The Potential of Variable Compressor Geometry for Highly Boosted Gasoline Engines

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. To generate the new compressor maps with different vane positions, a detailed model is developed, which includes several detailed models of earlier publications. The modification of the flow field of the compressor leads to a change in the forces on the rotor assembly. To take account of increased bearing loads another simulation model is developed that allows a detailed analysis of the lubricating film properties occurring. The variable maps are integrated into a 1-D engine simulation model to estimate the potential of this system for higher low-end-torque.