Time-Resolved Emission Sampling in a Direct-Injection Engine 1999-01-3309
Time-resolved measurements were made of the gas composition at the exhaust port of a direct-injection two-stroke engine operating at 2000 rpm and an air-fuel ratio of 30:1. A high-speed sampling valve capable of 1.0 ms (12 CAD) time resolution was used to collect samples 1 cm downstream of the exhaust port of the engine. The time-resolved NOx, CO2 and CO concentrations decreased continuously during the scavenging process due to the dilution by short-circuited air. The hydrocarbon emissions, however, behaved significantly differently from the other species. At the time of exhaust port opening the concentration was low, it reached a maximum value by BDC, then decreased slightly in the latter part of the scavenging event. The dilution rates calculated for the hydrocarbon data gave negative values, indicating that there was a significant production of hydrocarbons during the gas exchange period. By using the dilution rate obtained from the CO2 data, the fraction of the total measured hydrocarbon concentration which originated from the bulk cylinder contents at the time of exhaust port opening was estimated. The results indicated that a significant fraction, ∼50%, of the hydrocarbon mass flow can be attributed to the hydrocarbons released after the port opens. Thus, it is considered that the effect of wetted surfaces and injector sac volumes are a significant contributor to the hydrocarbon emissions for direct-injection engines under stratified operation.