DF - DEVELOPING FUELS
Energy Changes
Focus on Petrol 
Introduction to Organic Chemistry
What is Petrol?
Winter & Summer
The Problem of Knocking
Exhaust Emissions
What is petrol?
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.
Winter & Summer Petrol
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.
The Problem of Knocking
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.
- Use special additives. Lead containing compounds were used extensively but have now been phased out due to the toxicity of lead.
- Refining and blending. The shorter the alkane molecule, the higher the octane number, but the greater the volatility. Another factor is the degree of branching. The more branched the chain, the higher the octane number.
Crude oil does not contain enough branched chain hydrocarbons. Petrol companies overcome this by a number of processes.
- Isomerisation. This involves heating a straight chain alkane in the presence of a catalyst. This breaks the chains up and they then join up again branched.
- Cracking. Large molecular alkanes are broken down with heat and a catalyst into smaller molecules. These tend to be highly branched. In both these processes zeolites are used as a molecular sieve to separate the straight chain alkanes from the branched chain alkanes.
- Reforming. Reforming involves changing chains into cycloalkanes often by using a catalyst.
- Adding oxygenates. Alcohols and ethers increase the octane number. They both contain oxygen atoms in their molecules. They also tend to cause less pollution. The most commonly used oxygenates are methanol, CH3OH; ethanol, C2H5OH and MTBE (methyl tertiary butyl ether), CH3OC(CH3)3.
Exhaust Emissions
Exhaust emissions are causing world-wide concern. The main emissions from a car engine are:
- Oxides of nitrogen, e.g. NO and NO2. These arise from a reaction between nitrogen and oxygen from the air.
- Oxides of sulphur, e.g. SO2 and SO3. These come from sulphur compounds in the fuel.
- Unburnt hydrocarbons.
A number of ways have been developed to reduce the problem:
- Engine design. Lean-burn engines use more air than the stoichiometric ratio so less NO, NO2 and CO are produced but there are more unburnt alkanes. An oxidation catalyst in the exhaust converts CO and alkanes to CO2 and H2O.
- Three-way catalysts. A three-way catalytic converter oxidises CO and alkanes and reduces NOx to N2. This kind of catalyst system only works if the petrol-air mixture is carefully controlled so that it's exactly the stoichiometric mixture for the fuel. This means that cars fitted with three-way catalyst systems need to have oxygen sensors in the exhaust gases linked back to electronically controlled fuel injection systems. All catalytic converters only work when they are hot. They are also poisoned by lead.
- Changing the fuel. Aromatic hydrocarbons can make up as much as 40% of lead-free petrol. These can cause cancer. Butane is very volatile but also increase the octane number. Oxygenates are a possible solution.
- Methanol. Methanol burns cleanly in a car engine and does not release carcinogenic benzene vapour. It has a high octane number and is less volatile than petrol so it is less likely to explode in a collision. Some countries have used it as part of a mixture with petrol. The problems are that methanol does not mix easily with other hydrocarbons, and that methanol absorbs water vapour from the air. When it does absorb water, it causes corrosion to the engine parts which reduces the life of an engine and can lead to an unhappy ending! It is also toxic and less energy is produced from 1 litre of methanol than from ordinary petrol.
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