Experimental Investigation of Variable Geometry Compressor for Highly Boosted Gasoline Engines

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. A variable charging system consisting of VGC and VGT offers great potential to meet the future requirement for highly boosted engines.

The present paper shows experimental investigations of the potential of a variable geometry turbine with an additional wastegate on a small sized gasoline engine. To reach the torque and nominal power characteristic of a 2-stage boosted reference engine, the test engine is additionally equipped with a camshaft phasing system on the exhaust side. In addition two different variable geometry compressors are investigated.