48V Mild-Hybrid Architecture Types, Fuels and Power Levels Needed to Achieve 75g CO2/km 2019-01-0366

48V mild hybrid powertrains are regarded as a promising technology to meet future CO2 emission legislations in a cost-effective way. Current 48V powertrains with integrated belt starter generators (P0) are capable of 95g/km CO2 emissions in NEDC. However, in order to reach lower emission levels of 75g/km, different 48V powertrain architectures in combination with alternative fuels may be required. Therefore, this paper compares CO2 emissions from different 48V powertrain architectures (P0, P1, P2, P4) with different electric power levels under various driving cycles (NEDC, WLTC, RTS95). A vehicle model for a compact class passenger car with a 48V powertrain is created and experimental fuel consumption maps from engines running on different fuels (Gasoline, Diesel, E85, CNG) are used to calculate CO2 emissions. Simulation results are analyzed and reasons behind why a certain powertrain combination is more efficient under certain driving conditions are explained. As expected, increased power levels (20kW vs 10kW) allow more brake energy to be recovered, resulting in an additional 2-10% reduction in CO2 emissions depending on the driving cycle, but the main benefits come from powertrain architecture. Results reveal that P2/P4 architectures offer a substantial fuel efficiency improvement with an average of 18% over a conventional powertrain with start-stop, whereas P0/P1 architectures offer only 4% improvement on average, under different power levels and driving cycles. In a P0/P1 architecture, engine friction severely limits the energy recovered during braking and makes electric propulsion infeasible as well due to significantly increased power demands, whereas the engine can be decoupled from the powertrain in a P2/P4 system. Overall, 48V P2/P4 powertrains allow for significant improvements and combined with CNG, E85 and diesel fuels, CO2 emissions can be lowered down to 75g/km for certain power levels, under NEDC, WLTC and even RTS95 with renewable fuels like E85.