SYSTEM AND METHOD FOR CLEANING EXHAUST GAS WHILE AVOIDING NITROUS OXIDE

Abstract

The present invention relates to a method for cleaning exhaust gas, and a correspondingly designed exhaust gas system. The present method, or the corresponding system, serves to avoid the formation of nitrous oxide as a secondary exhaust gas, which may primarily be created during the loading of specific catalyst types with NH.sub.3.

Claims

1. Method for reduction of harmful automobile exhaust gas components with the aid of an exhaust gas system having at least two catalysts selected from the group consisting of NSC, TWC, and TWNSC, wherein the exhaust gas is diverted around the downstream catalyst of the at least two catalysts if this is within a temperature window in which it is capable of forming N.sub.2O from NH.sub.3, and the upstream catalyst of the at least two catalysts produces NH.sub.3.

2. Method according to claim 1, characterized in that the exhaust gas is produced by a gasoline engine that is operated predominantly with an A/F mixture that is lean on average.

3. Method according to claim 1, characterized in that the downstream catalyst of the at least two catalysts is selected from the group consisting of TWNSC and NSC.

4. Method according to claim 1, characterized in that the exhaust gas system downstream of the at least two catalysts has at least one NOx reduction catalyst.

5. Method according to claim 4, characterized in that the NOx reduction catalyst is made up of at least one SCR and/or NSC catalyst.

6. Method according to claim 5, characterized in that the at least one SCR catalyst is arranged upstream of the at least one NSC catalyst.

7. Method according to claim 1, characterized in that the diversion of the exhaust gas takes place when the downstream catalyst of the least two catalysts has a temperature of less than 350° C.

8. Method according to claim 1, characterized in that the at least two catalysts are located in the first half of the exhaust gas tract, as measured from the motor output to the end of the exhaust pipe.

9. Method according to claim 1, characterized in that at least one temperature sensor is located in the flow direction of the exhaust gas after the downstream catalyst of the at least two catalysts.

10. Method according to claim 1, characterized in that at least one temperature sensor is located between the at least two catalysts.

11. Method according to claim 1, characterized in that the diversion of the exhaust gas around the downstream catalyst of the at least two catalysts is effected by means of a device for activating and deactivating the diversion, which device is positioned at the merger of the diversion and the main exhaust gas tract.

12. Method according to claim 11, characterized in that the device for activating and deactivating the diversion is a valve or an exhaust gas flap.

13. System for exhaust gas aftertreatment, having at least two catalysts from the group consisting of NSC, TWC, and TWNSC, wherein the system is designed so that the exhaust gas may be diverted around the downstream catalyst of the at least two catalysts if this is capable of forming N.sub.2O from NH.sub.3.

14. System according to claim 13, characterized in that the downstream catalyst of the at least two catalysts is selected from the group consisting of TWNSC and NSC.

15. System according to claim 13, characterized in that the exhaust gas system downstream of the at least two catalysts has at least one NOx reduction catalyst.

16. A method for the aftertreatment of the exhaust gas of a gasoline engine that is operated predominantly with an A/F mixture that is lean on average, comprising passing exhaust gas within the system of claim 13.

Description

FIGURES

[0060] FIG. 1: The complete layout of a corresponding exhaust gas system is described

[0061] FIG. 2: Partial region of the complete exhaust gas system that is situated in the area near the motor, given flow through the cat BOX 2 (bypass closed).

[0062] FIG. 3: Partial region of the complete exhaust gas system that is situated in the area near the motor, given circumvention of the cat BOX 2 (bypass open).

[0063] FIG. 4: Complete exhaust gas system, given flow through the cat BOX 2 (bypass closed).

[0064] FIG. 5: Complete exhaust gas system, given circumvention of the cat BOX 2 (bypass open).

[0065] FIG. 6: NOx conversion in the load range of below 300° C. and relative N.sub.2O formation

[0066] FIG. 7: NOx conversion in the load range of below 350° C. and relative N.sub.2O formation; relation to FIG. 6 with regard to the N.sub.2O formation

[0067] FIG. 8: NOx conversion in the load range of above 400° C. and relative N.sub.2O formation; relation to FIG. 6 with regard to the N.sub.2O formation

[0068] FIG. 9: NOx conversion and N.sub.2O formation in the system of FIG. 1 in NEDC, given bypass control according to the claim.

