While not as in vogue as BEVs and other electrified vehicles, hydrogen propulsion systems have been in development since the mid-20th century by a wide variety of manufacturers for a multitude of applications. Several OEMs had exhibited prototypes of hydrogen-powered light vehicles, but none had reached production status until after the turn of the new millennium. Honda was one of the first OEMs to offer a hydrogen fuel-cell electric vehicle (FCEV) to retail customers in 2008 with the Clarity, which was available as a lease-only vehicle in Japan, Europe and southern California.
Over the next decade, retail availability of hydrogen fuel cell vehicles began to spread with companies like Hyundai and Toyota rolling out their own FCEVs. As of 2020, there were more than 30,000 hydrogen-powered vehicles on the road globally. However, hydrogen power has struggled to gain mainstream acceptance and use across the transportation industry. A market sector where this has been especially evident is in heavy-duty trucks and commercial vehicles.
Data from the 2020 survey shows that of all hydrogen-powered vehicles currently registered globally, just 25% are a commercial or heavy-duty truck. The largest number of those are buses, with more than 5,000 currently in use. That’s far more than light commercial vehicles (LCVs) or any other type of truck, which combined account for just over half that total. Despite this, there has been ongoing research and development to advance the technology and efficiency of hydrogen fuel cells as well as adapting existing internal combustion engine (ICE) technology to utilize hydrogen as a fuel for heavy-duty applications.
Hydrogen ICEs a mid-term solution
A recent SAE webinar headlined by presenters from Cummins, FEV, Stanadyne and Toyota highlighted several impressive technological breakthroughs in hydrogen technology and their potential impact across the industry for reduced fleet emissions while still providing reliable service with similar uptime as modern diesel trucks.
Jim Nebergall, general manager of hydrogen engines for Cummins, outlined the company’s progress on developing hydrogen-fueled ICEs installed in conventional chassis with existing drivelines, as well as hydrogen fuel cells for powering electric traction motors. Nebergall reported that Cummins is currently developing two hydrogen-fuel ICEs, a 6.7-liter and a 15-liter for commercial vehicles. These engines will complement the company’s BEV and FCEV technology and could be in series production by the middle of the decade.
These hydrogen ICEs are intended for difficult-to-electrify applications: trucks with daily usage ranges of 250 miles (400 km) or more, usage in areas where the ambient air quality is low, or high duty-cycle applications. Cummins believes that an ICE is still best suited to these environments. Though FCEVs could eventually provide a long-term solution to decarbonizing the long-haul trucking market, hydrogen-powered engines are an excellent mid-term, zero-emissions solution that utilizes current technology and chassis while also meeting customer needs.
In practice, a hydrogen-powered ICE truck is similar in operation to a CNG-powered unit, the main differences being that H2 engines will require different hardware and software. The second major difference lies in the fuel system. Cummins specifies that the carbon-fiber fuel tanks of an H2 truck will store the fuel at 700 bar (10,000 psi) versus 250 bar (3,625 psi) for CNG.
Cummins notes that this is not an industry standard; systems can range from 350 bar (5,000 psi) to 700 bar. However, the company claims that 700-bar pressure enables greater onboard fuel storage and range. This storage pressure does increase the number of windings required for each fuel tank, which increases weight and cost of the system versus natural gas, but these tanks can be used for either a hydrogen ICE or FCEV.
Production advantages of H2 ICEs include shared componentry with diesel and CNG engines, such as the engine block, crank, ancillaries, and installation parts (flywheels, piping, mounts) as well as utilizing existing manufacturing tooling and processes. An H2-specific motor will require a unique fueling system, electronics and ignition system.
“While there remain a few unknowns in the hydrogen engine space, we’ve witnessed a significant amount of interest within the industry for hydrogen engines over the last 12 months,” said Nebergall. “Fleet operators are very interested in leveraging hydrogen engines to accelerate and decarbonize their commercial vehicle fleets.”
H2 injection methods
Yousef Jeihouni, project manager of FEV North America’s engine and powertrain systems, expanded on the capabilities of hydrogen ICEs and the various types of H2 injection currently in development. While there are a wide variety of ignition methods that can be employed in H2 ICEs, Jeihouni states that the most common method currently in use is spark-ignition. “The majority of conversions we are seeing right now are utilizing spark-ignited engines and essentially using CNG engines to be converted into H2 combustion engines,” Jeihouni said. “This type of engine is applicable to light-duty passenger cars as well as heavy-duty long-haul trucks.”
