Petrol, a breakdown of the Facts

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What is Petrol?

Quite Simply Petrol is a misture of low boiling point hydrocarbons. Hydrocarbon molecules range from the smallest (in gaseous form) to the largest (or longest) which is the bitumen we have on our roads. The smaller the molecule, the lower the boiling point. For an internal combustion petrol engine to run properly, the fuel must have a range of varying sized hydrocarbon molecules. Remember, an engine does not run on petrol, it runs on a very finely atomised vapour. The hydrocarbon molecules have to boil to produce vapour. In cold-start conditions an engine runs on the smaller molecules, which boil at a lower temperature. Enriching the mixture during cold start increases the amount of petrol, and in so doing increase the available volume of small molecules. As the engine warms up it will start to burn the larger molecules, which boil at a higher temperature.


When the engine has reached its optimum operating temperature it will readily run on the largest molecules. It is the larger molecules that produce the greatest amount of power.
This explains why in areas of wide temperature variation from summer to winter, there is a need to change the ratios of the molecule sizes to cater for the seasonal changes. This is done at the refineries where they produce summer and winter fuel to suit the season.


What is petroleum?

Petroleum is a complex mixture of organic liquids called crude oil and natural gas that occurs naturally in the ground and was formed millions of years ago. Crude oil varies from oil field to oil field in colour and composition, from a pale yellow low-viscosity liquid to heavy black “treacle”consistencies. Crude oil and natural gas are of little use in their raw state, their value lies in what is created from them like fuels, lubricating oils, waxes,
asphalt, petrochemicals and quality natural gas.


Is petrol produced to a standard?

Yes. Petrol distributed and sold in Australia has to comply with the appropriate standards
as defined by the Australian Standards Association. Furthermore, all refineries have
their own specifications and each batch of petrol is tested to verify that it falls within the
tolerances of that standard.


How much does petrol vary in composition?

There are small changes in petrol composition as the crude supply varies from origin. Factors like world pricing and crude oil availability determines if petrol is imported
or produced locally. Importantly, in all instances the particular fuel should meet Australian Standards. Imported petrol is tested for compliance prior to being unloaded
from the ship.


What is “RON” or “Octane Number”?
RON (Research Octane Number) or Octane Number is commonly used to describe the same thing, which is a measure of a petrol’s ability to prevent detonation in a spark ignition engine.


What is “detonation” (also known as “pinging” or “knocking”)?
Firstly, we need to understand the basic combustion process in a petrol engine. Air and petrol are mixed, vaporised and drawn into the cylinder by the piston’s intake stroke. All valves in the cylinder are then closed and the vapour is compressed as the piston travels towards Top Dead Centre (TDC) on its compression stroke. Just before the piston reaches TDC the compressed mixture is ignited by the spark plug and it burns at a controlled rate in accordance with the design of the combustion chamber and piston crown. The progressive burning of the mixture exerts controlled pressure onto the top of the piston. As the piston travels upwards on its compression stroke the air/petrol mixture is subjected to increasing temperature and pressure. The mixture has to remain stable until it is ignited by the spark plug. Once ignited it must remain chemically stable for the flame to progress smoothly across the combustion chamber and in doing so exert even pressure
on the piston crown. If the mixture does not remain stable under the provocation of rising temperature and pressure during the piston’s compression stroke, it explodes and this causes an explosive rise in temperature resulting in shock waves. These shock waves are the “pinging” sound we hear. This explosion or, as we know it, “detonation” is inefficient, wasteful of fuel and can potentially cause severe engine damage.


When was leaded petrol removed completely from the Australian market?

Leaded petrol was withdrawn from the market in WA at the beginning of 2000 and withdrawn nationally in 2002.


My pre-1986 car does not run well on Unleaded Petrol. Why is this?

Originally leaded petrol had a RON of 96. Unleaded petrol has a RON on 91 which does not meet the minimum requirements of an engine designed to run on leaded petrol, resulting in pinging and poor performance. The use of higher octane unleaded fuel with the addition of a lead substitute.


What is an “Octane Booster”?

The purpose of an octane booster is to increase the octane number of the petrol, which in turn reduces or retards the fuel’s tendency to detonate. Octane booster ensures the fuel mixture burns properly so that the engine can function according to its design and at maximum efficiency.




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