DE10154974B4 - Method and device for switching an internal combustion engine from a fired operation to an unfired overrun operation - Google Patents

Abstract

Method for switching over an internal combustion engine (10) of a motor vehicle with at least one downstream catalytic converter (14, 16) from a fired operation (BLF, BSF) carried out in a load phase (τ _L ) or a coasting phase (τ _S ) to an unfired thrust mode (BSU) ) by interrupting a fuel supply, wherein during the overrun phase (τ _S ) a requested drive torque is less than or equal to a current thrust moment of the vehicle, wherein to switch from the fired operation (BLF, BSF) to the unspurged overrun mode (BSU) before interrupting the fuel supply a transition phase (PO) is performed, during the transition phase (PO) during the previous fired operation (BLF, BSF) present ignition angle (ZW) is adjusted in the direction of "late" and / or during the previous fired operation (BLF, BSF) set lambda setpoint (λ) in the direction of "lean" is shifted, thereby g ekennzeichnet that
- When a catalyst temperature (TK) of at least one catalyst (14, 15) exceeds a first predetermined temperature threshold (TSF), the fuel cut is not permitted and the overrun phase (τ _S ) is operated fired (BSF);
- When the catalyst temperature (TK) of at least one catalyst (14, 15) exceeds a second predetermined temperature threshold (TPO), but not the first predetermined temperature threshold (TSF), a switch from the fired load operation (BLF) in the unfired overrun (BSU) below Interposition of the transition phase (PO) is performed; and
- At the catalyst temperatures (TK) of the catalysts (14, 15) below the second temperature threshold (TPO) neither the transition phase (PO) nor the fired overrun operation (BSF) are allowed.

Description

The invention relates to a method and apparatus for switching an internal combustion engine of a motor vehicle from a fired load or overrun operation in an unfired overrun operation with the features mentioned in the preambles of the independent claims 1 and 23, respectively.

For the aftertreatment of exhaust gases of internal combustion engines, it is known to conduct the exhaust gas via at least one catalyst which carries out a conversion of one or more pollutant components of the exhaust gas. So-called oxidation catalysts promote the oxidation of unburned hydrocarbons (HC) and carbon monoxide (CO), while reduction catalysts support a reduction of nitrogen oxides (NO _X ) of the exhaust gas. Furthermore, 3-way catalysts are able to simultaneously catalyze the conversion of these three components (HC, CO, NO _X ). However, quantitative 3-way catalytic conversion can only take place at a strictly stoichiometric air / fuel ratio at λ = 1. In a low-consumption lean operation, in which the internal combustion engine with oxygen excess, that is, with λ> 1, driven, a complete 3-way catalytic conversion of NO _x, however, is not possible. As a remedy NO _x storage catalytic converters are used which, in addition to a catalytic component, contain an NO _x storage which stores NO _x in the form of nitrate in the lean operating phases and releases it again in intermediate fat regeneration phases at λ <1 and reduces it to nitrogen N ₂ , Often the NO _X storage catalyst is preceded by a precatalyst, for example a 3-way catalyst.

Compared with pure 3-way catalyst systems, NO _x catalyst systems are relatively temperature sensitive. As can take place above 800 ° C irreversible damage of the catalyst system is already at upstream of the NO _x storage catalytic converter the present exhaust gas temperature so that the catalyst activity over the life of the vehicle decreases significantly. This applies to both NO _x storage and regeneration during lean and rich service intervals as well as HC, CO and NO _X conversion behavior at stoichiometric loading. In order to avoid the exceeding of a critical temperature limit, exhaust gas cooling measures for reducing the exhaust gas temperature are known.

A particular problem with regard to the temperature load of all catalyst systems are unavoidable overrun phases during normal driving, which can occur, for example, when the vehicle is decelerating or when it is descending. In the overrun phase, a desired driver torque given by the driver is less than or equal to a momentary torque of the vehicle resulting from the driving resistances. In conventional systems, the fuel supply is interrupted for fuel savings during a coasting phase, the internal combustion engine thus not fired operated (fuel cut). However, high oxygen concentrations reach the exhaust gas and the catalyst system which, at the beginning of the overrun phase, in particular after a high or full load operation, still contains high masses of oxidizable exhaust gas components, in particular HC. As a result of the exothermic conversion reaction of these components with the oxygen, local temperature peaks arise, which lead to increased oxidation and / or sintering of the catalytic noble metal coatings and thus can permanently damage the catalytic activity (cf. 2 ). This problem is all the more serious, the higher the temperatures of the catalyst system reached during a preceding vehicle propulsion phase (load phase) during the coasting phase, that is, in particular after a high-load or full-load vehicle operation. The damaging potential of overrun fuel cutoff is shown in engine dynamometer tests, in which load cycles consisting of high loads and high exhaust gas temperatures alternating with unfired shear phases lead to greater deactivation of the NO _x storage catalytic converter system than corresponding duty cycles without intermediate overrun phases.

