Petrol is a complex mixture of at least 100 different compounds, mostly hydrocarbons (compounds which contain only carbon and hydrogen). The main source of petrol is from the gasoline fraction from the distillation of crude oil. This fraction contains mostly alkanes with between five and ten carbon atoms.
|Fraction||Boiling range /°C||Composition||% Crude Oil|
|Refinery gas||< 20||C1 - C4||1 - 2|
|Gasoline/naphtha||20 - 180||C5 - C10||15 - 30|
|Kerosene||160 - 250||C10 - C16||10 - 15|
|Diesel oil||220 - 370||C13 - C25||15 - 20|
|Residue||>370||> C25||40 - 50|
The gasoline fraction is in most demand. Some of it is used for producing motor fuels, the rest for manufacturing organic chemicals.
The gasoline from primary distillation makes poor petrol. The refinery converts it into petrol with the right composition and properties. The structure is modified in various ways. Heavier fractions such as kerosene and diesel oil, are 'cracked' to produce smaller molecules for use as extra petrol components.
The residue is also made into petrol by vacuum distillation, this is distillation under reduced pressure reducing the chances of cracking.
Petrol has to be blended to get the right properties, one important property is its volatility. Petrol is mixed with air in the car engine. When the weather is cold, petrol is difficult to vaporise, so the car is difficult to start. Petrol companies make four different blends for different times of the year. In the winter there are more volatile components, i.e. smaller hydrocarbon molecules. In the summer the blend has fewer more volatile components.
Another important property is the octane rating of petrol. This is a measure of the tendency of petrol to 'knock'. The petrol-air mixture must ignite at the right time, just before the piston reaches the top of the cylinder. The petrol-air mixture is compressed and gets hotter. There is a tendency for the petrol to auto-ignite, meaning for it to catch fire. Hence two explosions can occur, one due to compression the other due to the spark. This produces a 'knocking' or 'pinking' in the engine. It results in poor performance and damage to the engine.
2,2,4-Trimethylpentane (iso-octane) has an octane number of 100 and has a low tendency to auto-ignite. Heptane auto-ignites easily and is given an octane number of 0. Normal unleaded petrol has an octane number of 95 and contains 95% 2,2,4-trimethylpentane and 5% heptane.
Modern car engines have a high compression rating, hence knocking is a problem. There are two ways of preventing knocking.
Crude oil does not contain enough branched chain hydrocarbons. Petrol companies overcome this by a number of processes.
Exhaust emissions are causing world-wide concern. The main emissions from a car engine are:
A number of ways have been developed to reduce the problem:
Part of this site was last updated on 21st January 2009.
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