A petrol engine is a classic example of the utilisation of a machine to
produce work from the combustion of a fuel which is, in this case, petrol.
If we represent petrol as being composed of n-octane, we can write a global
reaction of n-octane oxidation:
C8H18 + 12.5O2 -> 8CO2 + 9H2O
The reaction above indicates the complete combustion of octane to give
CO2 and H2O. In the engine however, the reaction is simply not represented
by the above equation and in fact the products from the exhaust contains
more than just CO2 and H2O, which depends on the amount of air that has
been added with the fuel. The chemical processes involved in the engine are
complex. It entails a complex free-radical chain reaction mechanism,
involving many chain carriers and branching agents with the ability to
react in a number of different ways.
http://www.madsci.org/posts/archives/ma ... .Ch.r.html