DE10010031A1 - Regeneration of nitrogen oxides storage catalyst involves carrying out regeneration during at least portion of regeneration time with reduced exhaust gas return rate or with exhaust gas return - Google Patents

Abstract

Process for carrying out NOx regeneration of a NOx storage catalyst arranged in the exhaust gas channel (12) of an I.C. engine (10) comprises carrying out the regeneration during at least a portion of the regeneration time with a reduced exhaust gas return rate or with exhaust gas return. An increase or initiation of an exhaust gas return takes place before the end of the regeneration. An Independent claim is also included for a device for carrying out NOx regeneration of a NOx storage catalyst arranged in the exhaust gas channel of an I.C. engine comprising a control unit (22) integrated in an engine control device (20).

Description

The invention relates to a method and a device for carrying out a NO _x regeneration of a NO _x storage catalytic converter arranged in an exhaust gas duct of an internal combustion engine with the features mentioned in the preamble of claims 1 and 9.

In order to carry out an aftertreatment of exhaust gases from internal combustion engines, in particular internal combustion engines that run lean at times, it is known to arrange catalysts with an NO _x storage function (NO _x storage catalysts) in exhaust gas lines of the internal combustion engines. The catalytic converter reduces pollutant emissions by promoting the conversion of exhaust gas components such as carbon monoxide, unburned hydrocarbons and nitrogen oxides into less environmentally relevant compounds. Currently, the long-term possible operation of internal combustion engines in a lean operating mode with λ <1, that is to say with an excess of oxygen in an air / fuel mixture supplied, is sought. Under these conditions, fuel consumption is reduced. On the other hand, a complete catalytic reduction of NO _x cannot be achieved in lean mode due to the excess of oxygen. Instead, NO _{x is absorbed} into the NO _x storage catalytic converter during lean operation. Because of the limited Na _x storage capacity of the catalytic converter, it has to be subjected to a NO _x regeneration periodically, for which purpose it is usually subjected to a rich exhaust gas atmosphere, so that absorbed NO _x by reducing agents (HC, CO) now present in the exhaust gas in excess can be converted.

It is also known to operate an internal combustion engine with an exhaust gas recirculation system, a portion of the exhaust gas being admixed with the air / fuel mixture to be supplied to the internal combustion engine. This measure allows a combustion temperature and the associated NO _x production to be reduced. For this purpose, an exhaust gas recirculation valve arranged in a return line is usually controlled as a function of an operating point of the internal combustion engine.

The NO _x regeneration of No _x storage catalytic converters and the exhaust gas recirculation are usually regulated independently of one another, so that exhaust gas recirculation is permitted during NO _x regeneration. This poses various problems. First of all, a duration of the switchover from lean to rich operation of the internal combustion engine, which elapses from the point in time at which a rich air-fuel mixture is specified until the actual presence of a rich exhaust gas, is undesirably extended. This is due to the presence of oxygen-rich exhaust gas in the exhaust gas recirculation line, which is still fed to the combustion chamber of the engine, while a rich mode of the internal combustion engine is already being requested. Another problem arises from switching from shift to homogeneous operation of the internal combustion engine, which is usually associated with switching to rich mode. Associated with this, flammability problems and misfiring occur especially shortly after the change of operation if the mixture is too lean to ignite despite an increase in the fuel supply. This problem is exacerbated by the recirculation of the initially lean exhaust gas. Finally, the duration of a NO _x regeneration is considerably extended by the exhaust gas recirculation. This is due to the reduced exhaust gas mass flow and the associated reduced reducing agent flow, which is necessary for the conversion of the released nitrogen oxides on the NO _x storage catalytic converter. Extending the regeneration time leads to a significant increase in fuel consumption.

The invention is therefore based on the object of proposing a method and a device by means of which the problems described when carrying out a NO _x regeneration of a NO _x storage catalytic converter according to the prior art are overcome.

This object is achieved by the method and the device with the features mentioned in independent claims 1 and 9. Because the NO _x regeneration is carried out at a reduced exhaust gas recirculation rate or without exhaust gas recirculation and an end of the NO _x regeneration is preceded by an increase or activation of an exhaust gas recirculation, a rapid and intermittent-free switching of the internal combustion engine from a lean to a rich operation is ensured and the regeneration times shortened. Furthermore, as a result of the rapid switching, a NO _x desorption peak, which is observed when the lambda value passes from λ <1 to λ ≦ 1, is reduced. By increasing the exhaust gas recirculation rate before the end of the NO _x regeneration, the transition when switching the internal combustion engine from the regeneration mode to a lean mode is made more fluid, so that practically no change occurs in the operating behavior of the motor vehicle.

In a preferred embodiment of the method, the NO _x regeneration is carried out for at least part of its regeneration period at an exhaust gas recirculation rate of 0 to 15% by volume, in particular 0 to 10% by volume. Current recirculation rates are in the range of 25 to 35% by volume, depending on the motor type. The volume proportion relates to a proportion of the recirculated exhaust gas volume in a total air volume to be supplied to the internal combustion engine. Within the framework of advantageous specifications, the exhaust gas recirculation rate can be varied in steps or continuously during the regeneration.

