Development of Active Flow Control in a Turbocharger Turbine for Emission Reduction

Abstract

The increase in particle emissions restrictions in recent years coupled with improvements in such key automotive engine performance areas as low-end torque, increased boost and the elimination of turbolag now required, has led to conventional Variable Geometry Turbochargers (VGTs) to become quite popular in matching turbine inlet geometry to the characteristics of the exhaust gas stream throughout the engine operating range. However, over all these years of turbocharger development the fundamental issue of the less than ideal combination of a reciprocating engine providing energy to drive a rotodynamic machine such as a turbocharger turbine has not been addressed satisfactorily. This paper, therefore, introduces a new concept in turbocharger development, namely, that of active turbocharger flow control. It demonstrates the application of an active flow control device (nozzle) at the inlet to the turbine rotor to help harness the energy contained as a result of the pulsating characteristics of the incoming flow. In the Active Control Turbocharger (ACT), therefore, the nozzle (in this case a sliding vane) is able to alter the inlet area at the throat of the turbine inlet casing (volute) in phase and at the same frequency as that of the incoming exhaust stream pulses. Actuated by a high speed electrodynamic shaker the nozzle can adapt according to the engine exhaust gas pulse pressure variation, thus taking advantage of the lower energy levels existent before and after each pulse pressure peak, which the current systems do not take advantage of. Thus, ACT makes better use of the exhaust gas energy of the engine than a conventional VGT. The experimental work concentrates on the potential gain in turbine expansion ratio and eventual power output as well as the corresponding efficiency trace and nozzle area schedule. Exploration of the full turbine range was made possible by the use of a new eddy-current dynamometer, also, presented here. Early simulation work has, already, provided encouraging results with a theoretical potential gain in terms of cycle expansion ratio and actual power output of approximately 9% and 29%, respectively at typical engine operating points (40Hz or equivalent to 1600 engine rpm). Direct comparisons between VGT and ACT operation are also provided, at selected, typical, laboratory-simulated engine operating conditions, showing the benefits arising from the use of this innovative technology.