In a double-ended barrel engine, a compression cylinder (3) and a combustion cylinder (2) share a common axis, with a combustion piston (20) and a compression piston (21) being interconnected by a connecting rod (22). The compression end of the engine includes a valve plate assembly with a generally flat valve plate (6) with intake and exhaust passages defined therethrough. An intake flapper valve (8) is disposed on the inner surface of the valve plate and selectively covers the intake passage. An exhaust flapper valve (8) is disposed on the outer surface of the valve plate and selectively covers the exhaust passage. Additional cylinders and pistons may be provided. A compression plenum may be provided in fluid communication with the exhaust passages from one or more compression cylinders, and be in fluid communication with the intake system for the combustion end of the engine. A wastegate (14) may be provided for venting the compression plenum.