On shock structures in dynamic exhaust valve flows

Abstract

The gas dynamics of the flow past an exhaust valve has been investigated using Schlieren photography. An experimental setup was designed and constructed, which allowed optical access to the valve head and seat region as well as to the exhaust port. The setup was constructed so that the shock structures of a steady flow, with a static valve, could be compared to the structures found in experiments with a more realistic dynamically discharging cylinder, with a moving valve. The steady flow experiments were carried out at a valve lift to a port diameter ratio of 0.155 with cylinder pressures up to 325 kPa. The dynamic valve experiments were performed with an initial cylinder pressure of 300 kPa and at an equivalent engine speed of 1350 rpm. The steady flow experiments belonged to one of the two flow regimes, depending on the cylinder pressure: regime I, a wall-bounded supersonic jet (for low cylinder pressures) or regime II, a fully expanded supersonic nozzle-flow (for high cylinder pressures). By comparing the images from the dynamic valve experiment to those of the steady flow experiments, it was shown that the flow in the dynamic experiments exhibits more similarities with regime I. However, large differences in the shock structures between the steady flow in regime I and the dynamic valve flow remain. This indicates that experiments using a steady flow and a fixed valve lift do not encompass the essential physics found in real engine flows and should be avoided.