Kawasaki, Japan. Designated as the global hybrid development center for its parent Daimler Trucks, Mitsubishi Fuso has begun work on a diesel electric hybrid system for on-highway heavy-duty trucks. The first prototype has been installed in a Fuso Super Great, a cab-over tractor sold in Asia and Australia, but engineers at the hybrid center said it could be used in other Daimler heavy-duty trucks, including Freightliner and Western Star over-the-road tractors.

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Fuso’s hybrid expertise has already resulted in a new production version of its light-duty Canter. Officially introduced at the Tokyo Motor Show last week, the Canter Eco Hybrid goes on sale in Japan in next spring, followed by other global markets that could include North America.

The heavy-duty system is based on the parallel diesel electric hybrid technology used in the new Canter Eco Hybrid, placing a narrow electric motor/generator between the clutch and transmission. On deceleration, the unit generates electric power that is stored in lithium-ion batteries. That stored power is then used to drive the electric motor in low-speed acceleration conditions, augmenting the diesel engine and reducing fuel consumption.

It’s generally acknowledged that such a hybrid system works well in light- and medium-duty applications that require frequent stops resulting in high electrical power regeneration. But many have questioned its value in OTR applications that see fewer stops, instead focusing heavy-duty hybrid efforts on driving auxiliary systems.

However, Fuso believes it has uncovered significant fuel economy potential in heavy-duty applications that directly link the electric motor to the drivetrain, according to Genichiro Ishii, head of the global hybrid center. “We believe the parallel diesel-electric system is most suitable for truck applications, and it will be Daimler Trucks’ main hybrid strategy,” he said during a press briefing at the development center.

While an OTR tractor/trailer has far fewer stop events than a lighter truck in P&D service, it’s far greater overall weight means it can generate and capture high electric energy output on all brake inputs, even those on small gradient declines, Ishii said. That power can then be used to complement the diesel engine at slower speeds.

Tests with the Super Great Eco Hybrid on Japanese highway circuits have shown a 10% improvement in overall fuel economy. While well below the 30% gain seen with the Canter hybrid, an OTR heavy-duty truck’s far greater annual mileage should translate into practical cost benefits that would result in a three- to five-year payback on the cost of the system, according to Ishii. “That’s our goal and we are already close to it,” he said. “Now we’re working to get it closer to three years.”

While Fuso researchers declined to set a target date for production versions of their heavy-duty hybrid, Ishii said it sees the Super Great system being applied across the entire Daimler Truck heavy-duty line, including Freightliner.