Abstract: A variable expansion ratio internal combustion engine equipped with throttleless induction control and intake valve mechanism operable to induct an ambient pressure charge varying with load demand. Maximum compression pressure is preferably somewhat below that which produces combustion detonation. Constant compression ratio from idling through full load conditions is maintained by varying the clearance volume inversely with load demand. Charge induction at ambient pressure is provided with a throttleless carburetor and is manually controlled by a servo-power operator functioning to synchronize a linkage interconnecting the intake valve mechanism with mechanism for varying the cylinder clearance volume. This linkage operates to variably control closure of the intake valve during the intake stroke or, alternatively, to close the intake valve during the initial phase of the compression stroke.