EXAMPLE OF THE MODE OF OPERATION OF THE INVENTION

[0069] Complete Layout of Exhaust Gas System (See FIG. 1)

[0070] Mode of Operation: [0071] 1) For lambda greater than 1, the bypass is closed (FIG. 2 with the depiction of cat BOX 1 and cat BOX 2)

[0072] For motor operation given lambda greater than 1, the bypass is closed (FIG. 2). NOx from the motor exhaust gas is stored in the catalysts so that the concentration NOx 1 in the exhaust gas is greater than the concentration NOx 2, and the concentration of NOx 2 in the exhaust gas is greater than the concentration of NOx 3. This applies to the following temperatures: [0073] Temperature (Temp 2) cat BOX 1 is less than 350° C., and temperature (Temp 3) cat BOX 2 is less than 350° C. [0074] Temperature (Temp 2) cat BOX 1 is greater than 350° C., and temperature (Temp 3) cat BOX 2 is less than 350° C. [0075] Temperature (Temp 2) cat BOX 1 is greater than 350° C., and temperature (Temp 3) cat BOX 2 is greater than 350° C. [0076] 2) For lambda less than 1, the bypass is open; the minimum temperature for cat BOX 1 is greater than 350° C., measured at Temp 2.

[0077] For motor operation given lambda less than 1, the bypass is open if the temperature (Temp 2) of cat BOX 1 is greater than 350° C., and the temperature (Temp 3) of cat BOX 2 is less than 350° C. (FIG. 3). The termination of the operation below lambda 1 takes place via the NOx sensor 1 or via a model/map stored in the ECU. [0078] 3) For lambda less than 1, the bypass is closed if the minimum temperature for cat BOX 2 is greater than 350° C., measured at Temp 3.

[0079] For motor operation given lambda less than 1, the bypass is closed if the temperature (Temp 2) of cat BOX 1 is greater than 350° C., and the temperature (Temp 3) of cat BOX 2 is greater than 350° C. (FIG. 2). The termination of the motor operation given lambda less than 1 takes place via the NOx sensor 2 or via a model/map stored in the ECU. [0080] 4) For lambda greater than 1 and concentration NOx 2 is equal to concentration NOx 3; this means that no storage of NOx takes place via cat BOX 2.

[0081] For motor operation given lambda greater than 1 and a concentration NOx 2 that is equal to a concentration NOx 3 (meaning that no storage of NOx takes place in cat BOX 2), the bypass is open (FIG. 3). [0082] 5) For lambda less than 1, if the minimum temperature (Temp 5) for cat BOX 3 is greater than 350° C.

[0083] For motor operation given lambda less than 1, the bypass is closed if the temperature (Temp 2) of cat BOX 1 is greater than 350° C., the temperature (Temp 3) of cat BOX 2 is greater than 350° C., and the temperature (Temp 5) of cat BOX 3 is likewise greater than 350° C. (FIG. 4). The termination for the operation at lambda less than 1 takes place via NOx sensor 3 or via a model/map stored in the ECU. [0084] 6) For lambda less than 1, if the minimum temperature (Temp 2) for cat BOX 1 is greater than 350° C.

[0085] For motor operation given lambda less than 1, the bypass is open if the temperature (Temp 2) of cat BOX 1 is greater than 350° C., and the temperature (Temp 3) of cat BOX 2 is less than 350° C. The termination for the operation at lambda less than 1 takes place via NOx sensor 1 or via a model/map stored in the ECU. A combination of SCR and NSC catalysts is hereby preferred for cat BOX 3, wherein the SCR catalyst is arranged upstream of the NSC catalyst (FIG. 5).

[0086] Additional Examples and Exhaust Gas Measurement:

[0087] Stationary Tests at a Highly Dynamic Motor Test Stand for Obtaining the Results of FIGS. 6, 7, and 8:

[0088] In the stationary test on a system of FIG. 1, 10 rich/lean cycles were run in succession. Wherein the termination criteria for lean operation at 50 ppm NOx slip was [0089] a) position NOX Sensor 1 with open bypass; and [0090] b) position NOX Sensor 2 without bypass.

[0091] The regeneration of the NOx storage catalysts takes place via the rich operation of the motor test stand, over an established unit of time. The time unit is selected so that all catalysts are sufficiently regenerated.

[0092] Of the 10 rich/lean cycles, the last 5 are used to calculate the NOx conversion. This ensures that the system is in equilibrium. The person skilled in the art also knows this as a steady state.

[0093] The respective target temperatures at the catalyst are generated by the variation of the load at the motor test stand. In the test, 3 different load points were hit in order to generate the temperatures<300° C., <350° C., and >400° C. at cat BOX 1. Corresponding probe analysis is used to measure the secondary emissions—for example, of N.sub.2O.

[0094] Dynamic Run Cycle at a Highly Dynamic Test Stand to Obtain the Results of FIG. 9:

[0095] In the application of the run cycle NEDC at a highly dynamic test stand, motor data are read out from a production vehicle which is in testing operation and transferred to the controller of the highly dynamic test stand. It is hereby especially to be noted that the reproducibility of the applied tests reaches the highest degree of precision.

[0096] If the test conditions are applied as described above, the exhaust gas system is tested with and without bypass in the NEDC cycle.

[0097] It hereby applies that the termination for the respective lean or rich phases normally occurs via lambda sensor 3 or NOX sensor 3.