What component ignites the air-fuel mixture in a gasoline engine?

The SPARK PLUG ignites the air-fuel mixture in a gasoline engine, creating the spark that causes combustion during the power stroke. ASVAB Auto Information tests ignition. The spark plug receives high voltage (from the ignition coil) and produces a spark across its gap, igniting the compressed air-fuel mixture. (DIESEL engines have no spark plugs — they ignite fuel by compression heat.) Other parts: FUEL INJECTOR (delivers fuel); PISTON (moves in the cylinder). Knowing the spark plug ignites the mixture in a gas engine is commonly tested.