Pressure Amplitude Influence on Pulsating Exhaust Flow Energy Utilization

A turbocharged Diesel engine for heavy-duty on-road vehicle applications employs a compact exhaust manifold to satisfy transient torque and packaging requirements. The small exhaust manifold volume increases the unsteadiness of the flow to the turbine. The turbine therefore operates over a wider flow range, which is not optimal as radial turbines have narrow peak efficiency zone. This lower efficiency is compensated to some extent by the higher energy content of the unsteady exhaust flow compared to steady flow conditions. This paper experimentally investigates the relationship between exhaust energy utilization and available energy at the turbine inlet at different degrees of unsteady flow. A special exhaust manifold has been constructed which enables the internal volume of the manifold to be increased. The larger volume reduces the exhaust pulse amplitude and brings the operating condition for the turbine closer to steady-flow. The operating points are defined by engine speed and boost pressure. From these values the isentropic turbine work is calculated and with the measured compressor work the mean turbine efficiency is estimated. The results show that more energy has to be provided to the turbine at larger exhaust manifold volumes to maintain a constant boost pressure, indicating that the efficiency of the turbine decreases.