Several common ethanol fuel mixtures are in use around the world. The use of pure hydrous or anhydrous ethanol in internal combustion engines (ICEs) is only possible if the engines are designed or modified for that purpose, and used only in automobiles , light-duty trucks and motorcycles . Anhydrous ethanol can be blended with gasoline (petrol) for use in gasoline engines, but with high ethanol content only after minor engine modifications.
In a normal spark-ignition engine, the air-fuel mixture is heated due to being compressed and is then triggered to burn rapidly by the spark plug. During the combustion process, if the unburnt portion of the fuel in the combustion chamber is heated (or compressed) too much, pockets of unburnt fuel may self-ignite (detonate) before the main flame front reaches them. Shockwaves produced by detonation can cause much higher pressures than engine components are designed for, and can cause a "knocking" or "pinging" sound. Knocking can cause major engine damage if severe.