From the older German patent application DE 101 10 500 A1 is an optimized fuel cut strategy known, in which the overrun fuel cutout is suppressed during the overrun phase under certain conditions, that is, the internal combustion engine is still fired with a supplied fuel, especially when high catalyst temperatures are present. In this case, a lambda desired value of λ ≦ 1 in the fired overrun mode is preferred. In this way, catalyst aging due to high oxygen load at high catalyst temperatures is counteracted. Various strategies of Lambda setpoint input for controlling the temperature of the catalyst system and for compensating a Nutzmomentes generated in the fired overrun operation are proposed in this application. The earlier application, however, does not include a strategy for switching from a fired operation to the asynchronous overrun mode when, for example, there is a sufficiently low catalyst temperature. If, for switching over at the end of the fired operating phase, the fuel supply is at least almost instantaneously discontinued, there is a sudden drop in torque. Furthermore, the still flooded with reducing agents catalyst system with subjected to high oxygen mass flows, so that at least locally and temporarily certain Exothermiespitzen burden the catalyst system. This is particularly noticeable in a usually less strongly cooled pre-catalyst.

It is also known, in the transition from a fired to an unfired operation for a few working cycles (for example, at most 10, in particular about 5) of the engine to inject a reduced amount of fuel compared to the fired operation. Working cycle is understood here to be a single pass through the ignition sequence of the engine. The stored during the fired operation in the combustion chamber and optionally in the intake manifold fuel masses, which form a so-called wall film are removed by throttling the engine by evaporation. However, these compositions are insufficient to provide an ignitable mixture; they would therefore unburned apply the catalyst system without further fuel addition and cause there at least a local increase in temperature. Therefore, the Wandfilmabtrag is modeled and added during the Abtrag a difference in fuel for safe ignition. This function is usually integrated into a function called "dashpot" (throttle damper).

The DE 39 26 096 A1 also discloses a method of fuel cutoff in the transition from fired operation to overrun. The DE 44 45 462 A1 discloses switching off the fuel supply in overrun mode, wherein the ignition angle of the internal combustion engine is changed at the transition to this so-called fuel cut to improve comfort. Further, on the DE 196 19 320 A1 . DE 197 51 928 A1 . DE 199 43 914 A1 and DE 199 58 251 A1 directed.

DE 698 21 810 T2 and DE 196 30 944 A1 disclose that in overrun operation, a fuel cut is performed when a catalyst temperature is low enough.

Object of the present invention is therefore to provide a particularly catalyst-friendly method for switching an internal combustion engine from a fired thrust or load operation in an unfired overrun operation available, which largely avoids harmful temperature peaks and without significant change in driving behavior is feasible. In addition, an apparatus for carrying out the method is to be provided.

This object is achieved by a method and devices having the features of independent claims 1 and 23, respectively.

The inventive method provides that for switching from the fired operation in the unfired overrun operation before interrupting the fuel supply, a transition phase is performed, during the transition phase during the previous fired operation present ignition angle in the direction of "late" is adjusted and / or during of the previous fired operation set lambda setpoint is shifted in the direction of "lean".

• - wenn eine Katalysatortemperatur mindestens eines Katalysators eine erste vorgebbare Temperaturschwelle überschreitet, die Schubabschaltung nicht zugelassen und die Schubphase gefeuert betrieben wird;
• - wenn die Katalysatortemperatur mindestens eines Katalysators eine zweite vorgebbare Temperaturschwelle, nicht aber die erste vorgebbare Temperaturschwelle überschreitet, eine Umschaltung vom gefeuerten Lastbetrieb in den ungefeuerten Schubbetrieb unter Zwischenschaltung der Übergangsphase durchgeführt wird; und
• - bei Katalysatortemperaturen der Katalysatoren unterhalb der zweiten Temperaturschwelle weder die Übergangsphase noch der gefeuerte Schubbetrieb zugelassen sind.

Characteristically, it is provided that

• if a catalyst temperature of at least one catalyst exceeds a first predefinable temperature threshold, the fuel cut-off is not permitted and the coasting phase is fired;
• - If the catalyst temperature of at least one catalyst exceeds a second predetermined temperature threshold, but not the first predetermined temperature threshold, a switchover from the fired load operation in the unfired overrun operation with the interposition of the transition phase is performed; and
• - At catalyst temperatures of the catalysts below the second temperature threshold neither the transition phase nor the fired overrun operation are allowed.

The method according to the invention can generally be carried out during the transition from a fired (thrust or load) operation to an unfired overrun operation. In particular, the method finds application in the transition from a fired overrun mode to an unfired overrun mode.

