A spark ignition (SI) engine is the gasoline or diesel portion of the hybrid. It uses an air fuel mixture, compresses it, then ignites it using a spark plug (spark ignition). You control the engine with the throttle which limits the amount of air let into the engine.Although the SI engine has been efficient enough in the past, with higher and higher expectations all the time, it is quickly becoming too expensive to keep efficient. That is the whole purpose of hybrids. To make SI engines more efficient.