EP1307639B1 - Method and controller for operating a nitrogen oxide (nox) storage catalyst - Google Patents

Abstract

The invention relates to a method for operating a nitrogen oxide (NOx) storage catalyst (12') on an internal combustion engine (1), in particular in a motor vehicle (1). The nitrogen oxides (NOx) generated by the internal combustion engine (1) are stored in the storage catalyst (12') during a first operating phase and the nitrogen oxides stored in the storage catalyst (12') are released during a second operating phase. The beginning of the second operating phase is determined by means of a nitrogen oxide (NOx) level (mnosp) of the NOx storage catalyst (12'), which is modelled by means of a nitrogen oxide (NOx) storing model (30). According to the invention, in order to be able to determine the beginning and the end of the second operating phase as exactly and reliably as possible, a first value for nitrogen oxide (NOx) mass flow (msnonk s) behind the NOx storage catalyst (12') is determined and the NOx storing model (30) is corrected based on the determined first value (msnonk s).

Description

The present invention relates to a method for operating a nitrogen oxide (NOx) storage catalytic converter of an internal combustion engine, in particular of a motor vehicle, according to claim 1, part 1. In this process, nitrogen oxides generated by the internal combustion engine are stored in the storage catalytic converter in a first operating phase and nitrogen oxides stored in the storage catalytic converter are stored out of the storage catalytic converter in a second operating phase. The beginning of the second operating phase is determined on the basis of a nitrogen oxide (NOx) level of the NOx storage catalytic converter wherein the NOx level is modeled using a nitrogen oxide (NOx) storage model.

The invention also relates to a control device for an internal combustion engine, in particular of a motor vehicle, according to claim 7, part 1. The internal combustion engine can be switched back and forth by the control unit between a first operating phase, in which nitrogen oxides produced by the internal combustion engine are stored in the nitrogen oxide (NOx) storage catalytic converter, and a second operating phase, in which stored nitrogen oxides are expelled from the NOx storage catalytic converter become. The control unit has first means for determining the start of the second operating phase on the basis of a nitrogen oxide (NOx) level modeled by means of a nitrogen oxide (NOx) storage model of the NOx storage catalyst on. Furthermore, the present invention relates to a control, in particular a read-only memory or a flash memory, for such a control device.

Finally, the present invention relates to an internal combustion engine, in particular a motor vehicle according to the first part of claim 8. The internal combustion engine has a control unit and a nitrogen oxide (NOx) storage catalytic converter. The internal combustion engine can be switched back and forth by the control unit between a first operating phase, in which nitrogen oxides generated by the internal combustion engine are stored in the NOx storage catalytic converter, and a second operating phase, in which stored nitrogen oxides are expelled from the NOx storage catalytic converter. The internal combustion engine has first means for determining the start of the second operating phase on the basis of a nitrogen oxide (NOx) level of the NOx storage catalytic converter modeled by means of a nitrogen oxide (NOx) storage model.

Moreover, the invention relates to a control according to claim 6.

State of the art

In internal combustion engines which can be operated with a lean fuel / air mixture (lambda> 1), nitrogen oxide (NOx) storage catalysts are used to store the nitrogen oxide (NOx) emissions emitted by the internal combustion engine during a first operating phase (lean operation) , This first operating phase of the NOx storage catalyst is also referred to as Einspeicherphase. With increasing duration of Einspeicherphase the efficiency of the NOx storage catalyst decreases, which leads to an increase in NOx emissions downstream of the NOx storage catalyst. The cause of the decrease in efficiency is the increase in the nitrogen oxide (NOx) level of the NOx storage catalyst. The NOx level can be monitored and after exceeding a predetermined threshold, the second phase of operation of the NOx storage catalyst (Ausspeicherphase) are initiated. For determining the NOx level of the NOx storage catalytic converter, a nitrogen oxide (NOx) storage model can be used.

