Brake Power Availability Led Optimisation of P0 versus P2 48V Hybrid Powertrain Architectures

Through improving the 48V hybrid vehicle archetype, governmental emission targets could be more easily met without incurring the high costs associated with increasing levels of electrification. The braking energy recovery function of hybrid vehicles is recognised as an effective solution to reduce emissions and fuel consumption in the short to medium term. The aim of this study was to evaluate methods to maximise the braking energy recovery capability of the 48V hybrid electric vehicle over pre-selected drive cycles using appropriately sized electrified components. The strategy adopted was based upon optimising the battery chemistry type via specific power capability, so that overall brake power is equal to the maximum battery charging power in a typical medium-sized passenger car under typical driving. This will maximise the regenerative braking energy whilst providing a larger torque assistance for a lower battery capacity. Dynamic simulation models were developed using GT-DRIVE software, emulating a mid-sized car with a 48V battery, and different turbocharged gasoline engines with motor-generator unit positions along a drivetrain. The 1.3 kWh battery pack was developed using a 14 Ah Lithium Iron Phosphate cell arranged in a 14 series 2 parallel configuration. A fuel economy comparison was produced using the FTP, WLTP, and HEFET drive cycles. When the motor-generator unit was attached via a synchronous belt, a 10-17% fuel saving was achieved in the WLTP drive cycle. Comparatively, when placing the electric machine after the clutch in a “P2” position, a 17-21% fuel saving was attained. The energy loss analysis of both P2 and P0 configurations revealed up to 7% overall reduction in total energy losses for the P2 setup. This was despite an increase in the motor-generator unit and battery losses due to the extended use of both in the electric-only mode capability with the P2 layout.