Types of Fuel and Octane Ratings

Petrol's octane rating is a measurement of the fuel's ability to resist engine knocking. Knock occurs when the fuel-air mix in the cylinder explodes instead of burning in a controlled way. This shockwave moves within the combustion chamber, and creates a metallic 'pinging' sound.

An octane rating is often referred to as an 'anti-knock index'. If fuel has a high octane number, it will have a higher resistance to engine knocking.

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Octane Numbers

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.

Occasionally, some filling stations will confuse these different octane numbers in a bid to embellish on their octane rating claims, and advertise their fuel's Research Octane Number, which is higher than the Road Octane Number. In many European countries, the Research Octane Number is advertised on pumps, so a much higher octane value is common when travelling in certain countries.

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Diesel

Diesel automatically ignites and burns when it is compressed to a very high pressure. The released energy is contained by the engine and powers the vehicle. The key difference between diesel and petrol engines is auto-ignition. A spark plug ignites the fuel in a petrol engine whereas a diesel engine auto-ignites. Diesel has a Cetane Index and Number rather than the Research Octane Number (RON) that petrol has.