Abstract: Within an internal combustion engine (including a rotary engine), an apparatus and method for the pre-combustion of a rich fuel mixture within a spark plug to ignite a leaner fuel mixture within the cylinder. Substantially undiluted gaseous pilot fuel is supplied by a unidirectional valve through an integral passageway within the spark plug to a pre-combustion chamber recessed within the ignition end of the spark plug. The pilot fuel is supplied to the pre-combustion chamber during the downstroke of the piston. During the upstroke of the piston, lean cylinder gases are compressed into the pre-combustion chamber, mixing with the pilot fuel to yield a combustible mixture. Controlled electric ignition of the mixture within the pre-combustion chamber sends a flame jet into the leaner fuel mixture producing timed combustion within the cylinder.