Abstract: An internal combustion engine and a method of operating such an engine in which a power piston and a control piston move within a cylinder for defining between the pistons a combustion chamber. Charges of gas are drawn into an intake chamber defined within the cylinder by the control piston and are then transferred through the control piston into the combustion chamber where the charge is compressed and burned to deliver power from the engine through the power piston. In accordance with the present invention, the combustion chamber is maintained at a substantially constant volume during a combustion stroke of the power piston and as power is delivered from the engine.