Effect of top land width and engine operating conditions on exhaust hydrocarbon emissions from a spark ignition engine

In this paper, the formation mechanisms ofunburnt 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 land crevice width is 0.32 mm (1.3 times the two surface quench distances), thus the flame can get into the bottom of the crevice and burns out all of the accumulated unburnt hydrocarbons in it, consequently the engine exhaust hydrocarbons can be greatly reduced by 35-50 per cent with less penalty on engine power output and fuel economy (03-13 per cent loss in power output and 1 ~ 3 per cent increase in fuel economy).