Variable Turbine Geometry on SI Engines

Six closely matched turbochargers with variable geometry turbines, VGT, were benchmarked and investigated on a 2 litre spark ignited,SI, engine in order to evaluate their on-engine performance regarding the operating range of the VGT concepts and the ability of the turbine to deliver power to the compressor. The VGT turbines effect on engine operating conditions such as pre-turbine pressure that affects the engine output are also evaluated as well as the engine torque itself. The engine tests included performance at low engine-speed and at high engine-speed, transient response and a turbocharger flow capacity and matching check. Based on the engine test results three VGT concepts were further analyzed, in cold start emission testing and gas-stand testing. Gas-stand test result singled out one VGT concept as the most suitable for SI engines with regards to combined efficiency and flow capacity. The increase in engine performance with the VGT concepts compared to the fixed geometry turbine in this investigation are in the order of +10% low engine speed torque, +10% high engine speed torque and equal transient performance. The cold start testing showed that all variable turbines will most likely require improved exhaust after-treatment (such as secondary air injection (SAI), converter loading, cell density etc.) than a standard single scroll turbine. The main reason for this requirement is the increased temperature drop across the turbine.