What’s inside the pump – with Darren Moody

When it comes to demystifying petrol, the most important thing you need to know is the Research Octane Number (RON), which is a standard measure of performance of a fuel measured in a variable combustion engine.

In an internal combustion engine that runs on petrol, a fuel-air mixture is compressed in the combustion chamber and ignited by a spark plug to create an explosion and push the piston back down, creating turning torque on the crankshaft. In simple terms, the higher the octane number, the higher compression an engine can use.

If the compression ratio is too high and the octane rating is too low, the fuel-air mixture can start to ignite before the spark plug fires. This creates engine knock, which can damage pistons and cylinder heads and is generally very expensive to fix.

Starting with straight petrol only at this point, it is graded by octane number with 91, 95 and 98RON available at the pumps. The manufacturer will specify a specific grade of petrol for your car. They have designed and tuned the engine to run on this grade of fuel, so using a higher octane fuel won’t provide benefits. Similarly, prolonged use of a lower octane fuel will cause significant and expensive engine damage.

The majority of vehicles currently on sale in Australia use 91RON, but increasingly manufacturers are using smaller capacity engines fitted with turbo chargers that require 95RON, with the higher performance versions of these requiring 98RON. Ethanol blends are also available in E10 and E85, with each containing no more than 10% or 85% of ethanol respectively. E10 is rated as 95RON. Most vehicles manufactured after 1986 should run on E10, but if you are considering using E10 it is best to double-check with your vehicle’s manufacturer if it is compatible. E85 is a 107RON blend, far above anything else available on the market. There are a few vehicles capable of running E85, flex-fuel Commodores being one of them. E85 is also used in racing engines due to its superior octane rating.

The other technical component of fuel is its calorific value, or energy. It’s a bit like bang for buck, and this is where the difference between petrol and ethanol becomes more evident. Ethanol has around 30% less energy than an equivalent amount of straight petrol, but ethanol has a higher octane rating. Ethanol also burns cleaner than petrol so offers, in theory, reduced tailpipe emissions. However, because an E10-blend fuel increases consumption, this somewhat offsets the environmental benefits.

We often get asked ‘will I get more power and better fuel consumption if I use a higher octane fuel?’ The short answer is ‘possibly’, but what is probable is that any power increases will be unnoticeable, and any improvement in economy will be more than surrendered by the additional 10-12 cents per litre you pay for 95 over 91RON.