Thus, according to the invention, fuel cutoff does not start abruptly, but a transition phase (or a "phase out") is carried out by carrying out two measures, namely ignition retard and "leaning out", alternatively or preferably in combination with one another. The transition phase comprises at least twenty, preferably at least fifty, cycles of work and is thus significantly prolonged compared with the wall-film removal method explained in the introduction.

Due to the Zündwinkelspätverstellung a motor efficiency is increasingly deteriorated, thus causing a torque reduction during the transition phase. In this way, at the time of interruption of the fuel supply is a very late firing angle and thus only a small engine torque of the internal combustion engine, so that the fuel cut is associated with a relatively small torque dip. Thus is to expect at best a slight effect on driving behavior compared to known strategies for initiating a fuel cut.

The second transitional measure according to the invention consists in the lambda desired value shift during the transition phase in the direction "lean". This "emaciation" can be carried out before, during and / or after the ignition angle adjustment. With this measure, an almost sudden flooding of the catalyst system with oxygen at interruption of the fuel supply is avoided. Thus, a reducing agent mass present on the catalysts, in particular of unburned hydrocarbons HC, is at least largely reacted off in a controlled manner by gradually increasing the oxygen concentration acting on it. This is not or only to a very limited extent with the known dashpot function due to the short duration of time and the uncontrolled lambda setpoint shift. An extremely rapid combustion of HC with correspondingly high local temperature peaks, which otherwise occurs when the fuel supply is interrupted immediately, is prevented in this way. The switchover is therefore particularly gentle on the catalyst, in particular for a possible pre-catalyst.

Basically, it is desirable for fuel consumption reasons to operate overrun phases over as wide a range as possible, that is to say with interrupted fuel supply (overrun fuel cutoff). The thrust circuit is therefore preferably only suppressed, the thrust phase thus operated at least temporarily fired when particularly high catalyst temperatures and / or no strong downward slopes are present. Correspondingly, according to the present invention, it is preferably provided that the switchover from the fired overrun mode to the unfired overrun mode is effected as a function of the catalyst temperature, in particular if the temperature of at least one downstream of the internal combustion engine catalyst falls below a first predefinable temperature threshold. In this case, the temperature increase of the catalyst, which is caused by the oxygen supply at shutdown of the fuel supply, can be tolerated because due to the sufficiently cooled catalyst, a damaging, maximum allowable catalyst temperature is not reached.

Also for reasons of fuel consumption takes place in an advantageous embodiment of the invention, switching from a fired load operation in an unfired overrun operation only with the inventively intermediate transition phase, when the catalyst temperature a predetermined second temperature threshold, the 20 to 200 K, preferably 30 to 70 K below the first temperature threshold is exceeded.

The switchover from the fired overrun mode to the unfired overrun mode can also take place as a function of a gradient of a vehicle path, in particular if a predefinable critical gradient is exceeded. On the other hand, it can be advantageously provided to switch over from a fired thrust or load phase only with the interposition of the transition phase, when the gradient falls below a specifiable gradient. In this way, a useful torque generated in the fired overrun operation or in the transitional phase is suppressed, which may otherwise have an unfavorable effect with regard to a desired vehicle deceleration or a braking distance and can lead to dangerous driving situations. If, in such situations - even at high catalyst temperatures - the fuel cut is allowed immediately or at the latest after expiry of the dashpot function, hazards due to undesired useful moments are excluded.

According to an advantageous embodiment of the method, the ignition angle is controlled to a maximum late control value during the fired overrun operation before the beginning of the transition phase. This is chosen so that under approximately stationary conditions, a maximum allowable catalyst temperature of the at least one catalyst, preferably of all catalysts, is not exceeded. For example, here an ignition angle of 18 ° KWW can be controlled before the upper ignition point (ZOT) at an engine speed of 6000 min ^-1 . This measure causes a maximum low, generated in the fired overrun useful torque while ensuring harmless catalyst temperatures. On the other hand, it follows, however, that in the subsequently carried out during the transition phase Zündwinkelspätverstellung even later firing angles are driven, which would lead to steady-state catalyst temperatures, which would be expected to significant catalyst damage. Therefore, a further embodiment of the invention provides that a duration of the transition phase is chosen to be relatively short, namely at least 0.5 s and at most 5 s, in particular 1 to 3.5 s, preferably about 2 s. In this case, advantageously, the duration of the transition phase is defined as a function of an engine speed of the internal combustion engine and / or a measured or modeled exhaust gas temperature and / or a measured or modeled catalyst temperature. Under these conditions, extremely late firing angles, for example, 10 ° KWW before ZOT at an engine speed of 6000 min ^-1 , can be achieved during the transition phase without significant catalyst damage.

The adjustment of the ignition angle during the transition phase can be gradual or continuous. In the latter case, a specifiable adjustment speed of 1 to 20 ° crankshaft angle per second, preferably of about 5 ° KWW / s, has proven to be particularly advantageous in terms of a smooth transition as possible. After performing the Zündwinkelverstellung present at the end of the transition phase latest ignition angle can be held for a maximum of two seconds before the fuel supply is interrupted.

