**1. Introduction**

The demand for low fuel consumption and CO2 generation vehicles over the last few years has popularly increased the necessity of downsizing and increasing the overall thermal efficiency of Internal Combustion (IC) engines. Downsizing is the process of reducing the volumetric capacity of an engine for reduced throttling and friction losses while its boosting capabilities need to be increased for higher specific heat. This can be achieved by the implementation of a boosting device (turbocharger or supercharger) for increased air pressure at the intake of the engine and therefore higher volumetric efficiency.

A turbocharger is a device that recovers the waste energy from the engine's exhaust gasses and uses it to compress the air at the engine's intake. The level of compression is directly linked to the amount of air passing through the turbine, and it can be controlled by either bypassing part of the flow through a Wastegate (WG) or by changing the nozzle position of the turbine (VGT, Variable-geometry turbocharger). The proportion of the waste-gated flow can be up to 50% for high speed and load conditions, which imply a vast amount of unexploited energy. The drawback of this device is that due to the nature of the energy exchange between the engine and the turbocharger (filling of the intake and exhaust manifolds and low exhaust energy at low speeds/loads), the transient performance during engine load increase is relatively poor, which deteriorates the drivability of the vehicle [1]. On the other hand, a supercharger is a device mechanically driven by the engine to increase its volumetric efficiency. It has a very fast response time in transient conditions, but the power required for its operation is a parasitic loss for the engine; therefore, it is not widely used in recent technologies.

The parallel use of a turbocharger and a supercharger could potentially improve the transient performance of the system during load increase while reducing the engine losses compared to a solely supercharger boosted system. However, this will increase the complexity, as a two-stage boosting system is required [2,3]. An electrically-assisted turbocharger of a larger size and with higher efficiency could be used in case a simpler one-stage boosting system is needed. The motor can provide the electricity required at the periods of load increase for a faster response, while the turbocharger works as a conventional system during steady-state and tip-out conditions.

Katrasnik et al. [4] investigated the influence of an electric motor attached to the turbo shaft on the transient response of a diesel engine. It was found that the time required to perform transient power increase was reduced from 3.9 s for the original conventional turbocharger down to 1.7 s. Ibaraki et al. [5] tested a hybrid turbo developed by Mitsubishi Heavy Industries under transient operating conditions. An improved engine peak torque and enhanced transient response compared to a conventional system was demonstrated. Torque was enhanced by 18% at low engine speeds, while the response time was reduced by 70% when 2 kW of turbo motor assistance was provided. Millo et al. [6] investigated the potential of an electrically-assisted turbocharger for a heavy-duty diesel engine to evaluate the turbo-lag reductions and the fuel consumption savings that could be obtained in an urban bus at different operating conditions. The system allowed fuel consumption reductions of 6% to 1%, depending on the driving cycle, with lower values corresponding to congested traffic conditions.

Burke [7] applied various electric boosting systems to a gasoline and a diesel engine and evaluated their steady state and transient performance from the perspective of the air path. The author compared the performance characteristics of an electrically-assisted turbocharger with those of a two-stage system with an electrically-driven compressor placed before or after the main waste-gated turbocharger. 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. Bumby 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 krpm 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. Divekar 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.

Panting 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. Terdich and Martinez-Botas [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. Airse 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 CO2 emissions from small diesel passenger cars. The simulations showed a reduction in CO2 and fuel consumption of over 4% for the New European Driving Cycle (NEDC). Algrain [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%. Pasini 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. They found that by absorbing electrical energy from the battery, the ETC can lead to significant Brake-Specific Fuel Consumption (BSFC) reductions of up to 4% at the highest engine speeds and loads for the SI engine and up to 6% for the CI engine at 4000 rpm half load. Furthermore, calculations have shown that in the case of the CI engine, the maximum electric power that can be recovered by the ETC is around 4 kW at 4000 rpm full load, while in the case of the SI engine, the maximum power is 1.5 kW.

Tavcar et al. [16] presented a comprehensive study on engine performance improvement attributable to the application of different electrically-assisted turbocharger topologies, including a single-stage turbocharger, an electrically-assisted turbocharger, a turbocharger with an additional electrically-driven compressor and an electrically-split turbocharger (supercharger and turbo-generator). The results revealed that all of the electrically-assisted turbocharger topologies improve the transient response of the engine and, thus, the drivability of the vehicle. However, no electrically-assisted turbocharger topology could clearly be favoured in general.

In this paper, a detailed investigation of the potential of e-turbocharging to control load while providing energy recovery for increasing the overall system efficiency and if possible replace the wastegate boost control is provided. The current approach of e-turbocharging requires larger turbine systems that do not build up backup pressure and provide electrical assistance at low engine speeds. However, with this configuration, the energy regeneration occurs only at high speed and load areas of the engine, as shown in Figure 1a. The proposed study focusses on shifting the energy regeneration towards the low-speed area, which represents more realistic driving conditions, as shown in Figure 1b. Energy assistance is provided to the engine when the maximum power characteristics of the engine need to be met. The study is performed on a 2.0 L turbocharged SI engine under steady-state and transient driving conditions. The research work focuses on the availability of energy and the effects of component sizing, the transient behaviour for eliminating turbo lag and the overall system energy balance during various driving conditions.

After a comprehensive introduction in this section, Section 2 outlines the engine model used for the study and any relatively small modifications occurring for the purpose of electrifying the turbocharger. A detailed model validation against experimental data is also presented in Model Validation. Section 3 describes the methodology followed for all of the simulation studies conducted under steady-state and transient conditions. The simulation results for all of the studies performed are analysed and discussed in Section 4. Finally, Section 5 summarizes the main findings of this work and proposes future work in the area of the electrification of turbocharging systems.

**Figure 1.** Energy regeneration areas over the load/speed map of an engine: (**a**) current approach; (**b**) desired operation.
