A Numerical Study of the Benefits of Electrically Assisted Boosting Systems

An electric compressor and an electrically assisted turbocharger have been applied to a 2.0 L gasoline and a 2.2 L diesel engine 1D wave dynamic model. A novel approach is presented for evaluating transient response using swept frequency sine wave functions and Fourier transforms. The maximum electrical power was limited to 6% of the maximum engine power (12 kW and 5 kW, respectively). The systems were evaluated under steady-state and transient conditions. Steady-state simulations showed improved brake mean effective pressure (BMEP) at low-engine speeds (below 2500 rpm) but electric power demand was lower (3 kW versus 8 kW) when the electric compressor was on the high-pressure side of the turbocharger. This was due to the surge limitation of the turbocharger compressor. The electrically assisted turbocharger offered little opportunity to increase low-speed BMEP as it was constrained by compressor map width. Rematching the turbo could address this but also compromise high-engine speeds. BMEP frequency analysis was conducted in the region of 0.01–2 Hz. This was repeated at fixed engine speeds between 1000 rpm and 2000 rpm. Spectral analysis of the simulated response showed that the nonassisted turbocharger could not follow the target for frequencies above 0.1 Hz, whereas the electrically assisted device showed no appreciable drop in performance. When assessing the electric power consumption with the excitation frequency, a linear trend was observed at engine speeds below 1500 rpm but more complex behavior was apparent above this speed where BMEP levels are high but exhaust energy was scarce.