48V Mild-Hybrid Architecture Types, Fuels and Power Levels Needed to Achieve 75g CO2/km

Journal article, 2019

48V mild hybrid powertrains are promising technologies for cost-effective compliance with future CO2 emissions standards. Current 48V powertrains with integrated belt starter generators (P0) with downsized engines achieve CO2 emissions of 95 g/km in the NEDC. However, to reach 75 g/km, it may be necessary to combine new 48V powertrain architectures with alternative fuels. Therefore, this paper compares CO2 emissions from different 48V powertrain architectures (P0, P1, P2, P3) with different electric power levels under various driving cycles (NEDC, WLTC, and RTS95). A numerical model of a compact class passenger car with a 48V powertrain was created and experimental fuel consumption maps for engines running on different fuels (gasoline, Diesel, E85, CNG) were used to simulate its CO2 emissions. The simulation results were analysed to determine why specific powertrain combinations were more efficient under certain driving conditions. As expected, the greatest influence on emissions was from powertrain architectures. Increased electric power levels (from 8 kW to 20 kW) allowed more brake energy to be recovered, reducing CO2 emissions by 2 - 16% depending on the driving cycle. The P2 and P3 architectures with even low electric motor power level offered substantially better fuel efficiency (by 19% on average) than a conventional powertrain with a start-stop system, whereas the P0/P1 architectures offered average improvements of only 4% for different power levels and driving cycles. In the P0 and P1 architectures, engine friction severely limited energy recovery during braking and made electric propulsion infeasible due to significantly increased power demands. The P2 and P3 architectures allow the engine to be decoupled from the powertrain and so avoid this problem. Overall, the 48V P2/P3 powertrains allowed for significant improvements in CO2 emissions when used with CNG, E85 or diesel fuel. 75 g/km target value was predicted to be achievable with CNG-fuelled systems under the NEDC and WLTC cycles, and possibly even under RTS95 on a well-to wheel basis when using a renewable fuel such as E85.