Cost-Efficient Hybrid Powertrain System with 48 V Network

Abstract

Mission profiles show that a power of 12 kW in transient mode (20 s) is sufficient for hybrid functions. These rated powers are not compliant with a 12 V network. However, they are fully compliant with 48 V networks. Below 60 V, safety protections and insulated power networks are not required. In addition, with 48 V, the cell number of the storage unit is less than with 60 V. These improvements help to achieve a very low cost to CO2 ratio. For fast cost-effective vehicle integration, the first generation of the system is a belt driven architecture with standalone components. The electric motor replaces the conventional generator and is controlled by a standalone inverter supplied by a 48 V storage unit via an additional direct current (DC) network. Fixed transmission of the belt sets the maximum speed of the electric motor to 18,000 rpm. A specific belt tensioner was added to guarantee torque transmission over the full speed range. A 48 V/14 V DC/DC converter replaces the con ventional generator and supplies the standard 12 V power network. It provides 2.5 kW power with an efficiency of up to 96 %. Due to the limitations of the belt and Li-ion battery in cold conditions, a conventional starter and low voltage acid battery are retained. The 48 V electric motor covers the start/stop function without impacting the 12 V network. Hence, the electrical architecture, ➍, does not require a voltage stabilising system. Open image in new window ➍ System architecture of the hybrid concept The specifically developed air-cooled electric motor, with an efficiency of higher than 90 % throughout the operating range, provides 12 kW peak power and 55 Nm starting torque to optimise energy recovery. This definition is compliant with all mission profiles and helps to achieve CO2 targets. For the 48 V inverter driving the electric motor, a dedicated space vector control was developed to ensure accurate torque control. For the first generation, it is a standalone, air-cooled device operating in a voltage range between 35 and 60 V. The cost of the second generation hybrid architecture will be further reduced by removing wiring and EMC (electromagnetic compatibility) filters, combining the inverter with the electric motor and the DC/DC converter with the Li-ion battery. Further potential saving can be gained by removing the conventional starter and low voltage battery while adapting the belt drive for low temperature starting.