WO2002006653A1

WO2002006653A1 - Method for adapting a crude nox concentration

Info

Publication number: WO2002006653A1
Application number: PCT/DE2001/002577
Authority: WO
Inventors: Corinna Pfleger; Hong Zhang; Wolfgang Ludwig
Original assignee: Siemens Aktiengesellschaft
Priority date: 2000-07-18
Filing date: 2001-07-10
Publication date: 2002-01-24
Also published as: DE10034874A1; EP1301699B1; DE10034874B4; EP1301699A1; DE50105704D1

Abstract

The invention relates to a method for adapting a crude NOx concentration. During an adaptation process for a projected crude NOx concentration the duration of the lean phase is shortened in such a manner that no NOx is emitted during the lean phase. In a subsequent rich phase, the quantity of NOx stored during the shortened lean phase is calculated and compared with the projected NOx charge for said lean phase. The comparative value is used for the adaptation of the crude NOx concentration.

Description

description

Method for adapting a raw NOx concentration

The invention relates to a method for adapting a raw NOx concentration of an internal combustion engine working with lean and rich phases, which is provided with a NOx storage reduction catalytic converter which is arranged in an exhaust gas duct of the internal combustion engine and adsorbs NOx from the exhaust gas during the lean phase and during the rich phase can convert the adsorbed NOx.

In order to reduce the fuel consumption in motor vehicles with an petrol engine, internal combustion engines are increasingly being used which are operated with a lean mixture in selected operating areas.

A special exhaust gas aftertreatment is used in such internal combustion engines to meet exhaust gas limit values. For this purpose, NOx storage reduction catalysts, hereinafter also referred to as NOx storage catalysts, are used. Due to their coating, these NOx storage catalytic converters are able to adsorb NOx compounds from the exhaust gas during a storage phase. The storage phase of the NOx storage catalytic converter is also referred to as its loading phase. During a regeneration phase following the loading phase, the adsorbed or stored NOx compounds are catalytically converted into harmless compounds. CO, H ₂ and HC (hydrocarbons) are used as reducing agents for the conversion in lean petrol engines. The reducing agents are generated by briefly operating the internal combustion engine with a rich mixture and made available to the NOx storage catalytic converter in the form of exhaust gas components, as a result of which the stored NOx compounds in the catalytic converter are broken down.

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The object of the invention is to provide a method with which the raw NOx concentration of an internal combustion engine can be adapted as precisely as possible using simple means.

The object is achieved by the features of patent claim 1. Advantageous developments of the invention are specified in the subclaims.

The above process performs the following four process steps during an adaptation process for the raw NOx concentration. In a first step, the duration of a first lean phase is shortened in such a way that no NOx emission occurs behind the catalytic converter during the first lean phase. The NOx emissions in the course of a lean phase are known qualitatively: in a first section of the lean phase there is no or essentially no NOx emission behind the catalytic converter, which increases sharply from one point in time. The duration of the first lean phase is selected in the first process step so that the lean phase has ended before there is an increase in the NOx concentration after the catalyst. In the second step of the method according to the invention, the amount of storage stored in the NOx storage catalytic converter is calculated during the lean phase. The amount of storage corresponds to the corrected NOx supply, since there is no breakthrough percentage in the first lean phase. Because the first lean phase has ended before there is a breakthrough. In a further step, which can also take place at the same time as the second step, the calculated storage quantity is compared with a NOx loading of the NOx storage catalytic converter modeled for the first lean phase using a map, depending on the operating states. For the first lean phase, the NOx loading modeled neglecting the breakthrough should match the amount of storage, since there was no NOx emission after the catalytic converter during the first lean phase. The deviation of these two variables from one another is used in a fourth step for the adaptation. After adaptation of the raw NOx concentration, the internal combustion engine can be operated again using known methods. The advantage of the present method is that the shortening of the first lean phase enables simple determination of the NOx loading with simple means.

