Re: Which chemical processes are there in a petrol engine?

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.

A free-radical mechanism consists of the following steps:

1) initiation
2) propagation
3) termination

If we represent petrol as being R-H, where R is an alkyl group (R=C8H17,
for the case of n-octane), the initiation step would involve the H abstraction
by O2 to yield an alkyl radical and a hydroperoxy radical:

         R-H + O2 -> R + HO2

The process is entirely selective and depends on the activation energies
of the abstraction reactions.

Between 500 - 700 K, hydrocarbon oxidation can yield a number of alkenes
and oxygenated products. During the early stages of combustion, alkyl radicals
can react by an HO2 mechanism producing an alkene:

          R-CH2-CH2 + O2 -> R-CH=CH2 + HO2        

The remaining alkyl radicals can react with O2 to form alkylperoxy 
radicals:

          R-CH2-CH2 + O2 -> R-CH2-CH2-O2

OH radicals which are reactive are more significant during the later 
stages of the combustion.

Alkylperoxy radicals are stable below 600 K but above 700 K, they undergo
rapid thermal decomposition generating an alkene and HO2. Thus there is 
competition between the production of alkylperoxy radicals and its 
destruction, which very much depends on the temperature and affects the
initial product distributions. The principal chain propagation step involves
the intramolecular isomerisation of a H atom or an alkyl group. If it is
H isomerisation, then the alkylperoxy radical produces a hydroperoxyalkyl
radical:

          R-CH2-CH2-O-O -> R-CH2-CH-OOH

Such a radical can decompose or react with O2 producing carbonyls, alkenes,
O-heterocylces and OH radicals. The OH radicals can then attack the fuel molecule
abstracting a H to produce H2O therby regenerating an alkyl radical and 
completing the chain process.

Termination reactions involves the meeting of two radicals in the gas-phase
to yield inactive species which can no longer propagate the chain. Also,
the radicals may diffuse to the walls of the engine where they are
subsequently destroyed:

        HO2 + HO2 -> H2O2 + O2
        R-CH2-CH2-O2 + HO2 -> R-CH2-CH2-OOH + O2
        HO2 -> wall destruction

Hydrocarbon oxidation is a complex process and the details above that I 
have listed involves only the major parts of a much bigger picture. The
reference I have used is:

Gordon McKay
"The Gas-Phase Oxidations of Hydrocarbons"
Prog. Energy Combust. Sci., Vol. 3, pp. 105-126, 1977

I hope I have answered your question. If you need anymore information do
not hesitate to contact me.

Good Luck!