Method for reducing harmful exhaust emissions of gasoline engines operated with lean fuel-air mixtures

摘要

A method reduces harmful exhaust-gas emissions of a spark-ignition, direct fuel injection engine operated with a lean fuel/air mixture. Under part load, the engine is operated in the stratified-charge mode with a lambd value of 1, and to reduce nitrogen oxides adsorbed on a catalyst, a jump to a homogeneous mode with air ratios of lambda 1 is carried out at specific intervals by throttling of the air quantity supplied and/or by an increase in the fuel injection quantity. In a first step, the air quantity in the combustion space is reduced by changes in the valve control times and, in a second step, a transition is made from the stratified-charge mode to the homogeneous mode, with a lambd value of less than 1 being set.