Off-Highway Hybrids Are Entering Prime Time

October 1, 2024

Kami Buchholz

Vocational off-highway vehicles are piling on the miles with power provided by an internal combustion engine (ICE) with additional hybrid technology. “We often get asked the question, especially by those outside the industry, ‘Why don’t we just electrify everything?’ The reason is the current challenges we have with battery-electric vehicles,” Grant Van Tine, product manager for electric vehicles at John Deere, said during an August 22 webinar hosted by SAE Media Group.

Van Tine and Mihai Dorobantu, PhD, Eaton Mobility Group’s director of technology planning and government affairs, cited facts, figures and in-field and in-development examples of why hybrid innovations are especially relevant today in a session addressing “ The Future Role of Hybrids in Off-Highway Vehicles  .”

Since 2015, the John Deere 944 X-Tier electric-drive wheel loader has showcased hybrid technology instead of an all-electric powertrain, and with good reason. “Even if we were able to package enough energy onboard, which would make the machine extremely larger than what it is now, a 54,000-lb (24,500-kg) battery pack is not something that can be packaged within the current vehicle frame,” Van Tine said.

An all-electric version of today’s 944 loader, according to Van Tine, would need to be charged multiple times during a typical day’s 12-hour shift. A fully electric version would also cost three to four times higher than the current hybrid machine. “That’s just not practical or feasible for anyone,” he said.

Deere’s hybrid setup

The current 944 X-Tier wheel loader’s powertrain uses a John Deere 13.5-L diesel engine producing 536 hp (400 kW) that runs at either a limited speed range or a constant speed. The output shaft connects to a gearbox that houses the hydraulic pumps and two generators, which convert the engine’s rotational energy into three-phase AC electrical energy. There is no mechanical transfer of power. Each wheel’s electric motor converts the AC power back to rotational energy and torque in the final drive. Power electronics control the commands to the drive system based on operator input.

Recaptured energy is recycled into the system in certain situations, Van Tine said. “In certain applications, such as truck loading, we’ve seen up to a 33% reduction in fuel consumption compared to a conventional diesel machine,” he said.

Other benefits associated with the machine’s electric drive system include a longer lifespan for tires, which is important since the vehicle typically operates in a quarry. “In abrasive rock environments, wheel spin is bad and can deteriorate tires. So, independent control of each wheel can limit the torque when slippage begins to occur, and that extends tire life,” Van Tine explained.

The 944 and 644 X-Tier electric drive vehicles have accumulated more than three million customer hours. Additional X-Tier loader models, the 744 and 824, launched earlier this year. The 850 X-Tier dozer is next for market deployment. “We’ve learned a lot, and we’re applying that knowledge to future electric drive models and our battery-electric projects,” Van Tine said.

Hybrid technology is a powerful emissions-reduction resource. In a fuel emissions comparison of vehicles powered by one of three sources – diesel-electric, diesel-electric with renewable fuel, or battery-electric – each provided a double-digit percentage carbon emission savings versus a conventional diesel engine.

In a 2,500-hour usage example, an engine using renewable fuel (R100) provided a carbon emissions reduction estimate of 74% when compared to a conventional diesel engine. John Deere’s calculated values are based on machine telematics data and Argonne National Laboratory’s GREET (Greenhouse gases, Regulated Emissions and Energy use in Technologies) model for fuel emissions factors.

“From John Deere’s perspective, we don’t see a single solution that will win out and work in every single application,” Van Tine said, underscoring the relevance of combustion engines running renewable fuels, hybrid and all-electric technologies for off-highway vehicles. “We continue to invest in ICE technology, looking at solutions from both a compression ignition and spark ignition standpoint, to enable a variety of renewable fuels for the future,” Van Tine said.

‘Novel’ hybrid transmission

Looking back several years, hybrid technologies were essentially stalled out. “For vocational vehicles, hybridization makes sense today based on a combination of fuel savings and added performance, and this situation is fundamentally different than it was 15 years ago,” said Eaton’s Dorobantu.

Carbon emissions reduction and performance that exceeds what’s possible with conventional powertrains highlight a new technology Eaton developed in partnership with the U.S. Army. The hybrid transmission – an electrically power-shifting automated manual transmission (AMT) – is undergoing various field tests.

“This is a novel architecture,” Dorobantu said. “The transmission has the ability to switch between two standard hybrid modes. Electric power is being introduced ahead of the transmission in some modes of operation. And in other modes of operation, the electric power goes directly into the prop-shaft.”

Potential applications for the hybrid transmission include military and vocational vehicles. “For instance, with a traditional transmission that’s being used in difficult launches such as mud or on high grades, if you need to interrupt the torque while shifting, the vehicle will stop or it will start rolling backward,” Dorobantu said.

Initial test results with the electrically power-shifting AMT have demonstrated an ability to launch a 36-metric-ton vehicle on a 60% grade in third gear EV-only mode in both forward and rearward directions. The hybrid transmission also has shown low-speed maneuverability at speeds less than 2 mph (3.2 km/h), and it is 1.25 times better at reaching 55 mph (89 km/h) compared to a conventional transmission.

“Electrically generated torque helps with acceleration,” Dorobantu said. “For example, the wide-open throttle time from zero to 55 mph is 60-plus seconds with a torque converter automatic transmission compared to 50 seconds with this technology.”

Testing also reveals that the hybrid transmission can cut carbon emissions by 20% to 30%. No production plans have been announced yet for this electrically power-shifting AMT.

Government regulations for truck fleets tag a 40% reduction in carbon emissions by 2030. “The choice is open as to how to achieve the target,” Dorobantu said, noting that one battery-electric vehicle provides the same emissions reduction as three hybrid vehicles. “We can deploy a large fleet of hybrid vehicles as an industry much faster than we can save on carbon emissions with battery-electric vehicles,” he said.

For more on heavy-duty hybrid technologies and applications, read the following recent SAE articles:

Hybrids Are Having a Moment

Hexagon Agility Partners with Brudeli for Hybrid CNG Truck Powertrains

Detailing Kenworth’s Sleek SuperTruck 2

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