Antiknock, also known as octane rating, is a measure of the gasoline's ability to resist knocking or pinging during combustion. Since it is gasoline vapor rather than liquid fuel that ignites, the air-to-fuel mixture must be correct for smooth combustion. The higher the octane rating, the greater the engine's resistance to knock. Some of the problems associated with knock are overheating of critical engine parts – namely valves, pistons and spark plugs. To minimize the effects of knocking, modern engines are equipped with knock sensors, which temporarily delay the spark timing in the piston’s compression cycle. While this prevents audible knocking, delaying the ignition spark also sacrifices engine efficiency.