Octane rating or
octane number is a standard measure of the performance of an engine or aviation fuel. The higher the octane number, the more
compression the fuel can withstand before detonating (igniting). In broad terms, fuels with a higher octane rating are used in high performance
gasoline engines that require higher
compression ratios. In contrast, fuels with lower octane numbers (but higher
cetane numbers) are ideal for
diesel engines, because diesel engines (also referred to as compression-ignition engines) do not compress the fuel but rather compress only air and then inject the fuel into the air heated up by compression. Gasoline engines rely on ignition of
air and fuel compressed together as a mixture without ignition, which is then ignited at the end of the compression
stroke using
spark plugs. Therefore, high compressibility of the fuel matters mainly for gasoline engines. Use of
gasoline with lower octane numbers may lead to the problem of
engine knocking.