken into account in a licensing of the lean operation to reduce pollutant emissions ,

This object is achieved by a method having the features of claim 1. It is contemplated by the prior art that improper combustion events be registered and control of lean burn operation of the internal combustion engine be dependent upon a frequency of improper combustion events wherein lean operation is restricted or inhibited with increasing frequency of improper combustion events. Inadmissible combustion events of the internal combustion engine are a major cause of the impairment of the HC, but also of the CO and NO _x conversion rate of the catalyst system, in particular of a catalyst near the catalyst of the catalyst system. By the frequency of these events is determined, a collapse of HC , CO and NO _x conversion rate are estimated. As the incidence of improper combustion events increases, and thus the rate of conversion of the catalyst system decreases, an operating parameter range of the internal combustion engine in which lean operation is permitted is then restricted or completely disabled. The accuracy of According to the invention, the method is further increased by not equally weighting each impermissible combustion event, but rather by individually weighting according to its damage potential with respect to the HC conversion rate of the at least one catalyst, the weighting increasing with increasing damage potential.

According to a preferred embodiment of the method, the registered impermissible combustion events include delayed burns and / or misfires. Both in the delayed combustion, in which a flammability of the air-fuel mixture compared to the regular combustion far into the expansion stroke is moved into, as well as a misfire or ignition failure, in which the mixture burns only at the catalyst, extreme thermal stresses of Catalyst system, in particular the precatalyst, which reduce the HC conversion performance.

The detection of the impermissible combustion events is preferably carried out by evaluation of a speed or ripple signal which is output, for example, from a speed sensor. This procedure is known from the so-called misfire detection and will not be explained in detail here.

According to two particularly advantageous embodiments of the method, the lean operation of the internal combustion engine is controlled either as a function of an absolute or a specific frequency of impermissible combustion events. Both manipulated variables can also be combined with each other. The absolute frequency is determined by integration of the impermissible combustion events detected during a period of lean operation. By contrast, the specific frequency refers, for example, to a predetermined number of crankshaft revolutions or to a fixed time unit. A suitable range for the number of crankshaft revolutions is between 10 and 10,000, more particularly between 50 and 500. Ideally, the improper combustion events are determined based on 200 to 2000 crankshaft revolutions. The specific frequency of impermissible combustion events related to crankshaft revolutions is typically between 0.1 and 5% for a four-cylinder four-stroke engine, in particular between 0.5 and 3%, mainly between 0.8 and 3.5%. It should be noted that two ignitions occur per crankshaft revolution.

Different factors can be included in the calculation of a weighting of an inadmissible combustion event. For example, the weighting may increase with the level of a catalyst temperature and / or level of a load and / or level of fuel fraction in the air-fuel mixture and / or the frequency of improper combustion events, that is, the degree of pre-damage of the catalyst system ,

The restriction of the lean operation of the internal combustion engine is preferably carried out by a limitation of operating parameter ranges of the internal combustion engine, within which the lean operation is permitted. there Advantageously, a load range, a speed range, a range of a catalyst temperature and / or an exhaust gas temperature is restricted.

The method according to the invention is particularly suitably carried out in combination with the conventional control of the lean operation, which takes place as a function of a NO _x storage rate of an NO _x storage catalyst of the catalyst system. In this way, both a NO _x emission and the HC emission during lean operation can be effectively protected.

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

Figur 1

schematisch eine Anordnung einer Verbrennungskraftmaschine mit zugehörigem Abgaskanal und

Figur 2

ein Ablaufdiagramm einer bevorzugten Ausführung des Verfahrens.

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

FIG. 1

schematically an arrangement of an internal combustion engine with associated exhaust duct and

FIG. 2

a flowchart of a preferred embodiment of the method.