During the second operating phase, a reducing agent is added to the exhaust gas of the internal combustion engine, which reduces stored nitrogen oxides to nitrogen and oxygen. As the reducing agent, for example, hydrocarbon (HC) and / or carbon monoxide (CO) can be used, which can be generated by a rich adjustment of the fuel / air mixture in the exhaust gas. Alternatively, urea may also be added to the exhaust gas as the reducing agent. It is used to reduce the nitrogen oxide to oxygen and nitrogen ammonia from the urea. The ammonia can be obtained by hydrolysis from a urea solution.

Towards the end of the Ausspeicherphase a large part of the stored nitrogen oxide is reduced and less and less of the reducing agent meets nitric oxide, which can reduce it to oxygen and nitrogen. As a result, toward the end of the Ausspeicherphase increases the proportion of reducing agent in the exhaust gas behind the NOx storage catalyst, the proportion of oxygen in the exhaust gas downstream of the NOx storage catalyst decreases. By analyzing the exhaust gas behind the NOx storage catalyst by means of suitable exhaust gas sensors, the end of the Ausspeicherphase can then be initiated when the majority of the nitrogen oxide has been stored out of the NOx storage catalyst.

In one known from the prior art NOx injection model, the NOx level of the NOx storage catalyst depending on, inter alia, the NOx mass flow upstream of the NOx storage catalytic converter, the NOx mass flow downstream of the NOx storage catalytic converter and the temperature of the NOx storage catalytic converter. From these quantities, an efficiency of the NOx storage catalytic converter is determined, which multiplied by the NOx mass flow upstream of the NOx storage catalytic converter integrally supplies the current NOx level. As soon as the NOx level exceeds the predefinable threshold value, the second operating phase is initiated. The efficiency of the NOx storage catalyst decreases at constant boundary conditions with increasing NOx level.

According to EP-A-0 997 626, the amount of stored NOx is regulated in a storage catalytic converter 5 of an internal combustion engine, wherein a storage model is provided with a controlled parameter Alpha. This parameter alpha is integrally changed with a controller when the measured (oxygen) sensor voltage V downstream of the memory 5 during the emptying of the memory 5 at a time t <t3 (not) reaches the threshold for a rich mixture.

The present invention has for its object to be able to determine the NOx level of a NOx storage catalytic converter with the help of a NOx Einspeichermodells and thus the beginning and end of the second phase of operation (Ausspeicherphase) as accurately and reliably as possible to ensure optimum exhaust quality. This object is achieved with the features according to the independent claims.

Advantages of the invention

According to the invention, it is therefore proposed to correct the NOx injection model by a measured value. From the measured value, a correction factor for the NOx injection model can be obtained, which can be used for diagnostic purposes. Due to the measured value of the NOx level, the with

Help the NOx injection model modeled NOx level corrected and thus the beginning and the end of the second phase of operation can be determined with a much higher accuracy. This in turn allows to go to the limit of the storage capacity of the NOx storage catalyst, d. H. make full use of the storage capacity of the NOx storage tank without exceeding it, which leads to a significantly improved exhaust gas quality. With the aid of the method according to the invention, the NOx storage model or the start and the end of the second operating phase are adapted to the actual emissions of the internal combustion engine.

According to an advantageous development of the present invention, it is proposed that the first value of the NOx mass flow downstream of the NOx storage catalytic converter be measured by means of a NOx sensor.

According to a preferred embodiment of the present invention, it is proposed that a second value of the NOx mass flow downstream of the NOx storage catalytic converter is taken from the NOx storage model and the NOx storage model is corrected as a function of the two values of the NOx mass flow.

Advantageously, a difference between the two values of the NOx mass flows is formed and the NOx injection model is corrected as a function of the difference.

Advantageously, the NOx level is determined by integrating the product of the NOx mass flow upstream of the NOx storage catalyst and an efficiency of the NOx storage catalyst in the NOx storage model. The efficiency of the NOx storage catalytic converter becomes, for example, dependent on the NOx mass flow upstream of the NOx storage catalytic converter and on the temperature of the NOx storage catalytic converter determined.

