**5. Conclusions**

The present paper provides a detailed theoretical analysis on the potential of e-turbocharging to control load while providing energy recovery for increasing the overall system efficiency and if possible replacing wastegate boost control. The initial results show that at medium to high loads, significant amounts of enthalpy, up to 5 kW, are lost through the WG valve of a 2.0 L turbocharged spark-ignition engine at medium to high loads. A significant amount of this enthalpy loss can be recovered by the implementation of a motor-generator directly linked to the shaft of the turbocharger.

The study reveals that replacing the wastegate is only achievable when a 30% larger turbine is used for non-violating the pre-turbine pressure limit of the engine. However, a large turbine leads to very poor performance and energy provision needs at the low loads and speeds. A 10% smaller turbine when combined with increased pre-turbine pressures was found to provide efficiency gains and energy harvesting conditions across the whole area of the engine's load/speed map. However, this energy harvesting and efficiency gain come with a 5% penalty on the maximum power output of the engine. Increased power outputs are still achievable by reducing the pre-turbine pressure (opening the WG) and providing extra energy to the compressor.

The transient response time of the electrically-assisted engine showed an improvement between 70% and 90% depended on the engine speed and the power provided to the compressor (~1 to 5 kW). Testing the system under various driving cycles showed that the average amount of energy generated exceeds by up to 1 kW on average (6.6 kWh) the energy required by the motor during the transient events. The e-turbocharged engine demonstrated a fuel deterioration of up to 1.8% during the transient due to the increased pre-turbine pressures. However, this was compensated by the additional generated power. The maximum net energy gain of 5.5 kWh when considering the fuel consumption increase was for the combined driving cycle followed by the US06 cycle with a net gain of 3.8 kWh.

Finally, it needs to be mentioned again that none of the results presented in this study take into account any electrical losses, such as alternator, converter and battery losses, which will obviously reduce the net amount of harvested energy. Future work will include investigations on gasoline and multi-turbo systems with a comprehensive turbine/compressor matching and the electrification of VGT turbocharging systems. Finally, the simulation studies can be supported by experimental investigations with actual hardware testing and e-turbocharger emulation studies (using an externally-boosted engine transient facility located at the University of Bath) for a comprehensive in-cylinder combustion analysis.

**Acknowledgments:** The authors would like to acknowledge Ford Motor Company and Jaguar Land Rover for their funding support for this research work.

**Author Contributions:** Pavlos Dimitriou, Richard Burke and Harald Stoffels conceived of and designed the experiments. Pavlos Dimitriou performed the experiments, analysed the data and wrote the paper. Qingning Zhang and Colin Copeland contributed with their experience on electrically-assisted turbochargers and simulation tools.

**Conflicts of Interest:** The authors declare no conflict of interest. The founding sponsors have agreed to publish the findings of this work.


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