Numerical Investigation Into the Performance and Efficiency Trade-Off for a Mechanically Decoupled Electric Boosting System

Abstract

Electric turbochargers offer an additional degree of freedom in engine design to meet the conflicting needs of emissions, fuel consumption and performance targets. This paper presents a simulation study of the application of a mechanically decoupled electric turbocharger to a 2.0L Gasoline engine. The aim of this work is to investigate and quantify how the sizing of the turbine influences performance and efficiency both in steady state and over homologation and on-road duty cycles. Steady state simulations are performed in a GT Power 1D gas dynamics model that includes a simplified model of the electric turbocharger. The 1D model was used to create a mean value engine model for the drive cycle simulations. The turbine diameter was varied from 58mm to 78mm (representing 50%–150% of the baseline turbine size). In steady state conditions, the electric turbocharger achieved a maximum of 1.5% improvement in system efficiency. Larger turbines can improve efficiency by reducing the engine pumping loss, however, they compromise the amount of electrical energy that can be harvested in the low engine speed region. This is problematic as this represents a key area of engine operation in a passenger vehicle. A series of mean value engine models were created for different sized turbines to predict BMEP, turbine power generation and overall system efficiency — these were used to evaluate performance over key duty cycles. The mechanically-decoupled electric turbocharger can generate up to 0.38kW average power in the real road driving simulation which can greatly increase the engine fuel economy. In NEDC and WLTP cycles, the e-turbo system can always generate energy and store it into battery (0.21kW and 0.23kW average power in the whole cycle respectively). In several real road driving tests, the energy consumption of the compressor exceeds that of the turbine due to significant running in the low speed/high torque region (A 0.04kW in specific on-road cycle simulation). The sensitivity of these results to vehicle gear ratios and electrical system efficiency is also presented.