According to a further preferred embodiment of the present invention, it is proposed that the difference between the two values of the NOx mass flow downstream of the NOx storage catalytic converter is supplied to a controller and the NOx storage model is corrected as a function of a control variable of the controller. The controller is preferably designed as an integrating (I) controller. The output signal of the NOx sensor arranged downstream of the NOx storage catalytic converter is therefore not evaluated directly, for example via the absolute value, the gradient or the like, but serves to regulate the NOx injection model by means of the I controller.

Finally, it is proposed that the NOx injection model is corrected as a function of the efficiency of the NOx storage catalytic converter as the manipulated variable of the controller.

Of particular importance is the realization of the method according to the invention in the form of a control element which is provided for a control unit of an internal combustion engine, in particular of a motor vehicle. In this case, a program is stored on the control, which is executable on a computing device, in particular on a microprocessor, and suitable for carrying out the method according to the invention. In this case, therefore, the invention is realized by a program stored on the control program, so that this provided with the program control in the same way is the invention as the method to whose execution the program is suitable. In particular, an electrical storage medium can be used as the control, for example a read-only memory or a flash memory.

As a further solution to the problem of the present According to the invention, the control unit proposes second means for detecting a first value of the nitrogen oxide (NOx) mass flow downstream of the NOx storage catalytic converter and third means for correcting the NOx storage model as a function of the detected first value having.

Finally, to solve the object of the present invention, starting from the Brenrikraftmaschine of the type mentioned above, it is proposed that the internal combustion engine second means for detecting a first value of the nitrogen oxide (NOx) mass flow behind the NOx storage catalyst and third means for correcting the NOx Einspeichermodells in response to the detected first value.

drawings

Figur 1

ein schematisches Blockschaltbild einer erfindungsgemäßen Brennkraftmaschine gemäß einer bevorzugten Ausführungsform;

Figur 2

einen schematischen Signallaufplan eines NOx-Einspeichermodells; und

Figur 3

einen schematischen Signallaufplan eines erfindungsgemäßen Verfahrens gemäß einer bevorzugten Ausführungsform.

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

FIG. 1

a schematic block diagram of an internal combustion engine according to the invention according to a preferred embodiment;

FIG. 2

a schematic signal diagram of a NOx injection model; and

FIG. 3

a schematic signal plan of a inventive method according to a preferred embodiment.

Description of the embodiments

1 shows a direct-injection internal combustion engine 1 of a motor vehicle is shown, in which a piston 2 in a cylinder 3 back and forth. The cylinder 3 is provided with a combustion chamber 4, which u.a. is limited by the piston 2, an inlet valve 5 and an outlet valve 6. With the intake valve 5, an intake pipe 7 and the exhaust valve 6, an exhaust pipe 8 is coupled.

In the region of the inlet valve 5 and the outlet valve 6, a fuel injection valve 9 and a spark plug 10 protrude in the combustion chamber 4. Via the injection valve 9, fuel can be injected into the combustion chamber 4. With the spark plug 10, the fuel in the combustion chamber 4 can be ignited.

In the intake pipe 7, a rotatable throttle valve 11 is housed, via which the intake pipe 7 air can be supplied. The amount of air supplied is dependent on the angular position of the throttle valve 11. In the exhaust pipe 8, a catalyst 12 is housed, which cleans the exhaust gases resulting from the combustion of the fuel. The catalyst 12 is a nitrogen oxide (NOx) storage catalyst 12 'coupled to a 3-way catalyst 12 "as an oxygen storage.

