Autoignition and engine knock

The autoignition that leads to knock in spark ignition engines is extremely dependent on the chemical composition of the fuel. The knock phenomenon and the practical methods of characterising a fuel in terms of its octane number are described in Chapter 7, but it is pertinent to illustrate here how the understanding of alkane oxidation can help to elucidate the effect of fuel structure on its behaviour in engines. 

alkene ABO, oxygenated compound OR, oxygen-ring compound. 

R and AB contain a smaller number of carbon atoms than RH. 

Reaction numbers for specific radicals are usually given a further letter. For example, (5Ae) refers to the reaction of C2H5 radicals with O2 to give the conjugate alkene (AB), C2H4 and the HO2 radical. 

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