It is also preferably provided that the exhaust gas recirculation rate is increased to 5 to 25% by volume, in particular 15% by volume, before the end of the NO _x regeneration. The exhaust gas recirculation rate to be achieved at the end of the NO _x regeneration can in particular be made dependent on the recirculation rate to be set after the NO _x regeneration has ended.

A further advantageous embodiment provides for the exhaust gas recirculation rate to be increased gradually or continuously before the end of the NO _x regeneration. In this way, the transition can be leveled further.

According to a further embodiment, the exhaust gas recirculation rate is increased 5 to 0.5 s, in particular 2 s, before the NO _x regeneration has ended. In this context, it can further advantageously be provided that the time at which the exhaust gas recirculation rate increases before the end of the NO _x regeneration is measured as a function of the exhaust gas recirculation rate present before and / or after the increase. For example, if there is a large difference in the exhaust gas recirculation rates, an increase in the exhaust gas recirculation rate should be started relatively early before and after the NO _x regeneration has ended.

In the device according to the invention for carrying out a NO _x regeneration, means are provided with which the method steps of carrying out the NO _x regeneration can be carried out at a reduced exhaust gas recirculation rate or without exhaust gas recirculation and increasing an exhaust gas recirculation rate before the end of the NO _x regeneration.

In a preferred embodiment, these means comprise a control unit in which a procedure for controlling the method steps for carrying out NO _x regeneration and the NO _x storage catalytic converter is stored in digital form, the control unit being able to be integrated in an engine control unit.

Other advantageous embodiments of the invention are the subject of the rest Subclaims.

The invention is described below in exemplary embodiments based on the associated Drawings explained in more detail. Show it:

Fig. 1 is a schematic arrangement of an internal combustion engine with an exhaust gas duct and

Fig. 2 shows a time course of lambda during a NO _x regeneration with and without exhaust gas recirculation.

Fig. 1 is a schematic representation showing an internal combustion engine 10 with one of these associated exhaust channel 12. Downstream of the internal combustion engine 10 , a small-volume pre-catalytic converter 14 and an NO _x storage catalytic converter 16 are arranged in the exhaust gas duct 12 . A lambda probe 18 is used to detect an oxygen concentration in the exhaust gas and is still located in front of the catalyst system 14 , 16 in the exhaust gas duct 12 . The lambda sensor 18 reproduces a signal to an engine control unit 20 , which processes this and other measurement signals as well as operating parameters of the internal combustion engine 10 . Integrated in the engine control unit 20 is a control unit 22 , in which the procedure according to the invention for performing a NO _x regeneration of the NO _x storage catalytic converter 16 is stored in digital form. Depending on all incoming signals, engine control unit 20 or control unit 22 controls the operating mode of internal combustion engine 10 , for example by influencing an air-fuel mixture to be fed. For this purpose, an intake fresh air volume flow is regulated by setting a throttle valve 24 in an intake pipe 26 . Furthermore, the engine control unit 20 or the control unit 22 controls an exhaust gas recirculation valve 30 arranged in a return line 28 .

If a need for regeneration of the NO _x storage catalytic converter 16 is now established during a lean phase of the internal combustion engine 10 , the control unit 22 effects the switchover of the internal combustion engine 10 into a homogeneous, stoichiometric or rich operating mode. For this purpose, the control unit 22 specifies, for example, a more closed position of the throttle valve 24 , so that the proportion of oxygen in the supplied air / fuel mixture is reduced. According to the invention, the exhaust gas recirculation is reduced and / or completely prevented during at least part of the regeneration period. This is also done by a corresponding control of the exhaust gas recirculation valve 30 by the engine control unit 20 or the control unit 22 .

Fig. 2 shows a temporal course, in a simplified representation of a measured by the lambda probe 18 upstream of the catalyst system 14, 16 lambda value during an NO _x regeneration of the NO _x storage catalytic converter 16. The broken line 32 represents a lambda curve during regeneration with activation of an exhaust gas recirculation according to current customary practice (recirculation rate about 30 vol.%), While the solid line 34 shows a corresponding lambda curve without exhaust gas recirculation. During an initial lean phase, both profiles 32 , 34 have a constant level corresponding to a Lambda lean specification λ _m . After detection of a need for regeneration and switching of an air-fuel mixture supplied to the internal combustion engine 10 to a rich, that is to say fuel-rich composition, at a time t ₀ , the signal 32 measured with simultaneous exhaust gas recirculation still remains for a certain delay at the lean lambda value λ _m . This can be attributed to the oxygen-rich exhaust gas, which from the previous lean phase is still in the exhaust gas recirculation line 28 and is supplied to the fresh air drawn in. After this delay, the signal 32 begins to fall relatively flatly until a time t ₁ at which a value corresponding to the lambda fat specification λ _f is reached. The flatness of the previous drop in the lambda value is also a consequence of the exhaust gas being fed into the fresh air drawn in. During the following regeneration period, which extends up to a point in time t ₂ , the lambda fat specification λ _{f is} maintained and nitrogen oxides NO _x stored in the NO _x storage catalytic converter 16 are reduced with the reducing agents CO and HC present in the exhaust gas. After the memory has been emptied at time t ₂ , the internal combustion engine 10 is switched back to the lean mode, so that the lambda value reappears for the lambda lean specification λ _m .

