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Control of inlet valve deposit (IVD) formation in port fuel injected (PFI) gasoline spark ignition (SI) engines has been an on-going concern due to the deleterious impact that this material can have on engine performance (power, acceleration times, drivability and fuel consumption). However, decoupling the effects of IVD formation from the multiplicity of other changing engine parameters whilst quantifying the impacts on engine behavior has remained a challenging task to accomplish. A dedicated experimental methodology is presented that has been specifically designed to address this issue. It successfully decouples and quantifies those engine performance impacts that are attributable to IVD formation. Results are presented demonstrating the deleterious impact of IVD formation on fuel consumption in a PFI SI engine.

There is increasing concern that small flakes of combustion chamber deposits (CCD) can break lose and get trapped between the exhaust valve and the seat resulting in difficulties in starting, rough running and increase in hydrocarbon emissions. In this paper we describe experimental observations which might explain how this flaking of CCD occurs and the factors that might be important in the phenomenon. The experiments include thirty one engine tests as well as tests done in a laboratory rig and show that some CCD flake when they are exposed to water; indeed water is far more effective in bringing this about than gasoline or other organic solvents. The hydrophilicity of the deposit surface which determines the penetration of water and the inherent susceptibility of the relevant deposit layer to inter-act with water are both important. Consequently there are large differences between deposits produced by different fuels and additives in terms of their susceptibility to flake.

Combustion chamber deposit (CCD) formation in SI engines is a complex phenomenon which is dependent on a number of fuel and engine parameters. A mathematical model has been developed, based upon a previously proposed mechanism of CCD formation, which describes the physical and chemical processes controlling the growth of deposits in SI combustion chambers. The model allows deposit thickness to be predicted as a function of time, taking into account gasoline composition and factors influenced by engine operating conditions. Piston top deposit thicknesses predicted by the model for 38 unadditivated fuels show a strong correlation with data from three different bench engine tests. The model offers the possibility of predicting the amount of CCD produced by unadditivated gasolines for a range of engine designs, operating conditions and test durations.

The octane requirement increase (ORI) observed in spark ignition engines essentially occurs due to the effect of deposits in the combustion chamber changing the heat transfer characteristics between the end-gas and the combustion chamber walls. In addition, the volume occupied by deposits produces a change in compression ratio inside each cylinder which also contributes to ORI. However, all the ORI observed in spark ignition engines cannot be explained by these physical effects alone and for some time the existence of a chemical mechanism of ORI has been postulated. Evidence is presented from a laboratory experiment which demonstrates that deposits are indeed able to influence the ignition delay times of fuel-air mixtures by providing a source of active species which help initiate autoignition. Such effects have also been observed in some engine experiments, thus confirming the existence of chemically based ORI.

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.