Tsinghua team evaluates impact of types and arrangements of electric traction motors in fuel cell hybrid buses
Researchers at Tsinghua University have compared the performance of two different powertrains for fuel cell hybrid buses. Both buses use 50 kW PEM fuel cell stacks (from different manufacturers) as the primary power source, with LiMn2O4 battery packs as secondary power sources. A significant difference between the two powertrains lies in the types and arrangements of the electrical motor.
One powertrain employs a single induction motor (IM) to drive the vehicle via a reduction gearbox and differential (Powertrain A), while the other powertrain adopts two permanent magnetic synchronous motors (PMSMs) for near-wheel propulsion (Powertrain B). A further difference between the proposed powertrains is the supply path for the fuel cell accessories. A paper on their study is published in Journal of Power Sources.
Powertrain A—with the centralized IM—is low cost and easier to install in a conventional vehicle. With its two PMSMs, Powertrain B needs a new type of driving axle. However, the researchers noted, Powertrain B has advantages such as saving space and high vehicle handling and stability due to the dynamic optimal tractive force distribution control.
|Gao et al. Click to enlarge.|
For Powertrain A, the fuel cell accessories are connected to the input terminals of the DC/DC converter. If the fuel cell is not working, the accessories can be supplied by battery. Switching is automated due to the voltage being higher in the fuel cell than the battery. However, for Powertrain B, the FC accessories are connected to the output terminals of the DC/DC converter. The FC accessories in Powertrain B are always supplied by the battery.
The motors in both powertrains use field oriented control (FOC), which provides independent control of flux and torque by decoupling the direct axis current and the quadrature axis current. Because Powertrain B has two independent PMSMs to drive the bus, coordinated control and electronic differential technologies can be used to improve the dynamic performance of the vehicle. The two PMSMs may run at different speeds or torques depending on the road conditions and the purpose of the driver.
For safety and reliability, the two PMSMs coordinate and are controlled in real time using only one six-phase inverter. The IM in Powertrain A is controlled without considering coordinated control, and the torque is distributed to the two wheels by the mechanical differential. For the comparative study, both powertrains used an identical energy management strategy.
The researchers integrated the two powertrains into buses for a series of road on the road around Anting town near Shanghai, based on the China Typical Bus Driving Cycle (CTBDC)—a strictly urban bus cycle with a top speed of 60 km/h (37 mph).
With an identical energy management strategy, the power profiles of the components in Powertrain A and Powertrain B are similar. However, there were notable differences in the fuel consumption and energy flows between the two powertrains because of the configuration differences.
|Sankey diagrams ((a) for Powertrain A, (b) for Powertrain B). Gao et al. Click to enlarge.|
Among their findings:
The PMSM can achieve higher efficiency and has a wider high-efficiency area compared with the IM. Powertrain B (PMSMs) saved 1161.2 kJ of motoring energy and 51.2 kJ of generating energy compared to Powertrain A. Higher efficiency enables decreasing the cost and size of the cooling system for the electric motor. With no differential and final gearbox, the drive line of Powertrain B is more efficient than that of Powertrain A.
Although the driving system operates with more efficiency in Powertrain B, the conversion efficiency from hydrogen energy to traction energy of Powertrain B is lower than Powertrain A because of the lower average efficiency of the fuel cell system. The fuel consumption of Bus A was 13.29 km/kg, based on the CTBDC testing, but 14.21 km/kg for Bus B.
The smaller size, lower weight and torque control provided independently by the PMSMs for each wheel can lead to good, dynamic vehicle performance.
If all the vehicle accessories, including the air conditioner, are directly supplied by the fuel cell stack, the powertrain efficiency can be greatly improved.
Compared with the powertrain equipped with IM, the powertrain adopting PMSMs as near-wheel driving motors can achieve higher efficiency, flexible torque control for the wheels and space saving for a fuel cell hybrid bus.
The connected position of the fuel cell accessories has a great impact on the efficiency of the fuel cell system and the entire powertrain. The wider input voltage range of the accessories and switching circuit are necessary if they are directly connected to the output of fuel cell stack.—Gao et al.
Dawei Gao, Zhenhua Jin, Junzhi Zhang, Jianqiu Li, Minggao Ouyang (2016) “Comparative study of two different powertrains for a fuel cell hybrid bus,” Journal of Power Sources, Volume 319, Pages 9-18 doi: 10.1016/j.jpowsour.2016.04.046