Abstract: A process and apparatus are disclosed for improving the efficiency of combustion in a spark ignition four cycle internal combustion piston engine. After a fuel-air charge is inducted into the cylinder(s), gas is injected at moderate pressure into the cylinder at a level below the spark plug, while the pressure in the cylinder is low. Preferred gas injection timing is from about the time the piston completes the intake stroke until the piston is about half-way through its compression stroke. Air and/or engine exhaust may comprise the injected gases. Valves and conduit for controlling the injection are provided as a sub-assembly which can be fitted to existing engines and use the engine camshaft for correct actuation.