Abstract: A system is provided for varying the number of active cylinders in a multi-cylinder internal combustion engine, in response to the operating requirements of the engine. When the engine reaches that part of its operating range where the torque requirement is such that operation of all of the cylinders is not required to provide adequate and efficient power, certain cylinders are rendered inactive, thereby reducing the fuel consumed by the engine. A control system is provided which, upon sensing a reduced torque requirement upon the engine, activates a switch which de-energizes the spark generating electrodes of certain cylinders, and concurrently activates solenoid valve means whereby fuel vapor in the deactivated cylinders may be bypassed through the cylinders and returned to the fuel supply system of the engine for later use. In one embodiment of the system, novel spark producing means is provided which includes the fuel bypass valve means.