Method For Exhaust Gas Aftertreatment, And Exhaust Gas Aftertreatment System

Abstract

A method for exhaust gas aftertreatment in a gasoline engine and an exhaust gas aftertreatment system are provided. In the method, two catalytic converters arranged in the exhaust gas tract of the gasoline engine are operated in different states. A first three-way catalytic converter is operated in a slightly low-oxygen range, and a second three-way catalytic converter is operated in a slightly oxygen-rich range. Secondary air is furthermore blown into the exhaust gas tract between the two three-way catalytic converters. It is thereby possible to reduce the output of emissions of the gasoline engine to a great extent. An exhaust gas aftertreatment system is likewise explained.

Claims

1. A method for exhaust gas aftertreatment in a gasoline engine, the method including: operating a first three-way catalytic converter arranged in an exhaust gas tract of the engine in a slightly low-oxygen range; operating a second three-way catalytic converter arranged downstream thereof in the exhaust gas tract of the engine in a slightly oxygen-rich range; and blowing secondary air into the exhaust gas tract between the first and second three-way catalytic converters.

2. The method as claimed in claim 1, wherein an NOx sensor is used as an NH.sub.3 sensor in order to implement control of the first three-way catalytic converter in the slightly low-oxygen range (slightly rich range).

3. The method as claimed in claim 1, wherein a binary lambda probe is used to implement control of the second three-way catalytic converter in the slightly oxygen-rich range (slightly low range).

4. The method as claimed in claim 2, wherein two-point control of the first three-way catalytic converter is carried out by the NOx sensor.

5. The method as claimed in claim 1, wherein a linear lambda probe upstream of the first three-way catalytic converter is used.

6. The method as claimed in claim 1, wherein a secondary air supply is switched on and off alternately for control purposes.

7. An exhaust gas aftertreatment system for a gasoline engine, the exhaust gas aftertreatment system comprising: an exhaust gas tract; a first three-way catalytic converter arranged in the exhaust gas tract; a second three-way catalytic converter arranged downstream thereof in the exhaust gas tract; and a device for blowing secondary air into the exhaust gas tract between the first and second three-way catalytic converters.

8. The exhaust gas aftertreatment system as claimed in claim 7, further comprising an NOx sensor used as an NH.sub.3 sensor for control of the first three-way catalytic converter in the slightly low-oxygen range.

9. The exhaust gas aftertreatment system as claimed in claim 8, wherein the NOx sensor is arranged in the first three-way catalytic converter.

10. The exhaust gas aftertreatment system as claimed in claim 7, further comprising a binary lambda probe for control of the second three-way catalytic converter in the slightly oxygen-rich range.

11. The exhaust gas aftertreatment system as claimed in claim 7, further comprising a linear lambda probe upstream of the first three-way catalytic converter.

12. The exhaust gas aftertreatment system as claimed in claim 7, wherein the device for blowing in secondary air includes a secondary air pump and a secondary air valve.

13. The exhaust gas aftertreatment system as claimed in claim 7, wherein the second three-way catalytic converter is designed as an underfloor catalytic converter.

14. The exhaust gas aftertreatment system as claimed in claim 7, wherein the second three-way catalytic converter is designed as a four-way catalytic converter.

Description

DESCRIPTION OF DRAWINGS

[0043] FIG. 1 shows a schematic illustration of an exhaust gas aftertreatment system.

[0044] FIG. 2 shows an enlarged illustration of the system in FIG. 1.

[0045] Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

[0046] FIG. 1 shows schematically a gasoline engine (spark-ignition engine) 10, which has an air supply tract 11 and an exhaust gas tract 1. Arranged in the exhaust gas tract 1 is a first three-way catalytic converter 2, which is followed by a second three-way catalytic converter 3. A linear lambda probe 7 is arranged upstream of the first three-way catalytic converter 2. The first three-way catalytic converter 2 is provided with an NOx sensor 8, which is arranged in the catalytic converter at ⅔ of the length of the latter.

[0047] The second three-way catalytic converter 3 is situated downstream of the first three-way catalytic converter 2 and has a binary lambda probe 9. Opening into the exhaust gas tract 1 between the two catalytic converters 2, 3 is a secondary air line 4, by way of which secondary air is blown in by a secondary air pump 6 and a secondary air valve 5.

[0048] The mode of operation of the exhaust gas aftertreatment system described above is explained with reference to FIG. 2. The system has a conventional linear lambda probe 7, which, in a known manner via a line 18, outputs signals to a control unit (not shown), which controls the air-fuel mixture supplied to the engine 10. The first three-way catalytic converter 2 (main catalytic converter) is now operated in such a way that it can reliably reduce all the nitrogen oxides. For this purpose, it is operated in the low-oxygen range. Here, the NOx sensor 8 is used as a sensor for control, on the one hand detecting the proportion of ammonia (NH.sub.3) produced in the catalytic converter and on the other hand supplying a binary probe signal for the acquisition of a corresponding probe voltage of a comparable binary post-cat probe. Both signals are supplied via lines 19 and 12 to the control unit in order to perform the corresponding control.

[0049] As with the first catalytic converter 2, the second catalytic converter 3 must also be operated alternately in a lean and a rich mode, for which purpose control is likewise necessary in order to keep the second catalytic converter 3 very close to λ=1 on average. In order to control the oxygen loading of the second catalytic converter 3 in a corresponding manner, use is made of a binary probe 9 by way of which a lean binary probe setpoint is set. For this purpose, corresponding signals 13 are output to the control unit.

[0050] Since the exhaust gas is only slightly rich after the first catalytic converter 2, it is also necessary to add only a very small amount of secondary air. The system therefore requires very precise metering of the secondary air. Thus, the secondary air is switched on and off alternately by the secondary air valve 5 in order to implement the lean and rich phases. The duration of the on times or the secondary air quantities is/are controlled by the probe voltage of the binary probe 9. Here, the corresponding actuation of the secondary air pump 6 and of the secondary air valve 5 is indicated at 14 and 15.

[0051] A number of implementations have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the disclosure. Accordingly, other implementations are within the scope of the following claims.