Because of the short duration of the exposure time of a lambda end value present at the end of the lambda desired value shift during the transition phase, in the course of the "leaning out" an oxygen raw emission of the internal combustion engine of 1% by volume can be exceeded, which would already be critical for the catalyst system in the case of continuous loading. Preferably, the emptying takes place up to a final value of the desired lambda value of at least 0.97 and at most 1.08, the final value preferably in dependence on the engine speed, the catalyst temperature of the at least one catalyst and / or a lambdaabased exhaust gas composition with respect to a raw emission of O ₂ , HC and CO is set.

Furthermore, the implementation of the shift of the Lambda setpoint with a Ausmagerungsgeschwindigkeit of 0.01 to 0.3 s ^-1 , in particular of about 0.1 s ^-1 , has proven to be advantageous. The Ausmagerungsgeschwindigkeit is preferably determined depending on an exhaust gas mass flow, the catalyst temperature of the at least one catalyst and / or the lambdaabezogenen exhaust gas composition.

The lambda setpoint specification during the fired overrun operation prior to the transient phase may be accomplished by various strategies, such as those described in the earlier patent application DE 100 10 500 A1 are described in more detail. Preferably, the internal combustion engine is operated in this phase with a lambda setpoint of at most 0.95. In this case, the lambda desired value can be preset in a particularly advantageous manner as a function of the measured or calculated exhaust-gas temperature and / or the catalyst temperature (s). According to an alternative strategy, the desired lambda value is adjusted in such a way that a predetermined maximum permissible temperature of the exhaust gas and / or of the at least one catalytic converter is maintained, as a result of which the additional fuel consumption caused by the firing of the overrun phase is kept low.

Furthermore, it is particularly advantageous to raise a permissible maximum temperature for the catalysts given by non-shear phases (advancing phases) by 5 to 100 K, in particular by 10 to 50 K, preferably by 20 to 30 K, thereby reducing fuel consumption. This maximum temperature increase can be tolerated because, due to the inventive control of the overrun phase, the occurrence of stoppage-related temperature peaks in the catalysts is reliably prevented, at least to a large extent.

The object according to the invention is furthermore achieved by a device which is characterized by means which are set up to carry out the method according to the invention.

In particular, the means comprise an algorithm for carrying out the method steps in digital form, which is stored in a control unit, preferably in an existing engine control unit.

It is further preferred that the catalyst (s) used in exhaust systems of direct injection stratified gasoline engines operated in the non-thermally-damaged state of the New European Driving Cycle NEDC be operated for at least 250 seconds in stratified mode and less than 0.07 g / km HC emission and less than 0.05 g / km NO _x emission, a noble metal content of the noble metals platinum, palladium and / or rhodium of a maximum of 3.74 g / l catalyst volume (corresponding to 104 g / ft ³ ), in particular of not more than 2.87 g / l (80 g / ft ³ ). This means a decrease in precious metal content of at least 20% over prior art catalysts, which typically have a precious metal content of about 4.67 g / l (130 g / ft ³ ). This cost-effective precious metal loading of the catalysts is possible because the inventively optimized overrun cutoff catalyst aging is delayed compared to conventional methods and the usual aging does not have to be compensated by a correspondingly high proportion of noble metal.

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

• 1 schematisch einen Aufbau einer Verbrennungskraftmaschine mit nachgeschaltetem Abgastrakt;
• 2 zeitliche Verläufe eines Lambdasollwertes, einer Katalysatortemperatur eines Vor- und eines Hauptkatalysators, eines Zündwinkels sowie einer Sauerstoffrohemission einer Verbrennungskraftmaschine bei einer Schubphase mit Schubabschaltung gemäß Stand der Technik;
• 3 zeitliche Verläufe der Größen gemäß 2 nach einer vorteilhaften Ausgestaltung der erfindungsgemäß optimierten Schubphasensteuerung; und
• 4 schematisch die erfindungsgemäße Strategie zur Umschaltung eines gefeuerten Betriebs in einen ungefeuerten Schubbetrieb in Abhängigkeit von der Katalysatortemperatur.

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

• 1 schematically a structure of an internal combustion engine with downstream exhaust tract;
• 2 time profiles of a Lambda setpoint, a catalyst temperature of a pilot and a main catalyst, an ignition angle and a raw oxygen emission of an internal combustion engine at a coasting phase with fuel cut in accordance with the prior art;
• 3 Timing of the quantities according to 2 according to an advantageous embodiment of the inventively optimized overrun phase control; and
• 4 schematically the inventive strategy for switching a fired operation in an unfired overrun operation as a function of the catalyst temperature.

