Patent ID: 6941905

Claim:
A control unit for a multi-cylinder spark ignition-type engine in which a combustion cycle of each cylinder has a predetermined phase difference, said engine including: an inter-cylindrical gas passage between a pair of cylinders, an exhaust stroke of one of the cylinders and an intake stroke of the other of the cylinders overlapping each other, and through the inter-cylindrical gas passage, burnt gas discharged from the preceding cylinder at the exhaust stroke being introduced into the following cylinder at the intake stroke; a switching valve for switching the direction in which the burnt gas of the preceding cylinder is introduced, from the side of an exhaust passage to the side of the inter-cylindrical gas passage and vice versa; and a fresh-air introducing valve for introducing fresh air into the following cylinder by opening and closing a fresh-air introduction passage; said control unit comprising: an operation-mode controlling means for executing control of a special operation mode in a partial load range of the engine, in said special operation mode, the fresh-air introducing valve being closed and the switching valve being controlled so that the whole burnt gas discharged from the preceding cylinder can be introduced into the side of the inter-cylindrical gas passage, thereby the two cylinders being kept connected, the air-fuel ratio in the preceding cylinder becoming a lean air-fuel ratio, higher than a stoichiometric ratio and combustion being conducted, and fuel being supplied to the burnt gas having the lean air-fuel ratio introduced from the preceding cylinder into the following cylinder to conduct combustion in the following cylinder; and executing control of a medium operation mode in a higher load range than the partial load range, in said medium operation mode, the fresh-air introducing valve being opened to introduce both the burnt gas and fresh air into the following cylinder and fuel being supplied to conduct combustion in the following cylinder; and an air-fuel-ratio controlling means for controlling the air-fuel ratio in the following cylinder so that the concentration of oxygen in exhaust gas discharged from the following cylinder can be a value corresponding to a combustion state at a stoichiometric ratio.