Pulsating flow performance of a turbocharger compressor for automotive application

Downsizing with turbocharging is the most promising way, especially in terms of cost, to get reduced fuel consumption and CO2 emissions particularly in the case of Spark Ignition engines. In automotive applications the turbocharger turbine usually operates under heavy unsteady flow conditions due to the opening and closing of engine valves. However, in the case of extremely downsized engines with a reduced number of cylinders and a small intake circuit volume also the compressor performance can be affected by the unsteady flow generated by the engine intake valves. To make simulation models able to accurately predict engine performance, a better understanding of compressor and turbine pulsating flow performance can be accomplished through measurements performed on specialized test facilities, using suitable measuring equipment. As regards the turbocharger compressor, the surge line position under pulsating flow conditions is another important aspect to be considered. In the paper the results of a broad experimental investigation performed on a small turbocharger compressor matched to a downsized gasoline engine are presented. Measurements were developed on the test facility operating at the University of Genoa, which allows investigations on automotive turbochargers both under steady and unsteady flow conditions. Tested turbocharger compressor was coupled to the automotive engine intake circuit and the pulsating flow was generated by a motor-driven cylinder head fitted with a variable valve actuation system. Different levels of turbocharger rotational speed and different intake valve opening strategies were considered. For each operating condition compressor unsteady performance was evaluated starting from measurement of several instantaneous parameters (inlet and outlet static pressure, mass flow rate and turbocharger rotational speed). A significant deviation of compressor instantaneous performance from steady state was observed, resulting in a hysteresis loop surrounding the steady state curve.