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Downsizing of SI engines in combination with turbocharging is state of the art to reach future CO2 emission limits. A single stage turbocharged engine has a conflict of objectives between high rated power and high low-end-torque. To expand the stable map area of the compressor a variable compressor is investigated on an engine test bench supported by the use of engine process simulation. The measurements were carried out on a radial compressor with high trim compressor wheel. The limiting factor of the feasible low end torque is the surge line. Different inlet cross sections are investigated to shift the surge line to lower mass flow. The compressor maps are measured simultaneously on a hot gas and an engine test bench. A combination of both maps provides the input data for a 1D-simulation model of the test engine, which is presented in this paper. A predictive combustion model is validated for full and part load operating points up to 5000 rpm based on the serial production engine.

The supercharging of small-displacement gasoline engines requires high pressure ratios combined with a wide range of air flow rate. To resolve this conflict, two-stage turbo charging with two turbochargers or the combination of a turbocharger and a mechanical compressor is used. But this is associated with an increase in complexity. The highest potential for avoiding a multi-stage system is provided by the systematic modification of the turbo-machinery operating maps, e.g. on the turbine side by using variable turbine geometry. An additional promising approach is the implementation on the compressor side of a variable guide vane. The shape of the compressor map is directly affected and the requirements for highly boosted engines can thus be fulfilled. The present paper provides an assessment of the potential of a variable compressor in combination with a variable geometry turbine (VTG) and additional wastegate on a small-volume gasoline engine.