## 1 Introduction
He found that under steady-state operating conditions, there was significant system efficiency to installing the electric compressor downstream of the turbocharger's compressor. The author also concluded that an electrically-assisted turbocharger is not ideal as a replacement for the second compressor in a two-stage system, as it will push the compressor into surge. However, it was mentioned that this could be overcome through re-matching of the compressor. [PERSON] et al. [8; 9] investigated the technical problems in selecting an appropriate machine to use with an electrically-assisted turbocharger. The authors also demonstrated that the required time for accelerating an electrically-assisted turbocharger from 40 to 110 kppm could be around 50% less than a conventional turbocharger. However, albeit an electrically-assisted turbocharged engine requires less energy for its operation than a solemnly supercharged engine, the power required is still a parasitic loss for the engine, which results in an increased fuel consumption. [PERSON] et al. [10] proposed an electrical supercharging and a turbo-generation system integrated in a diesel engine for overcoming the issue of parasitic losses. The proposed system showed distinct transient response improvement benefits over a conventional turbocharged system and 7% improvement in fuel consumption over the Federal Urban Driving Schedule (FUDS) cycle. Furthermore, during transients and high load operation, the proposed system did not build up exhaust backup pressure in order to accelerate the supercharger, and as such, no additional pumping losses were incurred. However, the authors commented that the electrical energy required by the supercharger can be only partially obtained by the turbo-generation system, which still leads to additional energy losses. [PERSON] et al. [11] was one of the first research groups to implement the idea of a motor-generator electrical turbocharger for a 5.2 L truck diesel engine in a theoretical study. Adding a directly-coupled motor-generator offers tremendous advantages to the operation of the turbocharger. It abolishes the requirement of the turbine and compressor power to be matched under steady-state conditions while it assists the acceleration and deceleration of the shaft during transients with no engine energy needs. Over the last few years, keen interest has been shown in the numerical and experimental investigation of the motor-generator technology application in high-duty engines by several research groups. [PERSON] and [PERSON] [12] experimentally characterized a variable geometry turbocharger with a motor-generator technology. The authors found that the motor-generator is capable of delivering a maximum shaft power of 3.5 kW in motoring mode and 5.4 kW in generating mode. The peak electrical efficiency was more than 90% in both modes and occurred at 120,000 revs/min. [PERSON] et al. [13] developed a comprehensive powertrain model to evaluate the benefits of an electric turbo-compound, working in both generator and motor mode, in reducing CO\({}_{2}\) emissions from small diesel passenger cars. The simulations showed a reduction in CO\({}_{2}\) and fuel consumption of over 4% for the New European Driving Cycle (NEDC). [PERSON] [14] developed an advanced control system for a motor-generator fitted in a heavy-duty diesel engine for improving the overall fuel efficiency. The simulation results showed that at the rated power, the fuel consumption of a Class-8 on-highway truck engine could be reduced by almost 10%, while the overall reduction in fuel consumption was estimated to be around 5%. [PERSON] et al. [15] focused on the evaluation of the benefits resulting from the application of an Electric Turbo Compound (ETC) to a small-sized twin-cylinder Spark-Ignition (SI) engine and to a four-cylinder Compression-Ignition (CI) engine with the same power rating.