Drayage, is where old trucks go to die”

Anonymous Fleet Owner


With the road to class-8 long-haul clean freight currently being paved, what are the different powertrain architecture approaches and how do they compare? Hydrogen fuel cells claim no reduction in cargo weight, fast-fill times, and uncompromising range when compared with battery-electric trucks. In the article, Truck Hybridization—which is better, battery or fuel cell?, we concluded that a hybrid truck with both a battery and a fuel cell is a viable disruptor. In this article, we look deeper into identifying the best sizing of battery and fuel cell to achieve the goal of providing clean freight.


Not that long ago, fuel cell vehicles had one powertrain design with the fuel cell as the prime mover and an underutilized small battery. But between 2005-2010, there were prototypes and demonstrators using fuel cell APUs and in 2017, “Project Hesla”, conversion of a Tesla Model S to add a fuel cell battery charger by Holthausen Group, was picked up by the popular press. Today there are hundreds of fuel cell REX or series-REEV hybrids in operation. It is this powertrain architecture we have chosen for this article.

A series hybrid uses a range extender as a secondary power generator that is supplied by a different energy source than the primary propulsion method. Contemporary examples of these vehicles are electric. An excellent example of a range extender is the Chevrolet Volt (Holden Volt, Buick Velite 5, and Vauxhall/ Opel Ampera). Battery energy is the primary system, and the gasoline engine is the range extender that charges the battery. Yet, the range extender could be any power generator, such as an ICE engine. A range extender differentiator is that it does not follow the “load” of the motor. They deliver constant power, which may be relative to the energy needed by the powertrain. All this means simpler control software.

Below is the diagram of the chosen powertrain architecture used in this article. The battery provides the power for the traction motors, and a fuel cell engine provides power to the DC bus, either charging the battery or driving the motors. The battery and the motors both allow electricity to flow bidirectionally. You will notice that the only energy input to this powertrain is from hydrogen, which makes the term “range extender” seem misplaced. This relatively new configuration has yet to be uniquely named since it is derived from a similar design that included a battery charger.

Fuel Cell Powertrain Architecture (range extender)
Fuel Cell Powertrain Architecture (range extender)





80,000 lbs.

36,287 kg

Cargo Weight

40,000 lbs.

18,144 kg

Max. Speed

65 MPH

105 km/h

Gradeability, 12%, 2 min

35 MPH

56 km/h

Gradeability, 6%, Cont.

50 MPH

80 km/h


600 Miles

966 km


6×4 BEV/FC REX Hybrid





For this analysis, we have used the above requirements and made the following assumptions:

  • All configurations are 80,000-lbs. regardless of the size of the powertrain
  • Rolling resistance and coefficient of drag are from a conventional heavy-duty truck
  • All configurations use the same sub-system efficiencies and parasitic power demand
  • Fuel cell parasitic power only considers the air compressor

The drive cycle selected was from a Port of Los Angles terminal to a frequented distribution centre in Riverside, California. With its climb of approximately 1,000-feet, this run represents one of the most demanding runs for short-haul freight transport, and these results can be extrapolated to long-haul freight transport.

Hybridization Strategy

We have created four trucks configurations based on fuel cell engine sizing and battery sizing to make up for any energy deficit, and name them as follows:

  1. “Unlimited” – a range extender provides all power
  2. “Climber” – a range extender sized for continuous gradeability
  3. “Cruiser” – a range extender sized for high-speed cruise
  4. “Limper” – a range extender which only slows-down energy loss

Peak CapableFC

Hill Climb Capable FC

Cruise Capable FC










The table below presents shows each configuration.

