What do the numbers on fuel pumps mean?

Remember that milk ad on TV? The one with the world-weary woman serving behind the counter of a shop offers every distinct variation of milk – including one with an 'extra dollop'?

Well, petrol's a bit like that too.

On each service station forecourt there's a kaleidoscope of different colours and names for the pumps, each a variation on the same basic theme: smelly, highly-flammable liquid drawn out of the ground to combust in your car's engine.

Sometimes it will be a liquefied gas or diesel... but more often than not the various hoses are for different strains of petrol.

Forget the colours and names – the V-Powers, the Ultimates and the Vortexes (Vortices?) – what will help you find the fuel you want is the number on the pump. This number, which is the 'Research Octane Number', or 'RON', is a graduated scale indicating a fuel's resistance to pre-ignition inside an internal-combustion engine.

In practice, if you own a conventional passenger car, SUV or petrol-engined light commercial vehicle, you can most likely refuel using 91 RON petrol. This is the cheapest petrol freely available from fuel retailers (service stations). Be aware that many European brands do not recommend local customers run their cars on 91 RON. These brands include Citroen, Fiat, Peugeot, Renault, Skoda and Volkswagen.

An alternative to 91 RON petrol that is often acceptable for newer cars is E10. This is 91 RON petrol with a 10 per cent blend of ethanol. Fuel suppliers claim that the effective octane rating for E10 is around 94 RON.

While ethanol can raise the octane rating of a fuel, it can also corrode the fuel delivery system of older cars that weren't designed to run on the fuel. E10 should be no more expensive than 91 RON and it's not recommended unless your car's logbook tells you that the car will not endure any longer-term consequences running on E10.

One step up from 91 RON and E10 is 95 RON. This is sold as a 'premium unleaded petrol' in Australia, but is the only petrol you can purchase in Europe, which is why it's recommended by the European manufacturers listed above and prestige brands like Audi, BMW, Jaguar Land Rover, Mercedes-Benz, Porsche and Volvo.

It's a better fuel for your car, and many Japanese, Korean, American and Aussie cars that run perfectly well on 91 RON can deliver superior performance and fuel economy when the owner fills up with 95 RON. That fuel economy gain may not translate to lower running costs, however, not when 95 RON is currently up to around 20 cents a litre dearer than 91 RON.

The most expensive fuel available in Australia, aside from racing fuels of up to 110 RON, is 98 RON. This is a premium fuel for high-performance engines of the kind powering Ferrari and Lamborghini sports cars.

91 RON – cheapest fuel, lowest octane rating, not recommended for European cars and other high-performance models such as Nissan GT-R.

E10 – a blend of 91 RON and 10 per cent ethanol for a raised octane rating of 94 RON. Should be cheaper than 91 RON where available, but not recommended for cars that are susceptible to corrosion in the fuel lines.

95 RON – 'Premium' unleaded petrol that is recommended for volume-selling and prestige cars from Europe, plus high-performance cars from Japan, Korea, North America and other parts of the world.

98 RON – Higher-octane premium unleaded petrol for sports cars powered by high-compression engines. This is the most expensive petrol commonly available in Australia.

If you've ever pumped up the tyres on a bike, no doubt you'll have noticed that the tyre pump warms up as the air pressure builds inside the tyre.

It's the same with a petrol engine – which is a type of pump. As the piston moves up and down inside the cylinder, it's compressing air – and frequently fuel vapour with it – and raising the temperature inside the cylinder. The rate of compression inside the cylinder is known as the compression ratio, and the higher the ratio, the warmer the air and fuel mix – the 'charge' – inside the cylinder.

When low-octane fuel in a high-compression engine 'pre-ignites' before the spark plug ignites the fuel/air charge in the cylinder, it creates a 'knocking' or 'pinging' noise and can be damaging to the engine. This syndrome is named 'uncontrolled detonation' or 'engine knock'. What's happening is that the low-octane fuel is igniting prematurely, due to the temperature in the cylinder exceeding the fuel's flashpoint.

Higher octane fuels (95 and 98 RON, for instance) are specifically recommended for higher-compression engines. They have been formulated to resist pre-ignition. In theory, these fuels will not ignite until a spark passes across the electrodes of the spark plug, which projects into the cylinder.

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