1 shows a schematic representation of an internal combustion engine 10 with a downstream exhaust duct 12 , In the internal combustion engine 10 it is preferably a gasoline engine equipped with a fuel direct injection, not shown, which is capable of a lean stratified operation in the low and partial load range. In the fuel-efficient lean stratified charge mode, an ignitable fuel cloud is generated only in the region of a spark plug, while in the remaining combustion chamber there is approximately pure air. The generation and stabilization of the fuel cloud is effected by a late injection time and can be supported in a known manner further by structural measures, such as arranged in an air inlet duct charge motion flap and / or a trough-like configuration of a piston crown.

For cleaning one of the internal combustion engine 10 coming exhaust gas houses the exhaust duct 12 a near-engine, small volume precatalyst 14 , typically a 3-way catalyst, as well as a large volume NO _x storage catalytic converter 16 connected downstream of this. The NO _x storage catalytic converter 16 is charged discontinuously with lean and rich exhaust gas atmospheres, with nitrogen oxides NO _x being incorporated in the lean operating phases short intermediate fat operating phases a NO _x regeneration and conversion.

The regulation of the air-fuel ratio (lambda setpoint) of the internal combustion engine 10 typically occurs with the aid of an upstream of the precatalyst 14 arranged lambda probe 18 , in particular a broadband lambda probe, which measures an oxygen concentration of the exhaust gas. Another gas sensor 20 is installed downstream of the NO _x storage catalyst 16 and serves to monitor the conversion activity of the storage catalyst 16 , The gas sensor 20 may also be a lambda probe or preferably a NO _x sensor. An optional temperature sensor 22 determines an exhaust gas temperature upstream of the NO _X storage catalytic converter 16 from which its catalyst temperature can be determined. Alternatively, the catalyst temperature of the NO _x storage catalytic converter 16 as well as the pre-catalyst 14 be modeled by means of load and speed-dependent maps in a known manner.

The ones from the sensors 18 . 20 . 22 provided signals and various operating parameters of the internal combustion engine 10 find input into an engine control unit 24 , which uses stored algorithms and maps the operation of the internal combustion engine 10 controls. In particular, here is an algorithm 26 to control the switching of the internal combustion engine according to the invention, explained in more detail below 10 from a fired push or load operation to an unfired push operation of one of the internal combustion engine 10 driven motor vehicle (not shown) deposited.

First clarified 2 based on the course of various parameters, the catalyst-damaging temperature load during a conventionally controlled overrun phase. First, there is the internal combustion engine 10 for example, due to a high vehicle speed in a largely constant high-load phase τ _L in which the internal combustion engine 10 in a fired load operation BLF with a rich lambda setpoint with λ <1 and a load and speed-dependent ignition angle ZW from 15 to 20 ° KWW before ZOT is operated. Corresponding to one at the rich lambda setpoint λ predominant air deficit is a very low oxygen emissions O ₂ -RE the internal combustion engine 10 before, so the exhaust gas before the pre-catalyst 14 far less than 1% oxygen. The catalyst temperatures TK _V and TK _H of the precatalyst 14 or of the NO _x storage catalytic converter 16 are due to an operating point-related high exhaust gas temperature and a high proportion of reducing agent of the rich exhaust gas, which is limited by the Sauerstoffrohemission O ₂ -RE exothermic to the catalysts 14 . 16 is implemented at a relatively high level above 750 ° C. (Shown are the local temperature in a washcoat in a reaction zone of the catalysts 14 . 16 .) This is the temperature TK _V of the precatalyst 14 still above that of the main catalyst 16 , on the one hand at least part of the conversion power of the reducing agent (HC, CO) through the precatalyst 14 is provided and on the other hand, a loss of heat on the exhaust run until the NO _X storage 16 takes place.

To one through the vertical double line in 2 marked point sets a push phase τ _S a, for example, because the driver takes back a gas request, such as to bring about a delay. During the push phase τ _S is a requested driving torque is less than or equal to a momentary thrust torque generated by the driving resistances. According to usual procedure takes place during the pushing phase τ _S an overrun fuel cutoff by, optionally, after expiration of a dashpot function explained above, a fuel supply of the internal combustion engine 10 is interrupted. As a result, the lambda setpoint decreases λ a virtually infinite positive value and the raw oxygen emission O ₂ -RE reached - corresponding to an oxygen content of the air - a value of about 20%. Because of this now high oxygen concentration of the exhaust gas, there are intensive, very fast conversion reactions of the initially still high HC amounts of the catalysts 14 . 16 , As a result, at the beginning of the unfired shear phase occur τ _S intense temperature peaks in the catalysts 14 . 16 on. Depending on the initial temperature level present before the fuel cut, the catalyst temperatures may increase TK _V . TK _H thereby reach a critical temperature range in which the catalysts 14 . 16 irreversibly damaged. So find some place in _NOx storage catalysts sintering processes that can lead to a permanent separation of the _NOx storage-and catalyst components.

