The scavenging process in a direct-injection two-stroke research engine was examined. A physical model was developed to characterize scavenging based on known quantities. An electromagnetic valve was used to sample pre-combustion gases from the running engine. The valve proved to be very robust and was able to sample greater than 30massunder all test conditions. The concentration of either oxygen or carbon dioxide, the mass flow rate of the fuel, and the intake and exhaust temperatures were used to calculate the scavenging efficiency, trapping efficiency, trapped mass, and concentration of combustion products. A modified definition of delivery ratio, based on a reference trapped cylinder mass, was used for all scavenging calculations.
Good agreement was found between scavenging parameters calculated using the trapped oxygen concentration and those calculated using the trapped carbon dioxide concentration. Carbon monoxide was found to be a poor indicator of scavenging, because CO production continued after combustion. Oxides of nitrogen were theoretically good indicators of scavenging, but experimental problems with sampling were not resolved, and the NOx data were not considered to be reliable.
Tests were performed with a variable delivery ratio and: a constant air/fuel ratio, a constant fueling rate, or with fueling controlled by the engine control unit. Scavenging and trapping efficiency were shown to be strongly a function of delivery ratio. Scavenging efficiency increased with increasing delivery ratio, and trapping efficiency decreased. Tests were also run at constant delivery ratio with a variable air/fuel ratio or a variable engine speed. Scavenging efficiency decreased slightly with increasing air/fuel ratio, and with decreasing speed. Trapping efficiency did not seem to be affected by either parameter. Scavenging and trapping efficiency were near the non-isothermal perfect mixing model for all test cases.