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A key technology for further improving the efficiency of gasoline engines lies in downsizing in combination with turbocharging. Decreasing the engine displacement greatly increases the demands on the turbocharging system. The charging of the engine with a single-stage turbocharger leads to a compromise to fulfill the requirements of the nominal power of the engine and the low-end torque. To avoid the use of complex two-stage boosting systems, it is necessary to increase the pressure ratio and the air flow rate at the same time. The wide speed and airflow range of gasoline engines intensify this trade-off. The use of a variable geometry turbine (VGT), additionally equipped with a wastegate bypass, offers great potential to meet the requirements on the turbine side. The range of stable operation of the compressor is limited by choke at high mass flow rates and surge at low mass flow rates. The variable geometry compressor (VGC) is one promising approach to extend the compressor map.

Within a project of a research association (Forschungsver-einigung Verbrennungskraftmaschinen e.V.) a DI spark-ignition engine with small engine displacement was designed at the Institute of Internal Combustion Engines of the Technische Universitat Braunschweig. The objective of the project is to investigate the minimum bore diameter which allows the reasonable use of the advantages of gasoline direct injection. This article outlines the preliminary studies to identify suitable geometry variants for lateral and central injector position concerning effective engine operating data. Under consideration of current production possibilities and geometries of available injector and spark plug CAD studies were carried out. All suitable valve concepts, beginning with a two-valve (2V) concept and up to a four-valve (4V) concept, were examined in the CAD studies. The bore diameter was varied in a range from 56 to 62 mm combined with a variation of the stroke/bore ratio from 0.9 to 1.2.

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.