METHOD AND APPARATUS FOR CONTROLLING SUPERCHARGED ENGINE

A supercharged engine has a geometric compression ratio ser to 16 or more and is designed to perform a compression self-ignition combustion under an air-fuel ratio leaner than a stoichiometric air-fuel ratio at least in a low engine speed range. On a lower engine load side than a given engine load within an engine operating region at which the compression self-ignition combustion is performed, a fresh air amount is reduced and an effective compression ratio ( ′) is increased, as compared with a higher engine load side than the given engine load within the engine operating region, and, on the higher engine load side than the given engine load, a supercharging pressure based on a supercharger (25) is increased to increase the fresh air amount, and the effective compression ratio ( ′) is reduced, as compared with the lower engine load side than the given engine load. This makes it possible to perform the compression self-ignition combustion under a lean air-fuel ratio in a wider engine load range to effectively enhance engine thermal efficiency, while eliminating a need for an operation of forcedly raising/lowering a temperature of fresh air.