Numerical evaluation of an electric turbo compound for SI engines

To downsize a spark ignited (SI) internal combustion engine (ICE), keeping suitable power levels, the application of turbocharging is mandatory. The possibility to couple an electric drive to the turbocharger (electric turbo compound, ETC) can be considered, as demonstrated by a number of studies and the current application in the F1 Championship, since it allows to extend the boost region to the lowest ICE rotational speeds and to reduce the turbo lag. As well, some recovery of the exhaust gas residual energy to produce electrical energy is possible. The present paper shows the first numerical results of a research program under way in collaboration between the Universities of Pisa and Genoa. The study is focused on the evaluation of the benefits resulting from the application of ETC to a twin-cylinder small SI engine (900 cm³). Starting from the experimental steady flow performance of turbine and compressor, the complete model of a turbocharged engine has been created using the one-dimension code AVL BOOST. The first numerical results show that ETC is not beneficial over a conventional turbocharger with appropriate turbine geometry if the target is to optimize overall efficiency in one specific operating point of the ICE, like in the case of extended-range electric vehicles. Besides, ETC can slightly improve average overall efficiency when the ICE must provide variable power output, as in the case of conventional or hybrid vehicles. However, the major benefits coming from ETC are the boost range extension in the lowest engine rotational speed region and a wide reduction of turbo lag, which are key points in parallel-hybrid and especially in conventional vehicles.