Waste-gate turbocharging control in automotive SI engines: effect on steady and unsteady turbine performance

Turbocharging is becoming a key technology for automotive spark ignition engines (fed with both liquid and gaseous fuel) as a support to the downsizing concept in order to reduce fuel consumption and exhaust emissions. A waste-gate valve is usually fitted as turbocharger control system in these applications, due to its ability to work at very high exhaust gas temperatures. However, not much information is generally available on turbine behaviour in the opened waste-gate area. This paper presents the results of an experimental study developed on a waste-gated turbocharger for downsized SI automotive engines, performed on the test rig operating at the University of Genoa (Italy), extended both to steady and unsteady flow operation. Mass flow through the by-pass valve and turbine impeller was measured at different waste-gate settings in steady flow conditions. The analysis was then extended to unsteady flow turbocharger operation, focusing on the effects of by-pass opening on wave propagation phenomena in the turbine circuit and instantaneous mass flow rate.