A computational and experimental study of the scavenging flow in the transfer duct of a motored two-stroke cycle engine

This study was carried out to assess the ability of a computational fluid dynamics (CFD) code to predict the scavenging flow in the transfer duct of a two-stroke cycle engine. A two-stroke cycle engine was modified to allow laser Doppler velocimetry (LDV) measurements to be made in one transfer duct. It was operated under motoring conditions at 500 r/min with a delivery ratio of 0.7. Predictions were obtained from a dynamic CFD simulation of the flow within the cylinder, transfer duct and a portion of the exhaust duct. Boundary conditions for the CFD model were obtained from experimentally measured pressure-time histories in the crankcase and exhaust. A comparison of measured and predicted transfer duct axial velocities at various locations within the duct showed that the CFD model could replicate the general trend of the flow but not the details. From the LDV measurements and CFD predictions, velocity oscillations were observed between the end of crankcase blowdown and transfer port closing. A one-dimensional general engine simulation package was used to investigate the gas dynamic activity in the transfer duct. It was found that the observed oscillations were due to pressure wave reflections in the transfer duct. The general trend of the axial velocity profile in the transfer duct was well replicated by the one-dimensional simulation as were the exhaust and crankcase pressures.