It is safe to say that petrol plays a vital role in the way the whole of the western world lives. Other than water (and blood - of course!) gasoline is probably the most important liquid that we use everyday. Other than running our vehicles it also has a crucial part to play in stablising the worlds leading economies. As the worlds leading consumers, the U.S.A. users over 500 billion litres of gasoline per year.
But how does it work and what makes it so important?
The gasoline fraction contains striaght chained hydrocarbons that are between 5 and 10 carbons long which contain LOTS of energy. In fact 1 litre of gasoline contains over 36000 kilojoules of energy which is about the same as about 30 pints of lager. But before you rush off to order a pint of petrol remember, its quite poisonous and in no way do I endorse the drinking of petrol... Try absinth! Or Stella.
All we have to do is to utilise this energy that is "locked up" in the chains of carbon. One great way of doing this is to burn it in the presence of oxygen to produce carbon dioxide and water vapour as shown in the diagram below.
OK so we get the idea that gasoline releases a massive amount of energy when burned in oxygen, so imagine what effect this would have if we were to undertake this combustion in a cylinder containing a piston. The energy released would increase the pressure inside the cylinder and drive the piston. This is the basic principle of the modern day internal combustion engine used in cars, buses and motorcycles today. Almost all cars today use a four-stroke combustion cycle, or Otto cycle as it is otherwise known. This type of engine uses four different strokes to turn the gasoline into energy which then becomes motion. The diagram below shows the different parts of the piston which we can use to understand how the Otto combustion engine works.
The four stages in the internal combustion engine are:
Simple eh? But the average car engine undertakes this action over a hundred times every minute. The linear motion of the piston is converted to rotational movement of the crankshaft which is used to power the car wheels. This rotation of the wheels then is again converted into linear motion as the wheels move.
But why is this fraction so special?
When we look to choose a hydrocarbon fuel to use in our vehicle engines we should consider a number of their characteristics:
1.Volatility.
For greater efficiency the hydrocarbon fuel chosen must be very volatile so that it can mix easily with the air.
2.Octane value (pre-ignition knock resistance).
This determines the amount of energy needed to ignition the fuel in order for the fuel to burn. It would be of great advantage for the fuel to ignite with the minimum amount of energy expended.
3.Energy content.
How much energy can we get from how much fuel? The most efficient fuel is one which gives out the most energy from the least volume used.
4.Safety.
This follows from the energy content. Although we need to obtain the most energy we can from the least amount as possible, if the fuel contains too much energy it may explode too vigorously upon ignition... Not good!
5.Cleanliness.
How much pollution does the fuel emit? Can this pollution be easily overcome? The ideal fuel would emit no pollution at all, but that is very unlikely so the lower the pollution rate the better.
Basically the fuels that are available for transport purposes can be split into two catergories.