FIG. 1 shows an internal combustion engine 10 with a downstream exhaust gas channel 12. In a position close to the engine of the exhaust gas channel 12 there is a small volume precatalyst typically designed as a 3-way catalyst. The precatalyst 14 promotes an oxidative conversion of unburned hydrocarbons HC and carbon monoxide CO and simultaneously a reduction of nitrogen oxides NO _x . Excess NO _x , which can not be completely converted due to the small amount of reducing agent in the lean exhaust gas, is stored in a downstream NO _x storage catalytic converter 16, which is typically arranged at an underbody position of a vehicle. Besides the catalyst system 14, 16, various sensors are installed in the exhaust passage 12. A lambda probe 18 which is directly downstream of the internal combustion engine 10 outputs a signal proportional to an oxygen content of the exhaust gas to a control unit 20 which regulates an air-fuel mixture to be supplied to the internal combustion engine 10 (lambda control). A NO _x sensor 22 connected downstream of the NO _x storage catalytic converter 16 serves to monitor the storage catalytic converter 16 and to control the lean operation of the Internal combustion engine 10. Also its signal is processed by the controller 20. Is detected, for example by means of the NO _x sensor 22 has a decreasing NO _x storage capacity of the storage catalyst 16, so the operating range of the internal combustion engine 10, in which a lean operation is about is permitted or restricted or banned until a NO _x regeneration of the storage catalyst 16 can be carried out. In this way, NO _x breakthroughs can be suppressed. However, there are damage profiles in which, although sufficient NO _x conversion or storage takes place, the HC conversion rate of the catalyst system 14, 16, in particular of the precatalyst 14, but breaks. As already explained at the outset, this is mainly the result of inadmissibly occurring burns, in particular delayed burns, or sporadically occurring misfires, which lead to a high thermal exposure of the precatalyst 14. Since in lean operation, the HC conversion rate due to a relatively high proportion of hardly oxidizable HC components in the exhaust gas, in contrast to a stoichiometric operation at λ = 1 below 99.5%, a cumulative HC emission in lean operation is particularly sensitive to slumps of HC conversion rate and increases rapidly. In accordance with the invention, therefore, impermissible combustion events in the lean operation of the internal combustion engine are measured in order to detect damage to the catalyst system 14, 16 with regard to the HC conversion. For this purpose, for example, a speed sensor 24 is arranged at a suitable position of the internal combustion engine 10. The speed sensor 24 provides an agitation signal and also transmits this to the control unit 20. This evaluates in a manner to be described, the signal and controls the allowable operating range for the lean operation in response to the signal.

FIG. 2 shows a typical method sequence for controlling the lean-burn internal combustion engine 10. In a first step S1, the detection of inadmissibly proceeding combustion events takes place by means of evaluation of the running noise signal provided by the rotational speed sensor 24 by the control device 20. In a second step S2, a weighting of the Detected inadmissible combustion events based on their damage potential with respect to the HC conversion rate of the pre-catalyst 14 and / or the No _x storage catalytic converter 16 made. For example, a misfire, which occurs at a relatively high catalyst temperature anyway, be weighted higher than one at a relatively low catalyst temperature taking place dropouts. In a third step S3, integrating the weighted improper combustion events during a lean phase determines their absolute frequency. Alternatively or additionally, a specific frequency of the impermissible combustion events, for example per crankshaft revolution or time unit, can also be determined here. In a fourth step S4, a query is carried out in which it is checked whether the determined absolute and / or specific frequency of the impermissible combustion events exceeds a predefinable threshold. If this is not the case, step S1 is started again without taking further measures. Otherwise, the query in step S4 is affirmative, then in a fifth step S5, the lean operation of the internal combustion engine is either restricted or completely disabled. In this case, the restriction of the lean operation by restricting an allowable operating range for lean operation, such as by limiting an allowable load range and / or speed range, etc., take place. Particularly advantageous may also be provided a gradual restriction of lean operation. In this case, the step size, for example, for the allowable load and / or speed, either fixed or be determined variable depending on the frequency of inadmissible events. It is also conceivable, after elapse of a predetermined time interval and / or a predetermined fuel or Luftmassendurchsatzes the measures taken in step S5 cancel all or part. This allows a certain spontaneous regeneration of the internal combustion engine 10 or the catalyst system 14, 16 are taken into account.

