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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.

FCB-1
The two different hybrid powertrains. Both use a PEMFC system and a LiMn2O4 lithium-ion battery system. The fuel cell is the primary power source, the battery system is the secondary power sources, used to assist the driving of the vehicle during transients and to absorb the kinetic braking energy. The fuel cell stack is connected in parallel to the battery system via a DC/DC converter.

The supply path for the fuel cell accessories also differs. Gao et al. Click to enlarge.

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.

FCB-2
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.

Fcb-3
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.

Resources

  • 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

Comments

Arnold

If the accessories for example B were fed from the stack as is the case with system A, then there would be efficiency advantages over the I.M., If I red correctly.

Pleasing to see that two smaller (inwheel) P.M. motors as state of the art technology are capable of delivering performance efficiency and safety advantages.
Makes sense really.

I don't think these benefits would translate to light passenger vehicles in quite the same way.

Buses with large diameter wheels allow high e motor torque and slow rotational speeds.
They currently have very heavy robust axle requirements that would not be disadvantaged by well engineered arrangement of sprung outboard wheel motor drive to the same extent as LPV's.

My guess is that given the full treatment to upgrade the suspension axle etc of the heavy bus axle, the sprung weight could well come in below current designs.

The same seems unlikely (but not impossible)to me any time soon for the LPV.

Arnold

The technical parameters comparison show this is a comparison of just two ' best guess example' there would be many motor configuration and combinations possible in a more comprehensive study but there are clear inferences on the direction to be taken.

Alex_C

Instead of the two systems above, I'd use system from Wrightspeed with two induction motors per axle, multispeed (2 or 4 speeds, clutchless shifting) to avoid low rpm range of IM where it is very ineficient (and to maximize torque, say for hill climbing). The configuration is not patented by Wrightspeed, only method of dynamic control - there are tens of methods around, everybody uses similar way to control traction.
No differential either. Uses cheaper IM motors.

HarveyD

FC + batteries/super caps and various power train combo could replace current diesel ICE in most buses and trucks with positive effects on GHG and pollution.

Scaled down, it could do the same for cars and LDVs?

Brian Petersen

Whether you use one motor driving through a differential or two separate motors each with a separate reduction gearbox shouldn't have any meaningful effect on the efficiency of the powertrain as a whole. Using two separate motors with two sets of inverter outputs will cost more but allows a limited-slip effect which is better for slippery surfaces, that's about all. If you want to connect the motors via multi-speed mechanical transmissions as suggested by someone above (and there are some advantages to doing so) then using two separate such drivetrains starts costing a lot more because now you need two of those, too!

In-wheel motors aren't favorable in light-duty vehicles because there is a lot of equipment competing for space at the hubs (particularly with the front wheels) and the unsprung weight has adverse ride and handling effects on rough roads.

The Tesla P90D uses one chassis-mounted motor in the front driving through a diff and CV shafts, and two motors in the back driving through CV shafts, and there's good reason for it being done that way. The Chevy Bolt uses one chassis-mounted motor driving the front through a diff and CV shafts, and my money is on the Tesla 3 doing the same (but in the rear) in order to save $ $ $.

Alex_C

@Brian P.,
"The Tesla P90D uses ...and two motors in the back"

It's first time I read that P90D uses two motors in the rear.
Could you please provide some link where it is said.

I searched, everywhere it's said that there is only one drive motor in the rear of P90D (with a differential, obviously).
Haven't seen yet a Tesla drivetrain with 2 motors in the rear, without dufferential.

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