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.