Abstract: A fuel system for an internal combustion engine. Fuel is injected into a cavity formed between the cylinder head and the piston head of each cylinder and a relatively rich air-fuel mixture is created in a localized space. The amount of injected fuel and the timing of the injection remains constant for all speeds and loads. The injected fuel ignites, and supports the combustion of a lean air-fuel mixture drawn into the cylinder from a carburetor. The rich-fuel mixture is capable of being fired by conventional spark ignition, while the lean mixture is incapable of being ignited by this means. The preliminary combustion of the rich-fuel mixture is then used to fire the lean mixture. At all speeds and loads of the engine an excess of air is present so that virtually full combustion results.