4.1.4. Overall Engine Efficiency

The overall efficiency of the baseline engine, which is calculated using the brake power achieved over the total fuel power, can reach levels higher than 35% at medium to high load conditions. On the other hand, the overall efficiency of the electrically-assisted turbocharged engine is given by the brake power achieved plus the energy harvested or provided by the e-turbo divided by the total fuel power. Figure 20 shows the comparison charts between the overall efficiency of the baseline engine and four cases studied in Phase 2 and Phase 3 of the steady-state simulations.

It is evident in Figure 20 that the implementation of a motor-generator linked to the turbocharger leads to an overall engine efficiency increase at medium and high loads and engine speeds for all of the cases. The cases with a reduced turbine size (multiplier of 0.9) benefit from an increased overall efficiency across the whole map. Finally, the high pre-turbine pressures show a significant effect on the overall efficiency gain of the model.

By assessing the work presented in the three phases of the steady-state simulations, the ideal engine layout can be summarized as shown in Figure 21.

**Figure 20.** Absolute difference in overall efficiency between four different e-turbo configurations and the baseline engine; negative values indicate efficiency loss; positive values indicate efficiency gain; the area in red circle indicates non-convergence of the model: (**a**) model with 30% larger turbine and WG completely shut; (**b**) model with 20% larger turbine and WG half opened compared to the baseline engine; (**c**) model with 10% smaller turbine and 5% increased pre-turbine pressure compared to the baseline engine; (**d**) model with 10% smaller turbine and 15% increased pre-turbine pressure compared to the baseline engine.

**Figure 21.** Ideal component sizing for maximum power and energy harvesting.
