EFFECT OF TOP LAND WIDTH AND ENGINE OPERATING-CONDITIONS ON EXHAUST HYDROCARBON EMISSIONS FROM A SPARK-IGNITION ENGINE

In this paper, the formation mechanisms of unburnt hydrocarbons in the cylinder of a spark ignition engine are investigated and the effects of various top land crevice widths, engine speeds, air-fuel ratios and spark advance angles on engine exhaust hydrocarbons are investigated. The effects of engine top land crevice width on power output and fuel economy are also analysed. It is shown that the optimum engine top la nd crevice width is 0.32 mm (1.3 times the two surface quench distance s), thus the flame can get into the bottom of the crevice and burns ou t all of the accumulated unburnt hydrocarbons in it, consequently the engine exhaust hydrocarbons can be greatly reduced by 35-50 percent wi th less penalty on engine power output and fuel economy (0.5-1.5 per c ent loss in power output and 1 similar to 3 per cent increase in fuel economy).