What is Unleaded?

Petrol is a product derived from petroleum through a process called fractional distillation. Petroleum includes crude oil and many other fuels and needs to be separated through what is known as a fractionating column. Petroleum is found when drilling for oil and is then taken and distilled into many fuels such as petrol, kerosene to chemicals used to make plastic. It is estimated that we use around 95 million barrels of oil a day worldwide.

Engines that use petrol work in a different way to diesel engines. Petrol engines are spark ignition internal combustion engines, meaning that the fuel is ignited through a spark from a spark plug. If the ignition happens early or is mistimed then a metallic pinging noise is heard, known as knocking. This can damage the engine badly, and if you notice it let your garage know so they can advise on the next step.

The petrol that we see on the forecourts is a product of petroleum and is a different fuel to diesel, even though it is made from the same base product. Petrol types are measured in octane numbers and ratings depending on the resistance to autoignition.

What does an octane rating mean?

Petrols’ 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.

What are octane numbers?

Usually, there are three different octane numbers associated with all kinds of petrol. Petrol’s Research Octane Number (RON) is measured in a 600 RPM test engine under highly controlled conditions. Petrol’s Motor Octane Number (MON) is measured under tougher test conditions and at higher engine speed and temperature.

There is also the Anti-Knock Index (AKI), which takes the average of the RON and the MON but this is mainly used in North America and other countries. Just to confuse matters, the AKI is sometimes called the Posted Octane Number (PON).

The final kind of octane number is the Observed Road Octane Number which is tested on real-world multi-cylinder engines, normally at wide open throttle. This kind of testing has been around since the 1920s and is still reliable today, nearly 100 years on. While this was initially done on roads, it was moved to test centres to make the results more accurate.

A brief history of petrol and the prices

The first modern combustion engines were created in Germany in the late 19th Century, and used a fairly volatile fuel that boiled 40C lower than the current petrol we have. Over the years as knowledge and science developed better fuels that boiled at much higher temperatures and therefore ‘knocked’ less making cars more reliable and petrol a main source of fuel worldwide.

Unleaded petrol was introduced in 1988 in the UK after the leaded petrol that was sold previously was advised against by the EEC, the predecessor to the EU.

Petrol pricing started out when it first went on sale in 1896, and cost the equivalent of £9.00 per gallon. Nowadays the prices are a lot higher, and a lot more volatile. Market factors affect prices and over the years PetrolPrices has collected data on prices. The graph below shows how much can change year on year and what the difference can be depending on so many things.


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