A control unit 18 is acted upon by input signals 19, which represent operating variables of the internal combustion engine 1 measured by means of sensors. The control unit 18 generates output signals 20 with which the behavior of the internal combustion engine 1 can be influenced via actuators or actuators. Among other things, the controller 18 is to intended to control the operating variables of the internal combustion engine 1 and / or to regulate. For this purpose, the control unit 18 is provided with a microprocessor which has stored in a storage medium, in particular in a flash memory, a program which is adapted to perform said control and / or regulation.

In a first operating mode, a so-called homogeneous operation of the internal combustion engine 1, the throttle valve 11 is partially opened or closed depending on the desired torque. The fuel is injected from the injection valve 9 during a suction phase caused by the piston 2 into the combustion chamber 4. By simultaneously sucked on the throttle valve 11 air, the injected fuel is swirled and thus substantially uniformly distributed in the combustion chamber 4. Thereafter, the fuel-air mixture is compressed during the compression phase to be ignited by the spark plug 10. Due to the expansion of the ignited fuel, the piston 2 is driven. The resulting torque depends in homogeneous operation u.a. from the position of the throttle valve 11 from. In view of a low pollutant development, the fuel air mixture is adjusted as possible to lambda = 1.

In a second mode, a so-called shift operation of the internal combustion engine 1, the throttle valve 11 is opened wide. The fuel is injected from the injection valve 9 during a caused by the piston 2 compression phase in the combustion chamber 4, locally in the immediate vicinity of the spark plug 10 and in time at a suitable distance before the ignition. Then we ignited with the help of the spark plug 10 of the fuel, so that the piston 2 in the now following working phase by the expansion of the inflamed Fuel is driven. The resulting torque largely depends on the injected fuel mass during shift operation. Essentially, the stratified operation is provided for the idle operation and the partial load operation of the internal combustion engine 1. In stratified operation lambda is usually> 1.

During a first operating phase, the internal combustion engine 1 is operated in stratified operation and the storage catalytic converter 12 'is charged with nitrogen oxides and the 3-way catalyst 12 "with oxygen (injection phase) In a second operating phase (regeneration phase), the storage catalytic converter 12' and the third Pathway catalyst 12 "discharged again so that they can again absorb nitrogen oxides or oxygen in a subsequent shift operation (Ausspeicherphase). During the regeneration phase, a reducing agent is added to the exhaust gas upstream of the catalyst 12. As the reducing agent, for example, hydrocarbons (HC), carbon monoxide (CO) or urea can be used. Hydrocarbons and carbon monoxide are generated in the exhaust gas by a rich mixture adjustment (operation of the internal combustion engine in homogeneous operation). Urea can be metered controlled from a reservoir to the exhaust gas. During the regeneration phase of the catalyst 12, the following processes take place: The reducing agent reduces the stored nitrogen oxides to nitrogen and oxygen. These substances exit from the catalyst 12, so that there is an excess of oxygen behind the catalyst 12 during the regeneration phase, although the internal combustion engine 1 is operated with a rich fuel / air mixture (lack of oxygen).

Before the catalyst 12, an oxygen (02) sensor 13 and after the catalyst 12, a nitrogen (NOx) sensor 14 in the exhaust pipe 8 is arranged. After switching to Lack of oxygen (operation of the internal combustion engine 1 with a rich mixture) before the catalyst 12 at the beginning of the regeneration phase, the O2 sensor 13 reacts virtually instantaneously. Due to the prevailing excess oxygen during the shift operation in the exhaust gas, the oxygen storage locations of the catalytic converter 12 are initially almost all occupied. After switching to lack of oxygen at the beginning of the regeneration phase, the oxygen storage sites are successively freed of oxygen, which then emerges from the catalyst 12. After the catalyst 12 therefore prevails after switching to the Regeneratonsphase initially further excess oxygen. After a period of time dependent on the oxygen storage capacity of the catalytic converter 12, the entire nitrogen oxide stored in the catalytic converter 12 'is reduced and the total oxygen stored in the oxygen storage 12 "is removed, so that oxygen deficiency also occurs behind the catalytic converter 12.

