Octane rating or octane number is a standard measure of the performance of a motor or aviation fuel. The higher the octane number, the more compression the fuel can withstand before detonating. In broad terms, fuels with a higher octane rating are used in high performance petrol engines that require higher compression ratios. In contrast, fuels with lower octane numbers are ideal for diesel engines, because diesel engines do not compress the fuel but rather compress only air and then inject the fuel into the air heated up by compression, whereas petrol 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 petrol engines. Use of petrol with lower octane numbers may lead to the problem of engine knocking.