Effect of Piston Crevice on Transient HC Emissions of First Firing Cycle at Cold Start on LPG SI Engine

By changing the top-land radial clearance, this paper presents the effect of the piston crevice on the transient HC emissions of the first firing cycle at cold start on an LPG SI Engine. A fast-response flame ionization detector (FFID) was employed to measure transient HC emissions of the first firing cycle. At the same time, the transient cylinder pressure and instantaneous crankshaft speed of the engine were measured and recorded. The results show that increasing 50% crevice volume leads to 25% increase of HC emissions in the lean region and 18% increase of HC emissions in the rich region, however, the 50% increase of crevice volume contributes to 32% decease of HC emissions in the stable combustion region. For LPG SI engine, the HC emissions of the first firing cycle during cold start are relatively low in a wide range of the excess air ratio. The cylinder pressures are all reduced to the motoring cylinder pressure gradually with the spark angle retarded for different top-land radial clearance. For same concentration mixture, when the spark timing is retarded, the HC emissions rise to a maximum value at first, and then reduce greatly due to significant later combustion in the exhaust stroke, and the HC emissions rise hugely for the misfire cycle when the spark timing is too late. For the different top-land radial clearance, the HC emissions changing with the spark timing is display in three patterns, and the burning velocity is the key point for HC emissions at first, and then the late combustion and the oxidized in the exhaust stroke will be more important when the spark timing is too late.