FIG. 2 schematically shows a NOx storage model 30. As input variables, the NOx mass flow msnovk upstream of the catalytic converter 12 and an efficiency eta_sp of the NOx storage catalytic converter 12 'are applied to the NOx injection model 30. The efficiency eta_sp is dependent on u.a. the NOx mass flow msnovk before the NOx storage catalytic converter 12 ', a NOx mass flow msnonk behind the NOx storage catalytic converter 12' and the temperature of the NOx storage catalytic converter 12 'determined. The efficiency eta_sp is a non-linear function of the NOx filling level mnosp of the NOx storage catalytic converter 12 'and decreases as the NOx level increases.

In a multiplier 31, a product mnsospe of the NOx mass flow msnovk and the efficiency eta_sp is formed. The product mnsospe is stored in an integrator 32 integrated. As an output signal, the integrator 32 supplies the NOx filling level mnosp of the NOx storage catalytic converter 12 '. This is compared in a comparator 33 with a predefinable threshold schw. If the NOx level mnosp exceeds the threshold value schw, the regeneration phase of the NOx storage catalytic converter 12 'is initiated by means of a regeneration signal B_denox.

FIG. 3 schematically shows a method according to the invention. In the method, an output signal msnonk_s of the arranged behind the catalyst 12 NOx sensor 14 is used to control the NOx injection model 30. Thus, the beginning and end of the second phase of operation (regeneration phase) of the NOx storage catalyst 12 'can be determined much more accurate and reliable , which leads to a significantly improved exhaust quality.

A modeled NOx mass flow msnonk_m after the catalyst 12 is modeled. The modeled NOx mass flow msnonk_m results from the difference of the NOx mass flow msnovk before the catalyst 12 and the product of the NOx mass flow msnovk and the efficiency eta_sp, d. H. from msnovk · (1 - eta_sp). The NOx mass flow msnovk upstream of the catalyst 12 may be measured by a NOx sensor (not shown) or taken from the NOx model.

From a difference of the modeled NOx mass flow msnonk_m after the catalyst 12 and the measured by the NOx sensor 14 NOx mass flow msnonk_s after the catalyst 12, a control difference 34 of the control circuit shown in Figure 3 is formed. The control difference 34 is supplied to an integrating I-controller 35. Instead of an I-controller 35, any other suitable regulators can also be used.

A manipulated variable 36 of the I-controller 35 is passed to an actuator 37, which varies a manipulated variable 38 in order to act in a controlled manner on the NOx injection model 30. The manipulated variable 38 is the efficiency eta_sp of the NOx storage catalytic converter 12 '.

Claims (8)

