The Influence of Mixture Preparation on the HC Concentration Histories from a S.I. Engine Running Under Steady-State Conditions

Exhaust-port hydrocarbon (HC) concentration measurements were made using a Fast Response Flame Ionization Detector (FRFID) in order to investigate the mechanisms by which mixture preparation affects engine-out HC emissions. The mixture preparation was varied by: (a) using fuels of different volatility, (b) varying the injection timing, and (c) decreasing the coolant temperature. The observed increases in HC emissions which resulted from lowering the coolant temperature or employing open valve injection are primarily attributed to the resulting increase in the in-cylinder liquid fuel, which is deposited mainly on the cylinder walls and in the piston crevices. The HC attributed to the liquid fuel deposited on cylinder walls exit the engine cylinder roughly in the middle of the exhaust process. On the other hand, the HC attributed to the liquid fuel stored in the piston crevices, and which represent the largest fraction, exit the cylinder during the end of the exhaust process.