Military vehicles could benefit from hybrid electric engines.

Hybrid trucks that blend electric motors with internal combustion engines promise fuel efficiency, as well as plentiful, stealthy sources of electrical power for battlefield sensors, weapons, and command and control systems.

The National Automotive Center, which reports to the Army Tank-Automotive and Armaments Command, sponsored in recent years light, medium and heavy hybrid-electric truck demonstrators. The upcoming Future Tactical Truck System (FTTS) Advanced Concept Technology Demonstration will pursue motor, generator, energy storage and power control technologies. According to NAC Deputy Director Paul Skalny, "The beauty of doing it is we move from investment in components to full-up systems in the bands of soldiers."

The FTTS program will seek hybrid electric propulsion, intelligent load-handling, diagnostics for two-level maintenance and other advanced truck technologies. Phase I ACTD contracts cover modeling and simulation efforts for two versions--a heavy (Class 7-8) maneuver sustainment truck and a lighter (Class 2B) utility vehicle. The Humvee-like variant potentially could serve as a command and control vehicle.

Phase II contracts, to be awarded in February or March 2004, are expected to lead to vehicle deliveries in 2006. Program officials anticipate that multiple contract winners will be asked to produce seven maneuver sustainment trucks and 10 utility vehicles.

A Stryker brigade squad with seven maneuver sustainment and two utility demonstrators will assess FTTS technologies in the field. "You focus on what you can do with technologies to impact both the current force and the future force," says Skalny.

According to the Defense Science Board, fuel takes up about 70 percent of the logistical tonnage in a heavy armored division. The Department of Defense estimated that a gallon of fuel, if flown into the theater, can cost up to $600. The Army expects hybrid-electric trucks and the hybrid-electric Future Combat Systems (FCS) to help the service attain its stated objective of 75 percent lower fuel consumption by 2020.

One-off demonstrators show promise, contractors said. Oshkosh Truck Corp. expects its ProPulse hybrid-electric HEMTT (Heavy Expanded Mobility Tactical Truck) to burn up to 40 percent less fuel than a conventional HEMTT, depending on the mission.

Under contract to Boeing and SAIC, BAE Systems demonstrated a hybrid drive on an FMTV (Family of Medium Tactical Vehicles) truck. "If you don't see 30 to 35 percent [better] fuel efficiency, you probably don't have it adjusted properly," said Cliff Plummet, manager for advanced military systems at BAE.

While the pricetag of hybrid-electric propulsion is tough to justify in peacetime, Skalny believes that "in wartime, there is a big payback."

Fuel is a cost-driver for both the Army and the commercial truckers. NAC representatives routinely meet with the trucking industry to keep pace with commercial hybrid, electric developments. "We've always taken the approach that the big bang for the buck will be how we leverage off industry," says Skalny.

Hybrid-electric drives achieve greater efficiency in stop-and-go mission profiles than they do in long-haul commercial duty cycles. The regenerative braking that recovers and stores power as electrical energy stretches fuel farther, and electric motors can generate instantaneous power for better off-road maneuvering.

Hybrid trucks can also generate "exportable" power to run other equipment. The biggest generator typically towed by a Humvee provides 15 kW. In contrast, the hybrid Humvee demonstrator--developed by PEI Electronics--has a 75 kW generator resident in the vehicle. Common lead-acid batteries could also supplement the generator and give the vehicle 500 to 600 kW. A hybrid-electric Humvee exhibited at the Association of the U.S. Army annual convention carried a mockup of a 100 kW laser.

With hot, noisy truck engines turned off, the batteries, ultracapacitors or fuel cells of hybrid-electric vehicles provide "silent watch" power in stationary vehicles to avoid detection. Another benefit is the stored energy. United Defense estimates its hybrid-electric FCS-W (Future Combat Systems--Wheeled) demonstrator can travel 5 miles per hour for 30 minutes on level ground using only stored electric power.

General Dynamics Land Systems integrated hub-mounted electric motors on an 8X8 combat vehicle and incorporated the technology in a 4X4 reconnaissance, surveillance and targeting vehicle that the company is marketing to the military services.

While motor and generator technology are relatively well established, fuel cells and other hybrid vehicle advances are farther from production. One goal of the FTTS ACTD is to design an open drive architecture able to accommodate advancing technologies. "I think it's fair to say that the investments we've made over time are what we'll see in the Future Tactical Truck System," says Skalny.

Commercial Trucks

The Army also is evaluating hybrid-electric commercially based tactical trucks (COMBATT) or severe off road vehicle (SORV) demonstrators from General Motors and Daintier Chrysler.

GM estimates its diesel-electric Chevrolet Silverado can improve fuel efficiency 25 to 40 percent. Driven by a 210 hp diesel engine or a 5 kW fuel cell, a split-power continuously variable transmission with integral electric motors sends power to the wheels. The regenerative fuel cell electrolyzes water and stores the hydrogen in a metal hydride container. With the engine shut off, the fuel cell serves as an auxiliary power unit. With the diesel running, the generator system provides up to 30 kW exportable power.

Daimler Chrysler's COMBATT C-11 hybrid-electric truck gets about 15 percent better fuel efficiency than a conventional Dodge Ram 2500, the company says. At rest, the C-11 powertrain generates 12.5kW continuous or up to 30kW peak power.

The modified heavy-duty pickups will be available commercially in 2004, and the NAC is proposing a partnership to build up to 50 vehicles for an operational assessment in 2005. While the Army has no requirement for a COMBATT or SORV as of yet, the service and other government agencies may find such vehicles on the GSA schedules attractive for administrative or non-tactical roles. COMBATT hybrid electric technology may also be applicable to light, medium and heavy tactical vehicles.