According to FEV, Otto cycle premixed ignition is the most convenient method for smaller bore diameters. The bore limit for most CNG engines with open chamber ignition is 170 mm (6.7 inches) or less. Due to higher flammability limits and the increased flame speed required for hydrogen combustion, larger bore diameter applications can be equipped with only one spark plug.
Jeihouni elaborated that for medium duty (trucks equipped with 8-liter engines) and heavy duty (trucks equipped with 13-liter engines) the most favorable method of meeting the required engine outputs was the combination of low- to mid-pressure direct injection resulting in lean full load and ultra-lean part load operation. Turbocharging is still utilized for H2 combustion engines with boost pressure managed by a wastegate and supplemented by a variable geometry turbo. The culmination of all these methods is the best possible durability, efficiency and range available with current technology, he said.
There are some challenges with H2 ICEs such as management of NOx emissions, which are still a critical factor in hydrogen combustion. However, they can be reduced to extremely low levels with properly selected and dimensioned exhaust gas aftertreatment systems and dedicated control algorithms. Other challenges of developing hydrogen engines include avoiding preignition at high BMEP levels, as well as management of ignition hotspots.
FEV believes these concerns can be properly addressed. The company states that it could bring a PFI (port fuel injection) H2 system to market as early as 2023 and would be able to launch a DFI (direct fuel injection) system by 2025. FEV claims a five percent increase in efficiency from its PFI system to a DFI system .
World-class combustion efficiency
Bradlee Stroia, chief technology officer for Stanadyne, further championed hydrogen’s potential as an effective solution for carbon-free ICE fueling. “It’s important to understand the benefits of hydrogen as an internal combustion fuel,” Stroia said. “It has challenges, but it can enable not just decarbonization but also world-class combustion efficiency.”
According to Stanadyne, hydrogen has three times the energy density as diesel fuel, which provides the opportunity for very high combustion thermal efficiency. Hydrogen’s flammability range also allows for the reduction of “non-carbon” or NOx emissions from the combustion process. “Hydrogen has a wide flammability range, much wider than most traditional fuels that we use in engines,” Stroia said. “You can capitalize on that by adjusting your combustion parameters to run stratified lean to get NOx very low.”
Most applications will still require some type of NOx aftertreatment, but much less than is needed on a diesel equilivent. Stroia also highlighted that H2 ICEs retain the current manufacturing and supply base, including current fueling infrastructure (distribution, delivery, filling stations) with the potential for faster market adoption and infrastructure conversion than BEVs or FCEVs.
Stroia also covered the pros and cons of various H2 injection systems for ICEs. While PFI systems (7-20 bar/100-300 psi) are relatively easy to implement and low complexity, they suffer from up to 30% air displacement, result in the lowest BTE (brake thermal efficiency) and have a high probability of uncontrolled combustion. “PFI is very easy to implement [for H2] compared to PFI CNG. DFI H2 has advantages because you’re able to manage things after the valves are closed such as uncontrolled combustion and preignition,” Stroia said.
Mid-pressure DI systems (20-40 bar/300-600 psi) offer an intermediate amount of complexity and lower uncontrolled combustion but result in higher cylinder temperatures and may require a compressor for operation. High-pressure DFI systems (150-300 bar/2,000-4,300 psi) offer the greatest combustion control and are capable if diesel levels of efficiency courtesy of stratified and lean combustion. However, they are by far the most complex system to implement. It also generates the most combustion temperature and may also require a compressor to implement.
“From an injection standpoint, it’s hard to inject a liquid fuel at very high thermal efficiency because you have to get a lot of fuel into the cylinder in a short period of time,” Stroia said. “But since hydrogen is a gas, you optimize the thermal efficiency and reach even higher efficiency than diesel.”
Fuel cell kits for existing platforms
Chris Rovik, executive program manager for Toyota, outlined his company’s developments on hydrogen combustion and heavy-duty FCEVs. “Toyota announced Project Portal in 2017. This product uses two hydrogen fuel-cell stacks from our Mirai light vehicle. We refined that into a Class 8 heavy-duty truck powertrain kit. This kit is designed around existing OEM truck platforms,” Rovik explained. “This allowed quicker implementation rather than designing a new truck platform around the technology.”
Toyota’s FCEV prototypes spent significant time in service in the port of Los Angeles and Long Beach, California, in addition to being weather-tested at the company’s facility in Arizona, the upper peninsula of Michigan, and Pikes Peak in Colorado. Toyota states that there have been two generations of fuel cell stacks developed and nearly 20 trucks produced to date.
