Operating Method for a Motor Vehicle Diesel Engine Having an Exhaust Emission Control System

Abstract

An operating method is provided for a motor vehicle diesel engine having an exhaust emission control system that includes a three-way catalytic converter and an SCR catalytic converter situated one behind the other in the flow direction of the exhaust gas. The diesel engine is operated, at least intermittently, with an air-fuel ratio of approximately λ=1.0 in a first operating range in which the SCR catalytic converter falls below a predefinable minimum temperature and with excess air that is typical for normal diesel engine operation in a second operating range in which the SCR catalytic converter exceeds the predefinable minimum temperature. An output signal of an exhaust gas sensor situated downstream from the three-way catalytic converter and which is correlated with a NOx concentration of the exhaust gas is used to set the air-fuel ratio in the first operating range.

1 - 10 . (canceled) 
     
     
         11 . An operating method for a motor vehicle diesel engine having an exhaust emission control system comprising a three-way catalytic converter and an SCR catalytic converter situated one behind the other in the flow direction of the exhaust gas, the method comprising:
 operating, at least temporarily, the diesel engine with an air-fuel ratio target value of approximately λ=1.0 within an air-fuel ratio target range in a first operating range, wherein in the first operating range the SCR catalytic converter has a temperature below a predetermined minimum temperature; and   operating the diesel engine with excess air in a second operating range, wherein in the second operating range the SCR catalytic converter has a temperature exceeding the predetermined minimum temperature,   wherein in the first operating range a NOx conversion by the three-way catalytic converter is determined by an exhaust gas sensor situated downstream from the three-way catalytic converter, and a quantity of air and/or fuel supplied to the engine is adjusted, utilizing a characteristic curve indicating the NOx conversion as a function of the air-fuel ratio in such a way that a setpoint NOx conversion associated with the air-fuel ratio target value is at least approximately reached.   
     
     
         12 . The method according to  claim 11 , wherein the diesel engine is operated with the air-fuel ratio target value within approximately 0.95<λ<1.05 in the first operating range. 
     
     
         13 . The method according to  claim 12 , wherein an oscillating air-fuel ratio target value is set within the air-fuel ratio target range. 
     
     
         14 . The method according to  claim 13 , wherein the oscillating air-fuel ratio target value is set at an oscillation frequency of approximately 5 Hz to approximately 1 Hz. 
     
     
         15 . The method according to  claim 11 , wherein a pilot control value for the air-fuel ratio is set by pilot control by delivering a quantity of injected fuel necessary for a requested engine load, and delivering an at least approximate quantity of air necessary for combustion of the injected fuel, and for setting the air-fuel ratio target value, the pilot control value is reduced by at least one delayed fuel post-injection that leaves the engine load at least essentially uninfluenced. 
     
     
         16 . The method according to  claim 15 , wherein the set pilot control value is 1.05<λ<1.20. 
     
     
         17 . The method according to  claim 11 , wherein in the first operating range the diesel engine is operated with excess air in a combustion process within a first temperature range for the three-way catalytic converter. 
     
     
         18 . The method according to  claim 11 , wherein in the first operating range an electric heating element connected upstream from the three-way catalytic converter is energized within a second temperature range for the three-way catalytic converter. 
     
     
         19 . The method according to  claim 18 , wherein the electric heating element is energized before the engine is started. 
     
     
         20 . The method according to  claim 11 , wherein combustion air supplied to the diesel engine is selectively compressed (1) only by a compressor of a low-pressure exhaust gas turbocharger, or (2) is additionally compressed by a compressor of a high-pressure exhaust gas turbocharger. 
     
     
         21 . The method according to  claim 11 , wherein exhaust gas is supplied to the SCR catalytic converter is filtered by a particle filter situated in the exhaust emission control system downstream from the three-way catalytic converter. 
     
     
         22 . The method according to  claim 21 , wherein the particle filter has an SCR catalyst coating. 
     
     
         23 . The method according to  claim 21 , wherein a urea-water solution is into the exhaust gas by a dosing device situated upstream from the SCR catalytic converter. 
     
     
         24 . The method according to  claim 23 , wherein the dosing device is situated downstream from the particle filter and upstream from the SCR catalytic converter. 
     
     
         25 . The method according to  claim 24 , wherein the combustion air of the diesel engine exhaust gas is added via a low-pressure exhaust gas recirculation line that branches off downstream from the particle filter and upstream from the dosing device, or via a high-pressure exhaust gas recirculation line branching off from an exhaust manifold. 
     
     
         26 . The method according to  claim 11 , wherein for the minimum temperature at and above which the diesel engine is operated with excess air, a temperature is selected above which the SCR catalytic converter is operational.