REX Size

Battery Size




860 kW

100 kWh

600 miles



710 kW

100 kWh

600 miles



230 kW

500 kWh

600 miles



60 kW

2,100 kWh

600 miles

“Unlimited” and “Climber” – 710-kW+ fuel cell / 100-kWh battery

Both these trucks are similar sizing to a diesel ICE, but the hill climb speed we chose resulted in a slightly larger engine size. Moreover, the sizing of both trucks is not cooperative in that the fuel cell is doing the peak power work rather than the battery. we have not optimally sized the fuel cell for these two trucks because these powertrain architecture concept chosen uses the fuel cell as a range extender, not as the primary power source. A fuel cell operates at higher power with lower efficiency to achieve greater power output for true peak operation. This lower efficiency means the cooling system cannot keep up raising the temperature of the fuel cell close to the material limits. Afterwards, the fuel cell cooling system needs some time to rid the excess heat. This is how a primary power fuel cell powertrain operates, such as the Toyota Mirai. The fuel cell size could be reduced by 10-20% if we considered peak mode operation. Peak operation is limited to a few minutes in automotive applications, which is practical for the “climber” but not for the “unlimited” as the duration of extended hill climb of “unlimited” is, of course, limitless.

Best application: The drawing board

“Limper” 60-kW fuel cell / 2,100-kWh battery

The “limper” has the largest battery pack, and 2,100-kWh would weight approximately 30,000-lbs. (13,800-kg) with current technology. We assumed that the GCVWR was 80,000-lbs. with this battery, the cargo weight will have to be significantly reduced, which means it has a dismal cargo weight of less than 10,000-lbs. However, the fuel cell size of 60-kW was arbitrary, and the battery size varies with the fuel cell size and a resulting cargo weight (graphed below). With the lower range requirements of local deliveries, these trucks are currently in demonstrations by a few consortia.

Best application: Short-haul trucking such as drayage or last-mile freight delivery

"Limper" Truck Trade-Off
“Limper” Truck Trade-Off

In a future article, we will compare the fuel cell trucks around today and see which is a “cruiser” and which is a “limper”.

“Cruiser” — 230-kW fuel cell / 500-kWh battery

The “cruiser” meets all the performance requirements, and the battery does the high-power hill climb work while the fuel cell keeps the battery charged while cruising at highway speeds. The truck, however, is about 3,000 lbs. over the weight limit.

A 230-kW fuel cell represents the smallest fuel cell that keeps the truck in a positive energy state, will cruising at full speed (65-MPH), while a larger fuel cell allows a smaller battery, which ultimately is a cost vs weight trade-off.

Best application: All class-8 applications

And the Winner Is?

The “cruiser” represents today’s best strategy to displace long-haul diesel trucks. With the accuracy of the weight calculation and optimizations available, the 3,000-lbs. weight loss target is attainable. A 230-kW fuel cell is a practical sized fuel cell engine that can be packaged under hood/cab in the same location as a diesel engine with the 500-kWh battery pack between the frame rails keeping the centre of mass low to the ground.

Special recognition goes to the “limper” strategy for limited short-haul operation, which represents a transitional step towards a practical solution to replacing dirty diesel trucks. Since a “limper” has smaller fuel cells, its cost is lower, but since this truck won’t work in long-haul operation, the trucks will never be built in volume and cost will remain an impediment. And most trucks used in drayage are retired long-haul trucks. And as many insiders have said, “Drayage, is where old trucks go to die”.

Challenges, Opportunities and Optimizations

Truck weight is critical to manage since a heavy storage battery is needed to optimize this powertrain. The cargo weight of 40,000-lbs. plus a 12,500-lbs. trailer is constant, so the tractor needs to be less than 27,500-lbs.

Aerodynamics and maximum speed are the two main parameters to reduce energy needs; if optimized, they would reduce the overall energy consumption and decrease the size of both the fuel cell and the battery, also lowering the overall weight.

Energy conversion efficiency, specifically the fuel cell DC/DC converter, is a crucial factor in fuel cell size. For this sizing exercise, we used 85% efficiency, which means the effective fuel cell is 15% less than rated. A boost converter is less efficient than a buck converter. Still, there is boost converter technology available that will close the gap. For example, Combined Energies LLC. has pioneered boost power converter technology that 95% efficient at >10x boost ratios. The boost converter technology delivers an fuel cell engine cost savings of 10% and additional cooling system savings.

A high overall powertrain efficiency is also important to keep energy needs at a minimum. With the e-axle technology, energy conversion losses in transmissions and final drives are avoided.

Battery weight remains the main on vehicle challenge to commercial BEV adoption. Without new battery technology with better specific energies, fuel cell range extenders remain the feasibility enabler for electric trucks.

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