In order to effectively suppress the occurrence of temperature peaks in overrun phases, according to the invention, the overrun cutoff is in overrun conditions under certain conditions, in particular at high catalyst temperatures TK _V . TK _H , suppressed by the internal combustion engine 10 even during the push phase τ _S is operated fired. This principle is in a simple design in 3 represented starting from the same operating point as in 2 , After the beginning of the push phase τ _S becomes the internal combustion engine 10 in a fired push operation BSF to a (leaner) lambda setpoint raised compared to the previous load phase λ adjusted, which is chosen so that a critical oxygen content of the raw exhaust gas, in particular of 1%, is still below, to an uncontrolled burning of HC to the catalysts 14 . 16 to prevent. Preferably, a lambda setpoint of λ ≤ 1.00, preferably λ ≤ 0.95. Alternatively, a maximum allowable catalyst temperature for one or both catalysts 14 . 16 are specified and the lambda value λ be adjusted so that these temperatures adjust to the catalysts, but are not exceeded.

At the same time, the internal combustion engine 10 during the fired push operation BSF with a maximum late ignition angle ZW operated in order to generate the lowest possible moment of use. This is the ignition angle ZW preferably chosen so that an approximately constant temperature TK _V of the precatalyst 14 but does not exceed the critical catalyst temperature. For example, in a current engine speed of 6000 min ^-1, an ignition angle of 12 ° KWW is driven before ZOT.

As a result of in the push phase τ _S Only very low engine load occurs during the fired overrun operation BSF a more or less strong cooling at least on one of the catalysts 14 . 16 , Reaches the temperature-sensitive catalyst, here the NO _x storage catalytic converter 16, a predetermined for this lower temperature threshold TSF Here, for example, at 750 ° C, the suppression of the fuel cut is canceled and the fuel supply is interrupted to keep the fuel consumption in overrun as low as possible. Another reason for switching from the fired to the unfired overrun operation can be a noted steep gradient, in which the fired overrun operation BSF generated moment of use has a counterproductive effect on the desired delay.

If the fuel supply were suddenly discontinued at this point, there would be a brief but nevertheless noticeable torque drop for the driver due to the abrupt elimination of the in the fired overrun phase BSF generated Nutzmomentes. Such torque fluctuations are usually perceived by drivers as disturbing and irritating. Furthermore, this would still be flooded with reducing agents, especially with HC, catalyst system 14 . 16 subjected to high exhaust gas mass flows and high oxygen concentrations (20%), resulting in a vigorous burning off of the catalysts 14 . 16 would lead. This makes it possible, at least locally and for a short time, for catalyst-damaging temperature peaks of the catalyst temperatures TK _V . TK _H come.

To overcome these disadvantages, according to the invention before the interruption of the fuel supply, a transitional phase PO ("Phase Off") carried out, the duration of which depends on the engine speed and / or the exhaust gas temperature and / or the catalyst temperature / s TK _V . TK _H is set, and here is about 2 s. During the transition phase PO is the already very late ignition angle anyway ZW the fired thrust phase BSF (here 12 ° KWW before ZOT) even further towards late with a predeterminable adjustment speed preferably of about 5 ° KWW / s shifted. Then, deviating from the representation of the last ignition angle ZW to be held for a maximum of 2 s. The one at the end of the transitional phase PO present, extremely late ignition angle ZW for example, 8 ° KWW before ZOT at 6000 min ^-1 would lead to catalyst damaging temperatures under permanent use and is here tolerable only because of the shortness of its application. The ignition retardation during the transition phase PO leads to a continuous reduction of the useful torque of the fired overrun operation BSF , so that in the subsequent overrun cutoff (unfired overrun BSU ) barely a momentum is perceptible.

Further, during the transitional phase PO performed a Ausmagerung by the lambda setpoint λ with a predeterminable Ausmagerungsgeschwindigkeit of, for example, about 0.1 s ^-1 in the direction of "lean", that is, to an oxygen-rich mixture is raised. In this case, preferably a final value of at least 0.97 and at most 1.08 is reached, whereby even the critical oxygen concentration of 1% in the raw exhaust gas can be exceeded. By the Ausmagerung is a continuous oxygen enrichment in the exhaust gas and on the catalysts 14 . 16 and thus causes a controlled burning of the reducing agent present here. Although this leads to a certain warming of the catalysts 14 . 16 However, pronounced exothermic peaks are not observed.

The effect of violent burning off in the catalysts 14 . 16 Remaining reducing agent is generally present at the transition from fired (load or thrust) operation to unfired overrun operation. An interposition of the transition phase PO but may also be useful without previous fired overrun operation, especially when exceeding a second predetermined catalyst temperature threshold TPO , this threshold 20 to 200 K, particularly advantageous 30 to 70 K below that for the request of the fired overrun operation BSF applicable temperature threshold TSF (see. 3 ) lies.

