What goes into petrol?

Petrol contain mostly hydrocarbons. Hydrocarbons are basically anything which contain carbon atoms and hydrogen atoms. They're useful as a fuel, because a carbon atom can combine with two oxygen atoms to make carbon dioxide, and two hydrogens can comine with an oxygen to make water (H2O). Actually, as well as the carbon dioxide reaction, you get get a single carbon reacting with a single oxygen to make carbon monoxide (CO). This can then go on to react with more oxygen to make carbon dioxide. All these reactions release energy, which is what makes our piston move downwards.

Petrol is a mixture of about 500 different hydrocarbons, which contain between 3 and 12 carbon atoms. These hydrocarbons are liquid (obviously, since petrol is liquid) and they boil between 30 and 220 degree celcius. So, on a hot day the smallest molecules are going to start evaporating off if they're not in a sealed container.

Hydrocarbons are either saturated (only contain single bonds) or unsaturated (contains at least one double bond). Saturated hydrocarbons tend to burn cleanly, whereas unsaturated hydrocarbons burn smokily and are unstable.

Now, you want to get the biggest possible bang out of your fuel/air mixture, and you can do this by compressing it before igniting it. This pushes all of the fuel and air molecules closer together. A compression ratio of 6:1 gives 25% efficiency, whereas raising it to 12:1 gives 35% efficiency.

However, when you compress the mixture it gets hotter - up to 600 degrees celcius in a running engine.. If you compress it too much, it'll get hot enough to ignite itself, which is bad! If this happens in a running engine, it's called knocking or preignition and it can damage your pistons. Of course, if the engine is hot to start with, it'll heat up the mixture and make preignition more likely. We'll return to this in a minute.

So, the efficiency of your engine is related to the compression ratio, which is how much you squish the fuel air mixture before igniting it. The fuel/air mixture gets sucked into the cylinder on the intake stroke when the inlet value is open. Then it gets squished during the compression stoke, until the piston reaches its highest point. If there is 400 cubic centimetres above the piston at it's lowest point, but only 100cc at it's highest point then you have a compression ratio of 4:1. A real engine would have a compression ratios of something like 12:1.

We'd ideally want to have the compression ratio as high as possible, but we've seen that that'll cause the fuel/air mixture to ignite early (before the spark plug has sparked) and damage our engine. So, we look for some magic ingredient to stop this happening.

One such magic ingredient is Tetra-ethyl lead, which gives leaded fuel it's name. It reduces the tendency of the mixture to preignite. Unfortunately, it's bad for the environment and has been phased out in the UK. TEL acts to lubricate the exhaust value, so there's some concern that switching to unleaded fuel can lead to the exhaust valve being lubricated less.

Another problem with using tetra-ethyl lead (TEL) is that it would eventually gum up the engine and exhaust with lead, so you also have to add scavengers to the petrol. Scavengers are chemicals which react with the lead to make a volatile chemical which can pass out of the exhaust. An example would be lead halide salts.

It's worth noting that petrol usually contains some Sulphur before it gets refined. Sulphur defeats the anti-knock properties of TEL and so has to be removed. It's hard to remove it all though, and the sulphur which is left in the fuel reacts with air to form sulphur dioxide. The sulphur dioxide gets into the atmosphere via your exhaust and dissolves in water to form sulphuric acid - causing acid rain.