Unsteady flow behaviour of the turbocharging circuit in downsized SI automotive engines

The need to reduce CO2 emissions and the increasing cost of fuel will require the development of high efficiency automotive combustion engines, while complying with near-zero pollutant emissions. In recent years, the diesel engine was substantially improved, due to the introduction of electronically controlled fuel injection systems and advanced turbocharging units. On the other side, the Spark Ignition (SI) engine has to significantly reduce fuel consumption, especially at part load operation, to accomplish CO2 emission targets. Downsizing SI engines is a promising way to attain a better fuel economy: a potential of efficiency increase between 10 and 30 per cent can be achieved if a global approach is used, by integrating both available and innovative technologies (turbocharging, gasoline direct injection, variable valve actuation, etc), managed by proper control strategies. Within this frame, it is apparent that turbocharging is becoming a key technology for both gasoline and diesel automotive applications. However, a successful application of turbocharging to SI engines has to face 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 needed, particularly focusing on the turbocharger behaviour in the typical unsteady flow conditions occurring in automotive engines. To this purpose, measurements performed on dedicated test facilities can supply a lot of information to be used both in the development of simulation models and to assess correlation criteria between steady and unsteady turbocharger performance. In the paper the results of an extensive experimental investigation, developed on small turbochargers for downsized gasoline engines by using a flexible test rig operating at the University of Genoa, are presented. The study was focused on the unsteady flow behaviour of the turbocharging circuit, referring to different aspects. As a first item, the effect of exhaust line geometry on flow unsteadiness was deepened by comparing measured pressure diagrams in different circuit configurations. In the case of a typical ‘4 into 1’ exhaust manifold, a noticeable increase of turbine inlet pulse amplitude was achieved by using an appropriate dividing wall in the manifold mixing section. In the second half of the paper, the influence of the main pulsating flow parameters on the amount of available energy at the turbine inlet is considered, by evaluating this quantity on the basis of instantaneous turbine inlet parameters. The available energy confirmed to be related to the pulse amplitude at the turbine entry, which proved to depend both on the pulse frequency and the mean pressure level. Finally, turbine overall efficiency values in pulsating flow conditions are presented and compared with the levels calculated on the basis of mean measured parameters and with those referred to turbine steady flow operation.