Despite their clean-power potential, fuel cells still require long-term development to cut their cost and improve their durability. They also fall short of chemical batteries and ultra-capacitors for quick bursts of power. Under contract to AM General, PEI Electronics equipped four XM1124 Humvees with a hybrid-electric drive with one 75 kW generator, two 75 kW propulsion motors and conventional lead-acid batteries. "We have to be very sensitive to the initial cost of the vehicle," says Ken Winters, PEI vice president.

The cost of batteries rises with their performance. Nickel metal hydride batteries would provide twice the energy storage capacity but half the power surge of lead-acid batteries to drive the hybrid Humvee twice as far but not as fast. More advanced but less available lithium-ion batteries would give the vehicle four times the energy capacity with power density equal to lead acid. They could power a command and control shelter for 12 hours.

The Army cancelled funding in fiscal years 2005 and 2006 for hybrid Humvee production qualification testing, initial operational test and evaluation, and logistics. The demonstrators nevertheless achieved impressive performance. With their lead-acid batteries, they doubled the acceleration and attained higher speeds than conventional Humvees.

Following two proof-of-concept vehicles, four hybrid-electric Humvee demonstrators underwent testing at Aberdeen and Yuma Proving Grounds and Fort Greeley, Alaska. Two remain at Fort Benning, Ga., for continuing tests, and two more are being refurnished in Huntsville, Ala., pending future Army decisions.

Energy storage remains the principal challenge for integrators of hybrid electric trucks. "The batteries are the long pole in the tent," says Plummet, of BAE Systems. "We don't have batteries that could last the length of time we'd like to see them last."

The company drew upon its experience with aircraft control laws to maximize the energy density in the lead-acid batteries of the Hybri-Drive already used in New York City buses. A two-wheel drive hybrid FMTV truck was built under contract to Stewart and Stevenson in 1999. The six-wheel-drive hybrid prototype that BAE developed under the FCS Boeing contract recently completed 5,000 miles of performance tests at Aberdeen Proving Ground with acceleration and fuel economy better than conventional diesel FMTVs, Plummer said.

The two-motor series hybrid-electric drive in the truck is similar to the single-motor Hybri-Drive used in buses. While the civilian bus uses an air-cooled generator, the hardened truck uses a liquid-cooled generator that enables the FMTV to wade through 30 inches of water.

The FMTV power module can provide 200 kW of continuous exportable power, but future combat vehicles--packed with electronic systems--will require smart power management. The sensors, radios, computers, active suspension systems, electric gun turrets, nuclear/biological/chemical protective systems and other mission equipment on future vehicle can double the power demands on a hybrid-electric drive system. The ideal solution will automatically manage the load on available resources, and draw power from generators, batteries and other sources.

Oshkosh Truck Corp. successfully used Maxwell Technologies ultracapacitors in its HEMTT LAS proof-of-concept demonstrator to store the energy of regenerative braking without batteries. In 2001, the National Automotive Center gave Oshkosh a grant to integrate the company's ProPulse hybrid-electric drive into the so-called Transformation HEMTT. While the Army's next-generation HEMTT A3 is not yet definitely a hybrid-electric vehicle, Oshkosh officials say that the ProPulse technology can help increase vehicle performance.

The first of six or seven hybrid-electric HEMTT pre-production test units will be demonstrated during the next two years. Compared with the proof-of-concept vehicle, the production hybrid HEMTT would have ruggedized components, to meet military specifications. While the demonstrator has one electric motor per axle, a productionized vehicle might use one motor per wheel end.

The proof-of-concept vehicle uses a 450 hp constant-speed diesel engine to power a 335 kW electric generator. The generator in turn provides direct power to the truck wheels without torque converters and other heavy transmission components.

Electrochemical ultracapacitors store energy as an electrostatic charge in a polarized liquid layer between an electrolyte and an electrode. They can be used with chemical batteries in hybrid electric vehicles. "The capacitors are much better-suited to giving up and receiving large quantities of energy in a short time," says Gary Schmiedel, Oshkosh director of advanced products engineering.

Less vulnerable to extreme cold than batteries, ultracapacitors can last the life of the vehicle. The ultracapacitors in the proof-of-concept vehicle are expected to last 10 years, but the goal is 20 years. The technology in the Transformational HEMTT improves acceleration and grade-climbing performance compared to the standard truck, says Schmiedel.

The proof-of-concept vehicle has already powered hospital and airfield equipment, and a production hybrid-electric truck would have an engine sized to Army requirements. Unlike commercial hybrid vehicles that typically use smaller engines to maximize fuel economy, the hybrid HEMTT uses the same size diesel as a conventional HEMTT to preserve the original performance envelope. The 450 engine gives tire demonstrator 335 kW of exportable power. A 505 hp engine could generate 400 kW. "We're engine neutral," says Schmiedel.

The hybrid-electric drive with ultracapacitors makes it possible to run the engine at the most efficient speed, regardless of the road speed of the vehicle, he adds. It also makes it possible to tune the engine exhaust system to minimize noise signature when the engine is running. Engineers can bury hot mufflers and manifolds deeper in the vehicle to reduce infrared signatures.

Fuel-efficient, low-polluting hybrids have special commercial appeal in the stop-and-go duty cycles of municipal refuse trucks or "Wall-Mart Cycle" distribution trucks. "We always invested or partnered with industry because we knew where industry was moving," says Skalny.