To determine that in the first step a sufficiently short lean phase is the basis for the consideration, in the second step the amount of memory is additionally determined in a second lean phase, the duration of which is further shortened compared to that of the first lean phase. For this purpose, the process steps from the adaptation cycle are essentially repeated with the first lean phase. In this continuation of the method according to the invention, the entire process sequence is represented as the repetition of two adaptation cycles with lean phases of different lengths, the first and second method steps for the second lean phase being carried out in the second cycle. For the first and the second lean phase, the average storage amounts per time, i.e. calculated amount of storage divided by the duration of the lean phase, compared with one another in order to determine a missing NOx emission in the first lean phase in the event of an approximate match. In the event of a mismatch, the loading is not proportional over time, i.e. during the lean phase, for example, the NOx concentration after the catalytic converter has already risen and the breakthrough amount is therefore not negligible. If there is no match, the first and the second lean phase are further shortened, the second adaptation cycle preferably taking the place of the first as the basic cycle.

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Memory to be freed. The NOx / 0 ₂ ratio can be taken into account with the aid of characteristic curves.

As an alternative to using a 0 ₂ sensor, it is also possible to use a NOx sensor downstream of the NOx storage catalytic converter.

The adaptation of the raw NOx concentration is expediently carried out in the fourth step via a correction factor for a reduction factor of the raw NOx concentration or for a correction factor directly for the raw NOx concentration.

The change is preferably proportional to the difference between the storage quantity and the modeled NOx loading. It is also possible to make the change proportional to the difference of 1 and the quotient from the modeled NOx loading and the storage quantity.

A preferred embodiment of the method is described in more detail below using an exemplary embodiment. It shows:

1 shows the NOx concentration downstream of a NOx storage catalytic converter as a function of time,

2 shows the calculation of a correction factor for the raw NOx concentration and

3 shows the calculation of a correction factor for a reduction factor of the raw NOx concentration.

1 shows the development over time of the NOx concentration in a NOx storage catalytic converter. The curve marked 10 indicates the NOx concentration after the catalytic converter. At time t ₀ , which is identified by 12, regeneration of the NOx storage catalytic converter begins in normal storage / regeneration operation. The area marked with 14 specifies the amount of NOx (SM) stored up to. The proportion of the area below curve 10 indicates the breakthrough quantity DB, that is to say the quantity which has escaped up to the point in time t ₀ . If the raw emission is now to be adapted, the threshold 12 is initially moved to 18. For example, the duration of the lean phase can be halved.

In order to check whether there is a negligible breakthrough amount DB in the duration 18 of the lean phase, the lean phase is shortened to a point in time 20 in a further adaptation cycle. If the amount of storage in the shortened lean phase is in the same ratio to the amount of storage in the longer lean phase 14 as the duration of the shorter lean phase to the duration of the longer lean phase, then no breakthrough has occurred and the storage catalytic converter has stored NOx in proportion to the duration of the lean phase.

In general, the amount of NOx emitted during an engine lean phase can be split up into the following parts:

A first part is also converted into harmless substances by the exhaust gas cleaning system in lean operation, - a second part is stored in the NOx storage catalytic converter (SM) and a third part is released into the environment (DB) (breakthrough).

Regardless of the method for calculating the storage quantity SM used according to the invention, the integral of the corrected NOx concentration over the lean phase (IKK) is calculated based on the raw NOx concentration stored in characteristic maps and the reduction factor. The value of the integral is the sum of the modeled NOx loading and (modeled) post-cat emission. For the lean phases in which there is no breakthrough, SM should be equal to IKK if the mo- dellated, corrected raw NOx concentration corresponds to the actual raw NOx concentration reduced by the actual stationary reduction factor.

