METHOD FOR OPERATING AN EXHAUST-GAS PURIFICATION SYSTEM IN A LEAN-BURN SPARK-IGNITION ENGINE

摘要

To remove nitrogen oxides from the exhaust gas of a lean-burn engine known methods use an exhaust-gas purification system composed of a nitrogen oxide storage catalytic converter and an SCR catalytic converter, operating the engine with alternately lean and rich air/fuel mixtures. The ammonia required for the SCR reaction is generated during the regeneration of the nitrogen oxide storage catalytic converter. One disadvantage is that, at high temperatures, sufficient ammonia is no longer generated during the regeneration. It is therefore proposed to inject ammonia upstream of the SCR catalytic converter at exhaust gas temperatures above approximately 350°C. The injection of ammonia does not take place at exhaust gas temperatures below 350°C. The method is particularly suitable for lean-burn engines with jet-controlled direct fuel injection which can also be operated at high load with a lean air/fuel mixture.