Two properties are of interest: the volatility and the octane number.
•the vapour/air mixture must be correct for the engine
•in cold weather the vaporisation is difficult so a volatile fuel is needed; in hot weather too volatile a fuel leads to vaporisation in fuel lines and consequent vapour-lock which starves the engine of fuel (and can be dangerous if the engine stalls)
•petrol blends are altered four times a year; the colder the climate, the more volatile the hydrocarbons needed.
Octane number (RON):
•this measures the tendency of a fuel to auto-ignite (see below)
•2,2,4-trimethylpentane (iso-octane) has a low tendency to auto-ignite and is given an octane number (RON) of 100
•heptane auto-ignites easily and has an octane number of 0.
•the octane number of a fuel is the percentage of 2,2,4-trimethylpentane present in a mixture of 2,2,4-trimethylpentane and heptane that has the same auto-ignition characteristics as the fuel concerned
•unleaded fuel is 95 RON, i.e. equivalent to 95% 2,2,4-trimethylpentane and 5% heptane
•the octane number of 'straight run gasoline', i.e. immediately off the fractionating tower, is 70.
•the fuel is uprated using isomerisation, reforming and cracking.
This uses a Pt catalyst followed by separation and recycling of unchanged material. Thus pentane (RON 62) can be converted to the branched-chain isomer 2-methylbutane with RON 93.
This uses a Pt/Re catalyst (maybe Ł5 million worth in one reformer) which can convert alkanes to cycloalkanes, and cycloalkanes to aromatics. Thus hexane (RON 25) can be converted to cyclo-hexane (RON 83), cyclo-hexane to benzene (RON 106), and methylcyclohexane (RON 70) to methylbenzene (toluene; RON 120).
Heavy oils (C30 – C40) are heated over a catalyst in a fluidised bed, which gives
•alkane to branched alkane + branched alkene
•alkane to smaller alkane + cycloalkane
•cycloalkane to alkene + branched alkene
•alkene to smaller alkene
The conditions and the nature of the catalyst are varied to give the desired products.
Auto-ignition or pre-ignition.
•the fuel-air mixture is compressed rapidly in the cylinder and gets hot; the compression ratio (the ratio of maximum minimum gas volumes in the cylinder) in a petrol engine is between 8:1 and 10:1
•the mixture may catch fire because of the heat generated by this adiabatic compression; or because of residual heat in the cylinder which is greater if solid products of incomplete combustion are present in the cylinder;
•this auto-ignition is intentional in a diesel engine, but in a petrol engine leads to ignition before the spark: pre-ignition, knocking or pinking
•this reduces engine performance and causes damage to the engine, because the piston is still travelling upwards when the early explosion occurs;
•diesel engines are built to withstand the extra stresses arising from the higher compression ratio - that's why they're noisier.
•by the use of additives; the commonest is tetraethyl lead, Pb(C2H5)4, a colourless, covalent liquid; this is thought to prevent the radical reactions which lead to pre-ignition;
•however the products from the exhaust are toxic, and waste lead; lead destroys the catalysts designed to reduce nitrogen oxide and carbon dioxide emissions;
•the favoured route is to blend the fuel with aromatic and branched-chain hydrocarbons as an alternative since these have higher resistance to pre-ignition - RON 120 for methylbenzene;
•some unleaded fuels may be as much as 40% aromatics;
•alternatively alcohols can be used, for example methanol or ethanol, which also have the effect of reducing the inlet temperature;
•residual cylinder temperature can be reduced by using a better thermal conductor for the cylinder block - aluminium, for example.
[Thanks to Christopher Cassano for extra information on this topic.]
Diesels use a heavier fuel, C15 – C 19, that is intended to auto-ignite
•hydrogen is used to remove the sulphur from diesel, sulphur dioxide emissions having been a problem in the past.
•diesels are very economical to run but expensive to buy, because the comprssion ratio is high (about 26:1) and the engine is more substantial because of the greater stresses;
•the fuel is similar in price to unleaded petrol
•there are worries about small particulates from diesel exhausts, so new diesels have exhaust filtration systems.