Petrol

Octane rating

The octane rating of petrol is a measure of its resistance to knocking. The higher the octane number, the higher the resistance to knocking (uncontrolled burning of fuel).

• The pressure applied to the piston by the burning gases in the combustion chamber determines the amount of torque produced by the engine.
• The higher the pressure in the chamber the higher the torque output.
• Four-stroke cycle. 
• The job of the engine designer is to achieve the most efficient combustion by controlling the combustion process.
• Uncontrolled combustion causes detonation or pinging.
• Higher-octane fuels allow better control of burning (combustion) and less instances of knocking.
• Using a higher rated fuel than your engine needs will be of no real benefit and will cost your more.
• The best approach is to use what vehicle manufacturer recommends.

Lead and petrol

Lead compounds were first introduced into petrol over eighty years ago when it was discovered that they increased the octane rating significantly.

• The introduction of lead compounds eliminated the problem of engine knock that had been plaguing engine designers for some time.
• They also found out later that lead provided a lubricant for the engine valves.
• During the 1980s, it became compulsory in Australia for all new cars to run on unleaded fuel so that vehicles could be fitted with catalytic converters.
• This was in response to a global concern about the health effects of lead on children.
• Leaded petrol has now been phased out in Australia and replaced with LRP (lead replacement petrol).

LRP

The issues that needed to be addressed when LRP was introduced were:

• The fuel had to be the same octane rating as leaded to meet the requirements of pre-1986 vehicle engines.
• To provide the lubricating qualities of lead in a lead free fueland preventvalve seat wear.
• The fuel manufacturers overcame these problems by improving refining techniques to provide a fuel with the same octane rating as leaded and an anti-valve seat recession additive was included to provide the same lubricating properties as lead.
• Leaded petrol has not been available in Western Australia since 1 January 2000 and was phased out in all Australian states on 1 January 2002.

Which petrol?

While the lure of more power and a smoother drive may tempt you to fill-up with more expensive high-octane petrol, only some cars built after 1985 need premium (PULP) or 98-octane fuels.

• Try and use the petrol specified by your vehicle manufacturer. To avoid engine damage or voiding your warranty, never use a lower octane petrol than the one recommended.
• If the manufacturer recommends using ULP, a higher octane fuel won't provide any benefit.
• If the manufacturer suggests either PULP or ULP, PULP may provide better performance and fuel consumption. However, tests by the RACV show that while premium may provide better running in high temperatures and when towing, the improvement in fuel consumption is not enough to cover the extra fuel cost.
• Regular unleaded (ULP) - 91-93 octane:
  • Most new cars made or sold in Australia after 1985 run well on ULP.
  • Many cars made before 1986 can also use ULP. Find out if it's safe to use ULP in your pre-1986 car.
• Premium unleaded (PULP) - 95 octane:
  • Costs about 6 cents per litre more than ULP and makes up about 12 per cent of WA's fuel market.
  • Many European and Japanese cars need PULP for best performance. Using ULP in these cars may result in a slight decrease in power and increased fuel consumption.
  • Can be used with an anti-valve seat recession additive in many pre-1986 vehicles designed for leaded petrol.
• High-octane unleaded petrol - 98 octane:
  • Costs about 10 cents per litre more than ULP.
  • Is required by some high performance cars to achieve best performance.
  • Can be used with an anti-valve seat recession additive in some pre-1986 high performance cars.
  • While it is said to optimise performance and improve fuel consumption, tests by the RACV show for most vehicles the slight improvement in fuel consumption does not outweigh the extra cost of the fuel.
  • Some brands such as BP Ultimate have an environmental benefit as they contain lower levels of benzene, sulphur and aromatic hydrocarbons.