1. Method for operating a nitrogen oxide (NOx) storage catalytic converter (12') of an internal combustion engine (1) in particular of a motor vehicle, in which nitrogen oxides (NOx) generated by the internal combustion engine (1) are accumulated in the NO_x storage catalytic converter (12') during a first operating phase and nitrogen oxides which have accumulated in the NOx catalytic converter (12') are discharged from the NOx storage catalytic converter (12') in a second operating phase, the start of the second operating phase is determined on the basis of a nitrogen oxide (NOx) filling level (mnosp) of the NOx storage catalytic converter (12'), the NOx filling level (mnosp) is modelled on the basis of a nitrogen oxide (NOx) accumulation model (30), and a first value of a nitrogen oxide (NOx) mass flow (msnonk_s) downstream of the NOx storage catalytic converter (12') is recorded, and the NOx accumulation model (30) is corrected as a function of the recorded first value, characterized in that a second value of the NOx mass flow (msnonk_m) downstream of the NOx storage catalytic converter (12') is removed from the NOx accumulation model (30), and a difference between the two values of the NOx mass flows (msnonk_m - msnonk_s) is formed and the NOx accumulation model (30) is corrected as a function of the difference (msnonk_m - msnonk_s).
2. Method according to Claim 1, characterized in that the difference (msnonk_m - msnonk_s) between the two values (msnonk_m, msnonk_s) is fed to a controller (35), and the NOx accumulation model (30) is corrected as a function of a control variable (38) of the controller (35).
3. Method according to Claim 1 or 2, characterized in that the NOx filling level (mnosp) is determined by integrating the product of the NOx mass flow (msnovk) upstream of the NOx storage catalytic converter (12') and an efficiency (eta_sp) of the NOx storage catalytic converter (12') in the NOx accumulation model (30).
4. Method according to Claim 2 or 3, characterized in that the NOx accumulation model (30) is corrected as a function of the efficiency (eta_sp) of the NOx storage catalytic converter (12') as the control variable (38) of the controller (35).
5. Method according to one of Claims 1 to 4, characterized in that the first value of the NOx mass flow (msnonk_s) downstream of the NOx storage catalytic converter (12') is measured by means of an NOx sensor (14).
6. Control element, in particular read only memory or flash memory, for a control unit (18) of an internal combustion engine (1) in particular of a motor vehicle, on which is stored a program which can run on a computer unit, in particular on a microprocessor, and is suitable for executing a method according to one of Claims 1 to 5.
7. Control unit (18) for an internal combustion engine (1) in particular of a motor vehicle, it being possible for the internal combustion engine (1) to be switched by the control unit (18) between a first operating phase, in which nitrogen oxides (NOx) generated by the internal combustion engine (1) are accumulated in the nitrogen oxide (NOx) storage catalytic converter (12'), and a second operating phase, in which accumulated nitrogen oxides are discharged from the NOx storage catalytic converter (12'), and the control unit (18) having first means for determining the start of the second operating phase on the basis of a nitrogen oxide (NOx) filling level (mnosp) of the storage catalytic converter (12'), which has been modelled by means of a nitrogen oxide (NOx) accumulation model (30), second means (14) for recording a first value of the nitrogen oxide (NOx) mass flow (msnonk_s) downstream of the NOx storage catalytic converter (12'), and third means for correcting the NOx accumulation model (30) as a function of the recorded first value (msnonk_s), characterized in that the control unit (18) has fourth means for removing a second value of the NOx mass flow (msnonk_m) downstream of the NOx storage catalytic converter (12') from the NOx accumulation model (30) and fifth means for forming a difference between the two values of the NOx mass flows (msnonk_m - msnonk_s), and the third means correct the NOx accumulation model (30) as a function of the difference (msnonk_m - msnonk_s).
8. Internal combustion engine (1) in particular of a motor vehicle, the internal combustion engine (1) having a control unit (18) and a nitrogen oxide (NOx) storage catalytic converter (12'), and it being possible for the internal combustion engine (1) to be switched by the control unit (18) between a first operating phase, in which nitrogen oxides (NOx) generated by the internal combustion engine (1) are accumulated in the NOx storage catalytic converter (12'), and a second operating phase, in which accumulated nitrogen oxides are discharged from the NOx storage catalytic converter (12'), and the internal combustion engine (1) having first means for determining the start of the second operating phase on the basis of a nitrogen oxide (NOx) filling level (mnosp) of the NOx storage catalytic converter (12'), which has been modelled by means of a nitrogen oxide (NOx) accumulation model (30), second means (14) for recording a first value of the nitrogen oxide (NOx) mass flow (msnonk_s) downstream of the NOx storage catalytic converter (12') and third means for correcting the NOx accumulation model (30) as a function of the recorded first value (msnonk_s), characterized in that the internal combustion engine (1) has fourth means for removing a second value of the NOx mass flow (msnonk_m) downstream of the NOx storage catalytic converter (12') from the NOx accumulation model (30) and fifth means for forming a difference between the two values of the NOx mass flows (msnonk_m - msnonk_s), and the third means correct the NOx accumulation model (30) as a function of the difference (msnonk_m - msnonk_s).

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