Intake-air control system for engine

An intake-air control system for an engine enabling an intake-air quantity and a compression ratio to be variably controlled, includes sensors detecting engine operating conditions and the compression ratio, and a control unit electronically connected to the sensors for feedback-controlling the intake-air quantity based on the compression ratio as well as the engine operating conditions, while feedback-controlling the compression ratio based on the engine operating conditions. The control unit executes phase-matching between an intake-air quantity change occurring based on intake-air quantity control and a compression ratio change occurring based on compression ratio control, considering a relatively slower response in the compression ratio change than a response in the intake-air quantity change.