Petrol’s octane rating is a measurement of the fuel’s ability to resist engine knocking in spark-ignition internal combustion engines. Knocking is what happens when parts or all of the air-fuel mixture prematurely ignites before the flame from the spark plug can reach it. Knock occurs when the fuel-air mix in the cylinder explodes instead of burning in a controlled way. This can be caused by ignition timing that is too early or engine overheating, where the heat from the cylinder itself causes the mixture to combust before the spark plug can burn the mixture. This causes a decrease in performance and might also harm the engine. Engines are designed to have a minimum RON rating for its fuel. If fuel has a high octane number, it will have a higher resistance to engine knocking. Usually, there are three different octane numbers associated with all petrols. Petrol’s Research Octane Number (RON) is measured under simple test conditions. Petrol’s Motor Octane Number (MON) is measured under tougher test conditions and at higher engine speed and temperature. The average of these two values is what becomes related closer to actual driving conditions. This value is known as the Road Octane Number, and is what should be used in filling stations.