The overall strategy clarifies conclusively 4 , At low temperatures TK the catalysts 14 . 16 below the temperature threshold TPO is neither the transitional phase PO nor the fired push operation BSF authorized. Occurs in this lower temperature range, an operating point change from a load into a push phase ( τ _L → τ _S ), so suppression of the transitional phase PO from the fired load operation BLF in the unfired overrun mode BSU switched. Furthermore, in this temperature range, a shift phase τ _S basically unfurled ( BSU ).

Exceeds the catalyst temperature TK at least one catalyst 14 . 16 , in particular the precatalyst 14 , the temperature threshold TPO for the approval of the transitional phase PO , so the switch is from the fired load operation BLF in the unfired overrun mode BSU with the interposition of the transition phase according to the invention PO (but without upstream fired overrun BSF ) carried out. Furthermore, also in this temperature range below TSF a push phase τ _S basically unfired operated BSU ,

Exceeds the catalyst temperature TK at least one catalyst 14 . 16 even the temperature threshold TSF to request the fired overrun phase BSF , the fuel cut is no longer permitted and the overrun phase τ _S operate fired ( BSF ). A changeover of the internal combustion engine 10 from a load phase τ _L in a push phase τ _S takes place as based on 3 explained, first by transition into the fired overrun operation BSF , Only after the presence of a predefinable switching condition, for example, the temperature threshold TSF lower temperature TK the catalysts 14 . 16 , the transition phase according to the invention closes PO followed by the unfired fuel cutoff BSU ,

With this package of measures, an optimal catalyst protection can be achieved with the least possible disadvantages in terms of consumption and handling.

LIST OF REFERENCE NUMBERS

10

Internal combustion engine

12

exhaust duct

14

precatalyzer

16

Main catalyst / NO _X storage catalyst

18

lambda probe

20

NO _x sensor

22

temperature sensor

24

Engine control unit

λ

Lambda setpoint (air-fuel ratio)

τ _L

Last phase

τ _S

overrun phase

BLF

fired load operation

BSF

fired overrun operation (fuel cutoff)

BSU

unfired overrun operation

KWW

crankshaft angle

O ₂ -RE

Oxygen raw emission

PO

Transition phase (phase out)

t

Time

TSF

Temperature threshold for the request of the fired overrun operation

TK

catalyst temperature

TK _H

Catalyst temperature NO _X storage catalyst

TK _V

Catalyst temperature precatalyst

TPO

Temperature threshold for approval of the transition phase

ZW

firing angle

Claims (27)

