|MadSci Network: Chemistry|
Engine knock is a symptom of pre-ignition or auto-ignition. What happens is that the fuel-air in the combustion chamber ignites spontaneously resulting in a very rapid reaction of a portion or all of the fuel-air mixture. This fast release of heat results in pressure oscillations that cause mechanical vibrations to produce a knocking or pinging sound. One cause of engine knock is poor quality fuel. If the fuel is of insufficient octane, it can auto-ignite due to the high temperature and pressure in the combustion chamber, rather than from the spark, like it should.
The lead actually reacts chemically with the fuel during combustion to inhibit auto-ignition.
Alkyl lead interferes with hydrocarbon chain branching in the intermediate temperature range where HO2 is the most important radical species. Lead oxide, either as solid particles, or in the gas phase, reacts with HO2 and removes it from the available radical pool, thereby deactivating the major chain branching reaction sequence that results in undesirable, easily-autoignitable hydrocarbons.
Automotive Gasoline FAQ, Bruce Hamilton
The website I referenced above has a lot of great information on fuel and lots of references too.
Hope that helps you.
Your Mad Scientist,
Try the links in the MadSci Library for more information on Chemistry.