There are four main components of Toyota’s fuel cell drive kit: the fuel cell stack, a high-voltage battery, the hydrogen storage assembly and an eDrivetrain module. The fuel cell stack assembly consists of two integrated fuel cell modules providing a continuous net power output of 160 kW. This integrated module packages within a majority of heavy-duty OEM engine bays.
The high-voltage battery assembly is packaged below the cab and provides supplementary power when necessary and aid in regenerative braking. The hydrogen storage assembly packages behind the truck cab and contains hydrogen storage tanks, related components and up to 60 kg (130 lbs) of fuel. The eDrivetrain module contains two electric motors and transmission and is located below the hydrogen storage assembly. It can provide up to 450 kW of peak power and 2400 Nm (1770 lb-ft) of peak torque.
“This system can haul 82,000 lbs (37,000 kg) GVWR load over 300 miles (482 km) at the same or better performance as any similar diesel powertrain,” Rovik said. “I don’t want there to be any ambiguity about the range. The 300-mile range is fully loaded at 82,000 lbs and on real roads. It is also important to note that that is a minimum range.”
In 2019, a ten-truck pilot program utilizing the Toyota FCEV kit was launched in partnership with Kenworth Trucks. Kenworth assembled the trucks at its plant using Toyota’s kit, and they are now being operated by actual fleet customers and drivers. To date, these pilot trucks have logged over 50,000 miles (80,000 km). Toyota has stated that these modules will enter series production at the company’s plant in Kentucky starting in 2023.
“Our primary intention is to get these trucks into market as quickly as possible,” said Rovik. “Every [hydrogen] truck we sell displaces a diesel truck and improves the environment. This kit is designed as a drop-in replacement for diesel, allowing quick implementation for potential truck OEM partners.”
Other OEMs also are expanding their portfolios of hydrogen technologies. General Motors recently announced its intentions to further develop hydrogen technology by expanding the functionality of its Hydrotec hydrogen fuel cells with the integration of a DC rapid charger. GM’s Generation 2 Hydrotec power cubes have a power output range of 60-600 kW. The units had been originally developed for applications such as heavy-duty trucks, marine, aerospace and rail, but GM has broadened their use to include stationary power generation.
“Our vision of an all-electric future is broader than just passenger vehicles or even transportation,” said Charlie Freese, GM executive director of the global Hydrotec business. “Our energy platform expertise with Ultium vehicle architectures and propulsion components and Hydrotec fuel cells can expand access to energy across many different industries and users, while helping to reduce emissions often associated with power generation.”
In January 2021, GM announced an agreement with Navistar to supply the company’s forthcoming hydrogen-powered RH truck with its Hydrotec fuel cells. The partnership will reportedly lay the groundwork for an “ecosystem” of zero-emission long-haul trucks, which will be initially piloted by J.B. Hunt Transport Inc.
“Hydrogen fuel cells offer great promise for heavy-duty trucks in applications requiring a higher density of energy, fast refueling and additional range,” said Persio Lisboa, Navistar president and CEO. “We are excited to provide customers with added flexibility through a new hydrogen truck ecosystem that combines our vehicles with the hydrogen fuel cell technology of General Motors.”
Ballard Power Systems and Mahle also recently announced a partnership to develop hydrogen ICEs for long-haul commercial trucks. Mahle stated that it had taken delivery of a 120-kW module at its hydrogen test center in Stuttgart, Germany. Ballard says it will produce a future platform to suit it.
The target power output of these units will range from 180 to 360 kW to meet the needs of truckers in various global markets. Mahle will manage testing of the concept fuel cell module and integrate it with its own components, including thermal management and electronic control systems. “We are extremely optimistic about the value of our collaboration with Mahle,” said Seungsoo Jung, product director trucks for Ballard Power. “We are committed to fuel cell technology leadership, and to tailoring our fuel cell solutions to our customers’ needs.”
Hyundai also has been a leader in developing hydrogen propulsion for commercial applications. The company announced its Xcient FCEV in 2020 as a joint venture with H2 Energy. Production trucks have already been delivered to customers in Switzerland, and Hyundai is projecting a total of 1,600 units by 2025. The Xcient is powered by a 190-kW hydrogen fuel cell system and a range of approximately 400 km (250 miles).
“By putting this groundbreaking vehicle on the road now, Hyundai marks a significant milestone in the history of commercial vehicles and the development of hydrogen society,” said In Cheol Lee, executive VP and head of Commercial Vehicle Division at Hyundai. “Building a comprehensive hydrogen ecosystem will lead to a paradigm shift that removes automobile emissions from the environmental equation.”Continue reading »