Octane Rating
A measurement of a gasoline fuel's resistance to premature ignition or knocking during combustion.
An octane rating is a standardized measure of a fuel's ability to resist engine knocking, which is the premature, uncontrolled detonation of the air-fuel mixture in the cylinders. It is typically displayed as a number (e.g., 87, 89, 91, 93) on gasoline pump yellow decals.
Higher octane fuel does not contain more energy; rather, it has a higher self-ignition temperature, meaning it can withstand higher temperatures and pressures before igniting. In high-compression engines, turbocharged engines, or supercharged engines, high-octane fuel is required to prevent the fuel from igniting prematurely under compression before the spark plug fires. If low-octane fuel is used in these engines, it causes pre-ignition or knocking.
Consistent engine knocking can cause severe mechanical damage, including melted piston crowns and broken spark plug electrodes. Modern vehicles use knock sensors to detect this vibration and retard the ignition timing to protect the engine, though this reduces power and fuel economy. Drivers should always follow the manufacturer's recommended octane rating listed in the owner's manual.