Petrol is made up of alkanes, which are a type of hydrocarbon. These alkanes have carbon atoms connected four times to other carbon or hydrogen atoms by singular covalent bonds, and are therefore saturated. Alkanes have different structures. Some have a chain of carbons with hydrogens connected to them. They are called chain alkanes. Others have carbons interconnecting and are called branched alkanes. Petrol has short-chained alkanes and branched alkanes as these have high octane numbers. This means that, when the alkanes are put under pressure they explode smoothly and don’t cause the engine to knock.
If an alkane had an octane no. of 90 then it knocks the same amount as a mixture of 90% methylcyclohexane (knocks very little) and 10% n-heptane (knocks a lot). Petrol originally had mainly long chained alkanes. Therefore , as some alkanes have the same formula but different structures, they can be changed into other alkanes during isomerisation. This is the process where long chained alkanes are heated using a catalyst and due to this heat they break. The carbons then interconnect with other carbons to form mostly branched alkanes but sometimes two smaller chained alkanes.
If this wasn’t possible then the petrol would have a lower octane number and cause the engine to knock. Isomerisation : Petrol is formed when crude oil is seperated into its different components during fractional distillation. The crude oil is heated and sent into a distillation column. Here it is allowed to cool. Each component of the crude oil has a different boiling points and so the one with the lowest boiling point will evaporate the quickest. These evaporated alkanes go to the top of the column through bubble trays. Here they condense and are drained through pipes.
In this way, all the alkanes with boiling points lower then petrol are drained away until its boiling point is reached. The petrol is then drained away. After the petrol has been distilled, the process of getting the right alkanes from the gasoline to get the correct octane number for petrol takes place. First reforming takes place. This is when the alkanes are seperated into branched and straight chain by putting them through ‘zeolites’ a type of “sieve”. The straight chain molecules are recycled by heating them with a catalyst of platinum over finely dispersed aluminium oxide.
The chains break and form branched alkanes before becoming cycloalkanes, which are like rings of alkanes. All of these add to the octane number of petrol. Fractional Distillation : Reforming : When petrol is burnt in a car engine, many different pollutants are formed. The pollutants, which are released after petrol is burnt, are Carbon dioxide, Carbon monoxide, Sulphur oxide, Nitrogen monoxide and other various hydrocarbons. These are formed from the waste products of the petrol and the air that helped cause the petrol to explode. The reactions that take place in the engine are shown below :
Ordinarily, nitrogen won’t react with oxygen. However, inside the combustion engine, temperature can rise up to 1,200 C. These exhaust emissions are very harmful to the environment and can cause both acid rain and also photochemical smog. This is created when two of the pollutants from the car engine, nitrogen oxide and hydrocarbons, mix with oxygen, water vapour and sunlight. The final product is called ozone. This ozone is part of the smog. The smog causes haziness, poor visibility, eye and nose irritation and breathing difficulties for asthmatics, the young and the old.
To stop these pollutants adding to this smog, laws were drawn up to reduce emissions. Lean burn technology is the increasing of the air to fuel ratio. If there is more air then there is more oxygen and therefore more carbon monoxide will be turned into the less harmful carbon dioxide. To get this to happen, there had to be more oxygen in the combustion chamber where the pollutants are formed after the petrol has been burnt. As there is more oxygen, the hydrocarbons are fully burnt to form carbon dioxide and water.
As there is also a lot of nitrogen in air. A lot of nitrogen oxides are also formed in the combustion chambers. These are made safe in the catalytic converters. Any carbon monoxide or hydrocarbons left over from the combustion and also the nitrogen oxides are changed in the converter. Here, a catalyst is used to speed up reactions that would not otherwise happen, to make the emissions safer. The pollutants are sent through a piece of ceramic honeycomb with the catalysts on it. The metals used as catalysts were platinum and rhodium.
These metals speeded up the reaction between CO and oxygen and hydrocarbons with oxygen to form carbon dioxide and water. They also caused the CO and the NO to react together to form more carbon dioxide and nitrogen. Nitrogen, in theory, could react with oxygen in the air outside to form NO again but this reaction only takes place at high temperatures so it doesn’t happen. The converter will only last about six years as it will eventually become poisoned by the pollutants and have to be replaced. Catalytic converter : Reactions within a catalytic converter :
However, the carbon dioxide emissions from the car engine still harm the environment and this fact coupled with the possibility of a end to the supply of oil have left people looking for alternatives to the combustion engine to run cars. One suggestion has been to use methanol instead of other engines as it produces fewer pollutants than other hydrocarbons, it has a high octane rating so a smooth burn would be available and it’s cheap to change the engine and petrol pump and also to produce. However, when mixed with ordinary petrol, it won’t mix without a solvent to help.
Also, the mixture absorbs moisture and then corrodes the engine. The other option is to use hydrogen. The hydrogen could be used by creating protons and electrons. The protons would move to one side of a partially permeable membrane creating a potential difference across this membrane. Oxygen could then be used to link the protons and neutrons via electrolysis. This process would cause a flow of electrons, which could power the engine. The by-product is water, which is clean. Either way, the future looks cleaner for the engine.