48-V Mild Hybrid System for Commercial Vehicles
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MOTIVATION
The reduction of greenhouse gas emissions is a major challenge for the automotive industry. Hybrid drivetrains contribute to reducing fuel consumption. In [1] it is described that 48-V mild hybrid systems in different topologies offer a very good cost-benefit ratio compared to high-voltage full hybrid systems. Also, for MAN this showed to be an effective measure in city bus application to fur-
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ther improve fuel consumption of conventional powertrains based on diesel and gas engines. Due to the driving cycles with frequent braking, standstill and acceleration situations, city buses offer great potential for the recovery of braking energy, switching off in standstill situations and assisting during acceleration processes. For this purpose, a P1 mild hybrid (P1: parallel hybrid drive in which the electric motor is permanently connected to the crankshaft of
Markus Lampalzer, M. Sc. is Team Leader Design Engine Mechatronics in the Division Engineering Powertrain at MAN Truck & Bus SE in Nuremberg (Germany).
Dipl.-Ing. (FH) Michael Lechner is Team Leader Testing Inverter & E-Machine in the Division Engineering Electric/Electronic Systems at MAN Truck & Bus SE in Munich (Germany).
48-V Mild Hybrid System for Commercial Vehicles 48-V mild hybrid systems offer a significant fuel saving potential in city bus application with a good cost-benefit ratio. MAN has therefore developed a crankshaft starter-alternator for the Lion’s City bus. This electric machine converts mechanical energy into electrical energy during braking and supports the combustion engine when starting off.
the combustion engine), which is quickly amortized for the customer in real vehicle use, was developed. DESIGN AND FUNCTION OF THE 48-V MILD HYBRID SYSTEM
In addition to the conventional design of the powertrain, additional components are required for the selected system, FIGURE 1. The electric machine is installed between the combustion engine and the transmission and is designed as a Switched Reluctance Machine (SRM). The motor phases are controlled with the power electronics (inverter). The inverter is connected to a DC link with a direct voltage of 30 to 56 V. The DC link consists of an energy storage and a direct voltage converter (DC/DC converter). The energy storage provides the starting energy and the power supply of the 24-V electrical system via the DC/DC converter during the stop phases of the combustion engine. The MAN EfficientHybrid system enables the use of several hybrid functions in the powertrain. They are shown in the drive cycle in FIGURE 2. In overrun phases, the Crankshaft Starter-alternator (CSA) recuperates brake energy in generator mode. This energy is stored in the ultracapacitor. After reaching the vehicle’s standstill, the combustion engine is actively stopped to prevent vibrations and recover energy. During the engine ATZ heavyduty worldwide 03|2020
stop situation, the vehicle’s electrical system is powered by the ultracapacitor and the DC/DC converter. Immediately before the vehicle needs
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