Method for switching over an internal combustion engine (10) of a motor vehicle with at least one downstream catalytic converter (14, 16) from a fired operation (BLF, BSF) carried out in a load phase (τ _L ) or a coasting phase (τ _S ) to an unfired thrust mode (BSU) ) by interrupting a fuel supply, wherein during the overrun phase (τ _S ) a requested drive torque is less than or equal to a current thrust moment of the vehicle, wherein to switch from the fired operation (BLF, BSF) to the unspurged overrun mode (BSU) before interrupting the fuel supply a transition phase (PO) is performed, during the transition phase (PO) during the previous fired operation (BLF, BSF) present ignition angle (ZW) is adjusted in the direction of "late" and / or during the previous fired operation (BLF, BSF) set lambda setpoint (λ) in the direction of "lean" is shifted, thereby g ekennzeichnet that - when a catalyst temperature (TK) of at least one catalyst (14, 15) exceeds a first predetermined temperature threshold (TSF), the fuel cut is not permitted and the coasting phase (τ _S ) is operated fired (BSF); - When the catalyst temperature (TK) of at least one catalyst (14, 15) exceeds a second predetermined temperature threshold (TPO), but not the first predetermined temperature threshold (TSF), a switch from the fired load operation (BLF) in the unfired overrun (BSU) below Interposition of the transition phase (PO) is performed; and - at the catalyst temperatures (TK) of the catalysts (14, 15) below the second temperature threshold (TPO), neither the transition phase (PO) nor the fired overrun mode (BSF) are permitted. Method according to Claim 1 , characterized in that the transition phase (PO) over a period of at least twenty cycles of the internal combustion engine (10), in particular of at least fifty cycles, is performed. Method according to one of the preceding claims, characterized in that a switchover from the fired overrun mode (BSF) into the unfired overrun mode (BSU) takes place when a predefinable critical gradient of a vehicle path is exceeded. Method according to one of the preceding claims, characterized in that the second predefinable temperature threshold (TPO) by 20 to 200 K, in particular by 30 to 70 K, smaller than the first predetermined temperature threshold (TSF) for switching from the fired overrun operation (BSF) in the unfired overrun mode (BSU) is specified. Method according to one of the preceding claims, characterized in that a switchover from the fired overrun mode (BSF) or load mode (BLF) into the unfired overrun mode (BSU) only takes place with interposed transition phase (PO) if a determined gradient of a vehicle path is a predefinable Gradient falls below. Method according to one of the preceding claims, characterized in that during the fired overrun operation (BSF) before the beginning of the transition phase (PO) of the ignition angle (ZW) is driven to a maximum late control value at which under approximately stationary conditions, a maximum allowable catalyst temperature (TK ) of the at least one catalyst (14, 16) is not exceeded. Method according to Claim 6 , characterized in that during the fired overrun operation (BSF) before the beginning of the transition phase (PO) of the ignition angle (ZW) is set such that a precatalyst (14) has an at least approximately constant catalyst temperature (TK _V ). Method according to one of the preceding claims, characterized in that the adjustment of the ignition angle (ZW) during the transition phase (PO) stepwise or continuously with a predetermined adjustment speed, in particular with an adjustment of 1 to 20 ° KWW / s, in particular of about 5 ° KWW / s. Method according to Claim 8 , characterized in that a duration of the ignition angle adjustment at least 0.5 s and at most 5 s, in particular at least 1 s and at most 3.5 s, in particular about 2 s. Method according to Claim 9 , characterized in that the duration of the ignition angle adjustment in dependence on an engine speed and / or an exhaust gas temperature and / or the catalyst temperature (TK) of the at least one catalyst (14, 16) is set. Method according to one of the preceding claims, characterized in that after performing the Zündwinkelverstellung a present at the end of the transition phase (PO) ignition angle (ZW) is held for 0 to a maximum of 2 s before the fuel supply is interrupted. Method according to one of the preceding claims, characterized in that the displacement of the lambda desired value (λ) in the direction "lean" is carried out before, during and / or after the ignition angle adjustment. Method according to one of the preceding claims, characterized in that the lambda desired value (λ) during the transition phase (PO) is shifted to a final value of at least 0.97 and at most 1.08. Method according to Claim 13 , characterized in that the final value of the lambda desired value (λ) is determined as a function of the engine speed, the catalyst temperature (TK) of the at least one catalytic converter (14, 16) and / or a lambda-related exhaust gas composition. Method according to one of the preceding claims, characterized in that the displacement of the lambda value with a Ausmagerungsgeschwindigkeit of 0.01 to 0.3 s ^-1 , in particular of about 0.1 s ^-1 , is performed. Method according to Claim 15 , characterized in that the Ausmagerungsgeschwindigkeit depending on an exhaust gas mass flow, the catalyst temperature (TK) of the at least one catalyst (14, 16) and / or the lambdaabezogenen exhaust gas composition is set. Method according to one of the preceding claims, characterized in that during the fired overrun operation (BSF) before the transition phase (PO), the internal combustion engine (10) with a lambda setpoint (λ) of at most 1.00, in particular at most 0.95 operated. Method according to Claim 17 , characterized in that the lambda desired value (λ) during the fired overrun operation (BSF) before the transition phase (PO) depending on a measured or calculated exhaust gas temperature and / or catalyst temperature (TK) of the at least one catalyst (14, 16) can be predetermined. Method according to one of Claims 1 to 16 , characterized in that the lambda desired value (λ) during the fired overrun operation (BSF) before the transition phase (PO) is adjusted such that a predetermined maximum allowable temperature (T _max ) of the exhaust gas and / or the at least one catalyst (14, 16 ). Method according to one of the preceding claims, characterized in that the fuel supply by direct injection of the fuel into cylinders of the internal combustion engine (10). Method according to Claim 20 characterized in that the internal combustion engine (10) is a stratified charge gasoline engine. Method according to one of the preceding claims, characterized in that a NO _x storage catalyst (16) is used as the main catalyst. Device for switching over an internal combustion engine (10) of a motor vehicle with at least one downstream catalytic converter (14, 16) from a fired operation (BLF, BSF) carried out in a load phase (τ _L ) or a coasting phase (τ _S ) to an unfired thrust mode (BSU) ) by interrupting a fuel supply, wherein during the overrun phase (τ _S ) a requested driving torque is less than or equal to a momentary thrust torque of the vehicle, characterized by means that are arranged, the method according to one of Claims 1 to 22 perform. Device after Claim 23 , characterized in that the means comprise an algorithm (26) for performing the method steps in digital form. Device after Claim 24 , characterized in that the algorithm (26) in a control unit, in particular in an engine control unit (24), is deposited. Device according to one of Claims 23 to 25 , characterized by a noble metal content of at least one catalyst (14, 16) of at most 3.74 g / l (104 g / ft ³ ) catalyst volume. Device after Claim 26 , characterized by a noble metal content of at least one catalyst (14, 16) of at most 2.87 g / l (80 g / ft ³ ).

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