Abstract: A jet ignition engine, of the type in which lean mixtures in the main combustion chambers are ignited by flame jets formed by combustion of rich mixtures in adjacent small auxiliary precombustion or ignition chambers connected with the main chambers by restricted orifices, wherein the engine is provided with a main fuel system for supplying lean mixtures to the main chambers and a supplemental fuel injection system for adding fuel to the auxiliary prechambers to augment lean mixtures delivered from the main chambers and form the rich prechamber ignition charges. The simplified injection system injects fuel at low pressure during the piston exhaust strokes through timed injection valves and nozzles. Each fuel stream is directed to engage an extended spark plug electrode that forms an atomizing projection in the prechamber to break up the fuel stream, directing droplets to prechamber walls, formed by an insulated cup, for vaporization.