Time-Resolved Emission Sampling in a Direct-Injection Engine

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 NO_x, 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.