A Laboratory Simulation and Mechanism for the Fuel Dependence of SI Combustion Chamber Deposit Formation

The formation of combustion chamber deposits in modern SI engines is predominantly derived from hydrocarbon fuels and occurs as a consequence of the quenching action of the combustion chamber walls on the flame. A laboratory experiment has been designed which enables rapid generation of deposit material in the form of viscous brown liquids. Heating these deposits produces material that is consistent in composition and physical appearance with mature engine deposits. The deposit-forming tendency of a number of individual hydrocarbon species has been determined. The amount of deposit increases with i) the amount of unsaturation present in the molecular structure and ii) the boiling point of the hydrocarbon fuel being burned. A structurally derived parameter for each hydrocarbon molecule is found to correlate well with deposition rate, allowing a unified treatment of the different generic forms of hydrocarbons in which deposit-forming tendency is linked to molecular structure.