Overall, the method according to the invention makes it possible to tailor the permissible operating range for the lean operation of internal combustion engines, in particular of direct-injection engines, very precisely to the current combustion quality of the internal combustion engine 10 with little process outlay. In contrast to known methods, not only a NO _x charge of the NO _x storage catalytic converter 16, but also an HC conversion rate of the catalyst system 14, 16 can thus be taken into account. As a result, compliance with HC emission limits can be more effectively ensured.

LIST OF REFERENCE NUMBERS

10

Internal combustion engine

12

exhaust duct

14

precatalyzer

16

NO _x storage catalyst

18

lambda probe

20

control unit

22

NO _x sensor

24

Speed sensor

Claims (13)

1. Method for controlling an internal combustion engine (10) which is capable of lean-burn operation with a catalytic converter system arranged in an exhaust-gas passage (12) of the internal combustion engine (10), said catalytic converter system comprising at least one catalytic converter (14, 16), wherein inadmissible combustion events are recorded and lean-burn operation of the internal combustion engine (10) is controlled as a function of a frequency of the inadmissible combustion events, the lean-burn operation being restricted or blocked as the frequency of the inadmissible combustion events increases, characterized in that an inadmissible combustion event is weighted as a function of its potential damage to the HC conversion rate of the at least one catalytic converter (14, 16).
2. Method according to Claim 1, characterized in that the inadmissible combustion event is a slow combustion and/or a misfire.
3. Method according to either of Claims 1 and 2, characterized in that the inadmissible combustion events are detected by evaluating an engine speed signal or a smooth running signal.
4. Method according to one of Claims 1 to 3, characterized in that an absolute frequency, based on the duration of the lean-burn operation, of inadmissible combustion events is determined, and the lean-burn operation of the internal combustion engine (10) is controlled as a function of the absolute frequency of inadmissible combustion events.
5. Method according to one of Claims 1 to 3, characterized in that a specific frequency of inadmissible combustion events, in particular based on a predetermined number (n) of crankshaft revolutions or on a predetermined time unit, is determined and the lean-burn operation of the internal combustion engine (10) is controlled as a function of the specific frequency of inadmissible combustion events.
6. Method according to Claim 5, characterized in that (n) is between 10 and 10 000, in particular between 50 and 5000, preferably between 200 and 2000, crankshaft revolutions.
7. Method according to Claim 5, characterized in that the specific frequency is between 0.1 and 5%, in particular between 0.5 and 3%, in particular between 0.8 and 2.5%.
8. Method according to one of the preceding claims, characterized in that the weighting of the inadmissible combustion event increases with at least one of the parameters consisting of catalytic converter temperature, engine speed, load, proportion of fuel in an air/fuel mix and frequency of inadmissible combustion events.
9. Method according to one of the preceding claims, characterized in that the lean-burn operation of the internal combustion engine (10) is restricted by limiting operating parameter ranges within which the lean-burn operation is permitted.
10. Method according to Claim 9, characterized in that the operating parameters comprise a load and/or an engine speed and/or a catalytic converter temperature and/or an exhaust gas temperature.
11. Method according to one of the preceding claims, characterized in that the restriction of the lean-burn operation of the internal combustion engine (10) takes place in steps, with a step width being fixedly predetermined or being set variably, in particular as a function of the frequency of inadmissible combustion events.
12. Method according to one of the preceding claims, characterized in that the restricting or blocking of the lean-burn operation is completely or partially cancelled after a predeterminable time period has elapsed and/or after a predeterminable mass throughput of fuel or air has passed through.
13. Method according to one of the preceding claims, characterized in that the determination of the admissibility of the lean-burn operation of the internal combustion engine (10) is additional dependent on an NO_x storage rate of an NO_x storage catalytic converter (16) of the catalytic converter system (14, 16).

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