Unsteady Flow Turbine Performance in Turbocharged Automotive Engines

Turbocharging is becoming a key technology for both Spark Ignition (SI) and diesel automotive engines. As regards gasoline engines, turbocharging can help to reduce CO2 emissions when used in conjunction with other technologies, such as engine downsizing, direct injection and variable valve actuation, all managed on the basis of appropriate control strategies. However, the successful application of turbocharging to SI engines must address different problems related to the specific operating environment (exhaust gas temperature level) and to functional aspects (torque curve configuration, transient response). Research work on this subject is required, particularly as regards the unsteady flow operation typical of automotive applications. To this end, a great deal of information can be provided by measurements made on dedicated test facilities as these allow the correlations between unsteady flow characteristics and turbocharger behaviour to be investigated over a broad range without the operating restrictions imposed by the engine. The intake and exhaust components test rig operating at the University of Genoa can be used to investigate automotive turbochargers under steady and pulsating flow conditions, analysing the effect of unsteady flow parameters, circuit geometry and exhaust valve actuation strategies on turbine performance. This paper presents the results of an extensive investigation performed on small turbocharger turbines for gasoline engines, focusing on unsteady flow operation. Initially, the assessment of turbine unsteady flow performance is deepened as regards the evaluation of available energy at the turbine inlet and the procedures to calculate turbine efficiency. The influence of the main pulsating flow parameters on the amount of available energy at the turbine inlet is then analysed, taking into account both theoretical and experimental waves. Lastly, the results of an experimental test programme investigating instantaneous turbine mass flow behaviour and the efficiency levels provided by various different procedures are presented.