Abstract: An engine includes an engine block (10) provided with cylinders (12,14) having two pistons (16,18) and a cylinder head (19) having a piston (20) biased by means of torsion bars (24). The biased piston (20) is held in position by an arm (22) which is restrained by torsion bars (24). During the compression stroke of the engine the biased piston (20) is seated with faces (30,32), in contact and on ignition of the compressed mixture the biased piston (20) is lifted off the seat (32) creating a common combustion chamber in which separate charges are mixed. The biased piston (20) limits the combustion space and thereby controls the compression ratio to suit the desired performance.