Reduction of N2O in the exhaust gas of lean-burn petrol engines

Abstract

The present invention relates to the use of different regeneration strategies for nitrogen oxide storage catalysts (NOx storage catalyst, LNT or NSC), depending on the exhaust gas temperatures, to reduce in the total exhaust gas the greenhouse gas N.sub.2O (nitrous oxide) that is produced as a secondary emission during the regeneration of the storage catalyst. If the exhaust gas temperature is below 275° C.-290° C., regeneration takes place using a strategy with short pulses of around 2 seconds and λ Lambda 0.95 rich.

Claims

1. A method of reducing N.sub.2O formation during regeneration, comprising utilizing different strategies for the regeneration of at least one nitrogen oxide storage catalyst to reduce the N.sub.2O formation during regeneration, wherein the at least one nitrogen oxide storage catalyst is used in an exhaust gas system of a spray-guided stratified operating petrol engine, wherein the different strategies are specifically selected based on the temperature of the exhaust gas such that below a temperature range of 275° C.-290° C. regeneration occurs with a shorter, but richer pulse of reducing agents than it does at temperatures above this temperature range.

2. The method according to claim 1, wherein, with regeneration strategies above this temperature range, a modulation of an λ-amplitude of a regeneration pulse is provided in such a way that a rich pulse is followed by a phase with an exhaust gas mixture at λ=1 (rich/λ=1 regeneration strategy).

3. The method according to claim 1, comprising regeneration times of under 10 sec.

4. The method according to claim 1, wherein regeneration is not carried out below λ=0.87.

5. The method according to claim 1, wherein the exhaust gas composition is set by injecting fuel into a cylinder of the engine or into the exhaust gas system upstream of the nitrogen oxide storage catalyst.

6. The method according to claim 1, wherein the regeneration of the at least one nitrogen oxide storage catalyst is controlled by one or more sensors or based on modeling.

7. The method according to claim 1, wherein the at least one nitrogen oxide storage catalyst is contained in an exhaust gas cleaning system having one or more three-way catalytic converters arranged in proximity to the engine.

8. The method according to claim 7 wherein said at least one nitrogen oxide storage catalyst includes a plurality of nitrogen oxide storage catalysts.

9. The method according to claim 8 wherein at least one of the plurality of nitrogen oxide storage catalysts is arranged in proximity to the engine.

10. The method according to claim 1 wherein said at least one nitrogen oxide storage catalyst includes a plurality of nitrogen oxide storage catalysts.

11. The method of claim 2 wherein, during the modulation, at least one phase with an exhaust gas mixture at λ=1 has a duration of 2 to 4 seconds.

12. The method of claim 11 wherein the modulation comprises repeated phases with the exhaust gas mixture at λ=1 above the temperature range of 275° C.-290° C.

13. The method of claim 2 wherein the modulation comprises repeated phases with the exhaust gas mixture at λ=1 above the temperature range of 275° C.-290° C.

14. The method of claim 2 wherein the modulation comprises λ=1 phases between the rich pulses, with the time duration of the λ=1 phases and the rich pulses being equal.

15. The method of claim 1 wherein the regeneration below the temperature range of 275° C.-290° C. occurs at least one second shorter in phase time than regeneration occurring above that temperature range.

16. The method of claim 15 wherein the regeneration occurring below the temperature range of 275° C.-290° C. is 1 to 6 seconds shorter in phase time than regeneration occurring above that temperature.

Description

DESCRIPTION OF THE FIGURES

(1) FIG. 1 shows various single-pulse symmetrical regeneration strategies and the N.sub.2O formation during regeneration for different temperatures. The relative N.sub.2O formation is given in relation to the maximum measured in the test, which is set at 100%.

(2) FIG. 2 shows various rich/λ=1 regeneration strategies and the N.sub.2O formation during regeneration for different temperatures. The relative N.sub.2O formation is given in relation to the maximum measured in the test, which is set at 100%.

(3) FIG. 3 shows the exhaust gas system used based on a three-way catalytic converter arranged in proximity to the engine (cc) (1st cc brick) and two downstream nitrogen oxide storage catalysts (2nd and 3rd cc brick). The special analysis positions (SU) for measuring N.sub.2O and NH.sub.3 are also marked along with the analysis sampling points (AMA-L½) for measuring HC, CO and NOx.

(4) FIG. 4 provides a schematic view of the course of the trials for determining the N.sub.2O formation at different temperatures and when applying different regeneration strategies.

EXAMPLES

(5) As a criterion for the quality of regeneration with a view to low secondary emission formation, it is ensured that the A) lean phase (storage phase) is comparable in all applied regeneration strategies. Particular attention is given to the temperature, exhaust gas composition and lean NOx storage parameters so that there are reproducible conditions in the subsequent rich phase.

(6) A fixed B) regeneration strategy (Table 1) follows to react the stored NOx, preferably to give N.sub.2. Nine regeneration variants were set in the test sequence. These sequences differ in terms of the regeneration duration and the A-depth of immersion from which a different exhaust gas composition (HC, CO and NOx) results during regeneration. All strategies exhibit exhaust gas conditions λ≦1.

(7) There then follows a brief additional C) lean phase in which the subsequent storage is assessed. Alongside the hopefully low N.sub.2O formation, it now becomes apparent whether or not the quality of the regeneration was sufficient. Quality is used here as a synonym for the ability to store NOx anew or again.

(8) In order to ensure comparable conditions in the trial sequence, a conditioning takes place in D) which ensures that all storage centers are available for the fresh A) lean phase. This conditioning comprises a regeneration controlled by probe-related stoppages which prevent the catalyst from becoming full during a strategy with a poorly selected regeneration duration and/or λ-depth of submersion with increasing test sequences. This ensures that a high reproducibility of the individual measurements is obtained.

(9) The procedure (FIG. 4) is repeated 7 times per regeneration strategy, wherein the last 4 test sequences are recorded and relevant mean values are formed. In this way it can be ensured that, with respect to the catalyst temperature, the system has already achieved a steady state in the first test sequence recorded. It should be stressed that the deviations between the individual test sequences within the same regeneration strategy are very small.