4.2.3. Energy Balance Study

The energy balance study was performed for the NEDC, WLTC, US06 cycles, as well as real driving conditions cycles shown in the Methodology Section.

Figure 27 reveals that all of the driving cycles tested provide a negative average power, which indicates energy harvesting. Energy is provided to the compressor during transient load and speed conditions. The motor-generator is almost inactive when running at low to medium load steady-state conditions. Finally, at extra-urban driving conditions, the generators can harvest a high amount of energy. The average amount of energy harvested is highly dependent on the driving behaviour. The combined driving conditions and the US06 cycle, which represents aggressive driving conditions, demonstrated the maximum amounts of energy gain (6.6 and 5.9 kWh, respectively). The NEDC and WLTC cycles that represent mild driving behaviour provided energy gains between 0.4 and 0.9 kW.

**Figure 27.** Average power provided/harvested by the motor-generator; negative values indicate energy harvesting.

The fuel consumption of the model in transient mode cannot be highly trusted, as it may be affected by the tuning of the different PID controllers used for the baseline and the electrically-assisted models, as shown in Figure 28.

**Figure 28.** Fuel flow mass flow rate comparison between e-turbo and baseline model for the WLTC cycle; highlighted area indicates the disturbance of PID tuning on the accuracy of the results.

The obtained results for the transient cycles show that with the implementation of the e-turbocharger, the fuel economy of the engine is deteriorated by up to 1.8% (Table 7) due to the increased pre-turbine pressures. However, this fuel consumption increase is compensated by the generated power of the e-turbocharger. The combined driving cycle demonstrates the maximum net gain of 5.5 kWh, followed by the US06 cycle with 3.8 kWh. The WLTC is the only cycle that provides a marginally negative energy gain when considering the increased fuel consumption.

**Table 7.** Fuel consumption difference between baseline and e-turbo models for various cycles; energy gain = average power values − heat loss (*Q*) due to increased fuel consumption, *Q* (kJ) = Calorific value of fuel (CV) (kJ/kg) × fuel (kg), CV of gasoline = 47,300 kJ/kg; \* considering the increased fuel consumption.