The lambda curve 34 (solid line) measured according to the invention without exhaust gas recirculation shows considerable deviations from the curve 32 explained above, measured with the exhaust gas recirculation switched on. Without exhaust gas recirculation, the lambda curve 34 has a curve that begins almost immediately after switching the internal combustion engine 10 into the rich operating mode at the time t ₀ and drops off relatively steeply, so that the lambda fat specification λ _{f is} already reached at a time t ₁ ^′ . Furthermore, the regeneration time, which must be applied for a complete emptying of the storage of the NO _x storage catalytic converter 16 , is considerably shorter than with an approved exhaust gas recirculation, so that the end of the NO _x regeneration is already reached at a time t ₂ ^' . The very long regeneration period with simultaneous exhaust gas recirculation (curve 32 ) is a result of the constant exhaust gas extraction, so that only a reduced mass of reducing agent is available to convert the nitrogen oxides of the NO _x storage catalytic converter.

Overall, the regeneration time without exhaust gas recirculation is compared to that with simultaneous exhaust gas recirculation, the regeneration time is approximately the same Shortened by half. The preferred exhaust gas recirculation rates according to the invention of a maximum of 15% by volume have lambda curves that show an intermediate state between the represent explained courses.

By connecting the exhaust gas recirculation or increasing the exhaust gas recirculation rate before the end of the NO _x regeneration, the regeneration period is increased slightly. However, this disadvantage is offset by the advantage of a largely torque-neutral transition in the operating mode of the internal combustion engine.

REFERENCE SIGN LIST

10th

Internal combustion engine

12th

Exhaust duct

14

Pre-catalyst

16

NO _x

Storage catalytic converter

18th

Lambda sensor

20th

Engine control unit

22

Control unit

24th

throttle

26

Intake pipe

28

Exhaust gas recirculation line

30th

Exhaust gas recirculation valve

32

Lambda curve with exhaust gas recirculation

34

Lambda curve without exhaust gas recirculation
λ _f

Lambda grease specification
λ _m

Lambda target

Claims (11)

1. A method for carrying out a NO _x regeneration of a NO _x storage catalytic converter arranged in an exhaust gas duct of an internal combustion engine, the internal combustion engine being able to be operated in different operating modes and exhaust gas recirculation being carried out in an intake air of the internal combustion engine in at least one of the operating modes, characterized in that the NO _x regeneration is carried out during at least part of its regeneration period at a reduced exhaust gas recirculation rate or without exhaust gas recirculation and an end of the NO _x regeneration is preceded by an increase or activation of an exhaust gas recirculation. 2. The method according to claim 1, characterized in that the NO _x regeneration of an exhaust gas recirculation rate of 0 to 15% by volume, in particular at 0 to 10% by volume, is carried out. 3. The method according to claim 1 or 2, characterized in that the exhaust gas recirculation rate is increased to 5 to 25% by volume, in particular 15% by volume, before the end of the NO _x regeneration. 4. The method according to any one of the preceding claims, characterized in that the exhaust gas recirculation rate is increased gradually or continuously before the end of the NO _x regeneration. 5. The method according to any one of the preceding claims, characterized in that the exhaust gas recirculation rate is increased 5 to 0.5 s, in particular 2 s, before the NO _x regeneration has ended. 6. The method according to claim 5, characterized in that the time of the increase in the exhaust gas recirculation rate before the end of the NO _x regeneration is measured as a function of the before and / or after the increase in the exhaust gas recirculation rate. 7. The method according to any one of the preceding claims, characterized in that the NO _x regeneration is carried out in a stoichiometric or rich operation of the internal combustion engine. 8. The method according to any one of the preceding claims, characterized in that the NO _x regeneration is carried out with homogeneous operation of the internal combustion engine. 9. Device for carrying out a NO _x regeneration of a NO _x storage catalytic converter arranged in an exhaust gas duct of an internal combustion engine, characterized in that means are provided with which the method steps carry out the NO _x regeneration at a reduced exhaust gas recirculation rate or without exhaust gas recirculation and increasing one Exhaust gas recirculation rate can be carried out before the end of the NO _x regeneration. 10. The device according to claim 9, characterized in that the means comprise a control unit ( 22 ) in which a procedure for controlling the method steps for carrying out a NO _x regeneration of a NO _x storage catalytic converter is stored in digital form. 11. The device according to claim 10, characterized in that the control unit ( 22 ) is integrated in an engine control unit ( 20 ).