Control device for spark-ignition engine

Abstract

The invention is intended to provide improved emission-cleaning performance by use of a three-way catalyst alone, without the need for a lean NOx catalyst, while ensuring a fuel economy improvement effect of lean burn operation. A multicylinder spark-ignition engine is constructed such that, in a pair of preceding and following cylinders whose exhaust and intake strokes overlap each other, burned gas discharged from the preceding cylinder (A,D) which is currently in the exhaust stroke is introduced directly into the following cylinder (B,C) which is currently in the intake stroke through an intercylinder gas channel () and gas discharged from only the following cylinder (B,C) is led to an exhaust passage () provided with a three-way catalyst () in a low-load, low-speed operating range. Fuel supply to the individual cylinders is controlled in such a manner that combustion in the preceding cylinder (A,D) is made under lean mixture conditions at an air-fuel ratio larger than the stoichiometric air-fuel ratio by a specific amount and combustion in the following cylinder (B,C) is made under conditions of the stoichiometric air-fuel ratio created by supplying fuel to the burned gas introduced from the preceding cylinder (A,D).