In the flow chart shown in FIG. 2, a correction factor for the raw NOx concentration (RK) is calculated. The correction factor (RKKF) is determined depending on the modeled NOx loading (IKK) and the amount of storage (SM). In the flow chart shown in FIG. 2, the factor:

1-IKK / SM

amplified with an amplifier 22 and added with a corresponding correction factor RKKF (N-1) in order to determine RKKF for the subsequent correction of the raw NOx concentration. The adaptation is carried out iteratively, the correction factor of the (n-l) th adaptation being changed in the case of an nth adaptation step. The raw NOx concentration is multiplied by the factor (1-RF), where RF denotes the reduction factor for the raw NOx concentration RK. Depending on the catalyst temperature, the reduction factor takes into account, for example, the steady state conversion in the catalyst. The product is called the corrected raw NOx concentration.

3 shows a flow chart for an alternative determination of the corrected raw NOx concentration. In this method, size 1-SM / IKK is amplified with an amplifier 24 and added with a correction factor for the reduction factor. The correction factor for the reduction factor RFKF is multiplied by the reduction factor RF, so that a corrected reduction factor is present. With the factor (1-RFKFxRF) the corrected raw NOx concentration is calculated from the raw NOx concentration (RK).

In both methods, the difference between the values can be used as an alternative to the quotient of SM and IKK.

Claims

claims

1. A method for adapting a raw NOx concentration (RK) of an internal combustion engine working with lean and rich phases, which is provided with a NOx storage reduction catalyst which is arranged in an exhaust gas duct of the internal combustion engine and adsorb NOx from the exhaust gas during the lean phase and during the Fat phase can implement the adsorbed NOx, characterized in that the following process steps are carried out during an adaptation process for the raw NOx concentration: in a first step, the duration of a first lean phase is shortened in such a way that no NOx emission behind the catalyst during the first Lean phase occurs, in a second step the amount of memory (SM) is calculated during the first lean phase, in a third step a comparison value is calculated from the amount of memory (SM) and a NOx load (IKK) modeled for the lean phase using maps in a fourth step the raw NOx concentration (RK) is adapted depending on the comparison value.

2. The method according to claim 1, characterized in that in the second step the amount of memory (SM) is determined in a second lean phase, the duration of which is shortened compared to the duration of the first lean phase, the average amount of memory per time in the first and second lean phase are compared with one another in order to determine a missing NOx emission in the first lean phase in the event of an approximate match, and to repeat the procedure with a shortened duration for the first and second lean phase in the event of a missing match.

3. The method of claim 2, d a d u r c h g e k e n n z e i c h n e t that when the lean phases are shortened to below a predetermined minimum duration, a signal is generated which indicates that the NOx storage catalytic converter is not suitable for lean operation.

4. The method according to any one of claims 1 to 3, so that the storage amount of the NOx storage reduction catalyst is determined in a rich phase for the previous lean phase in the second step.

5. The method according to claim 4, characterized in that in the second step, the storage amount for the previous lean phase is calculated depending on the time between the beginning of the rich phase and a detection of a complete NOx regeneration, the stored oxygen amount, the fuel mass flow and the substance ratio the reaction fuel and NOx.

6. The method of claim 5, d a d u r c h g e k e n n z e i c h n e t that the complete NOx regeneration by a downstream of the NOx-

Storage catalyst arranged 0 ₂ sensor is detected.

7. The method of claim 5, so that the complete NOx regeneration is detected by a NOx sensor arranged downstream of the NOx storage reduction catalytic converter.

8. The method according to any one of claims 1 to 7, characterized in that a correction factor (RFKF) is determined in the fourth step for a reduction factor (RF) of the raw NOx concentration.

9. The method according to any one of claims 1 to 7, so that a correction factor (RKKF) is determined in the fourth step for the raw NOx concentration (RK).

10. The method according to any one of claims 1 to 9, so that the difference between the storage quantity (SM) and the modeled NOx loading serves as a comparison value in the fourth step.

11. The method according to any one of claims 1 to 9, so that in the fourth step the difference between the storage quantity (SM) and the modeled NOx load divided by the storage quantity (SM) serves as a comparison value.

12. The method according to claim 8, so that in the fourth step the difference between the modeled NOx loading and the storage quantity (SM) divided by the modeled NOx loading serves as a comparison value.