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Hybrid-Electric Trucks on Army’s Horizon 


by Sandra I. Erwin 

Ongoing efforts to expand the use of hybrid propulsion systems in military ground vehicles are gaining some momentum. But experts caution that the technology is not yet mature enough to allow the U.S. Army to make long-term buying decisions.

Both government and industry experts agree that the Army could save fuel and reduce air pollution by using hybrid propulsion systems. Another side benefit of hybrids is the availability of on-board power generation.

The most widely known hybrids are the so-called hybrid-electric systems, which are commercially available. A hybrid-electric vehicle combines two power sources. In most cases, the vehicle is powered by electric motors combined with a small, on-board combustion engine-generator.

Generally, hybrid systems feature a small conventional engine, such as a diesel or a fuel cell, which powers a generator, which in turn charges the battery. Because the internal combustion engine is smaller, the vehicle burns less fuel and emits less toxic fumes. The on-board generator means the battery constantly is recharged.

In an all-electric vehicle, the battery would have to be recharged approximately every 100 miles. With a hybrid system, the power still comes from the engine, while the batteries help it run more efficiently.

“The Army would like to contribute a reduction in emissions,” said Harold Samborn, project leader at the Army’s National Automotive Center (NAC), in Warren, Mich. The center, in partnership with private firms, is pursuing several projects to develop and demonstrate hybrid systems in military tactical and combat vehicles.

The umbrella program for these efforts is the so-called 21st century truck initiative, a government-industry venture seeking to develop fuel-efficient vehicles for commercial and military use. U.S. federal agencies and automotive firms are expected to invest $5 billion in this program during the next decade.

“We hope to have a variety of platforms that demonstrate advanced propulsion techniques, whether it’s hybrid drive or fuel cells,” said Samborn in a recent interview. “We may have alternative fuel vehicles running on natural gas. We may have several competing architectures for a particular technology, to see which one is most mature.”

Last month, during the Society of Automotive Engineers annual truck and bus convention in Portland, Ore., the NAC showcased a number of hybrid prototypes, such as a hybrid-electric 5-ton military truck, part of the Army’s family of medium tactical vehicles (FMTV). Another prototype at the show was a commercial Class 8 Volvo tractor-trailer, which the NAC believes can be a test bed for the future introduction of hybrid drives into heavy military trucks.

“There are many hybrid systems out there, particularly in the automotive industry. But the heavy truck industry is just now exploring some of these hybrid systems,” said William Haris, chief engineer at Radian Inc., in Alexandria, Va. The company is working with the NAC on the Volvo project. “It may be one to two years before we see commercial hybrid systems finding their way to heavy trucks,” said Haris in an interview.

The major obstacle for bringing hybrid systems into military vehicles is not the technology, but “energy management,” said Samborn. “All of our studies so far, in various applications, have shown that the hardware is not really the issue, in terms of making hybrids applicable to certain platforms.” In any hybrid vehicle, Haris explained, the operator should not have to think about the propulsion system.

Battery Problem
The Achilles heel of hybrid systems today, however, is the battery, Haris added. “You need to have a source of energy to propel the electric motors. Traditionally that has been batteries.” The most commonly used batteries today are lead-acid, which are the least expensive. But they also are heavier and less efficient than more advanced chemistry batteries.

A more desirable alternative would be nickel-metal-hydride batteries, which have twice the energy density of lead-acid. Energy density is the amount of energy that can be stored per pound of material. In the long-term, experts are looking at lithium-ion batteries, which have four times the energy density of lead-acid.

As far as range is concerned, Haris, said, “you can certainly design a hybrid vehicle to accomplish a certain range. Lead-acid can have the same range as any advanced chemistry battery, but you have a weight penalty.” To travel a given distance, for example, a vehicle would require four lead-acid batteries but only two nickel-metal-hydride, or one lithium-ion.

The “big question,” Samborn explained, is whether the hybrid systems will have adequate energy management capabilities to keep energy flowing to the battery without upsetting it. “Batteries are a very sensitive technology,” said Haris. The energy storage in hybrid systems occurs in batteries or in ultra-capacitors.

An ultra-capacitor is a high-performance electric energy storage system, which can improve the performance of a battery/engine-powered electric vehicle. Whether ultra-capacitors should be treated as supplements or replacements for batteries is “up for debate,” Haris said. Ultra-capacitors, he explained, can help force energy into the battery without damaging it.

The hybrid system on the Volvo prototype is designed to take advantage of regenerative braking to make more efficient use of the energy. In conventional trucks, regenerative braking is done to make the engine slow down the vehicle, thus saving wear on the brakes. With hybrid systems, Haris noted, regenerative braking not only saves on brake wear but also helps create “free power” that can be put back into the batteries.

“While aiding in the deceleration of the vehicle, the hybrid system engages regenerative braking to convert kinetic vehicle energy into gained electric energy, rather than energy lost in the form of friction and heat,” he said.

Energy management, Haris said, is “an issue for the manufacturers” to address. “To be quite blunt, no one has adopted what consumers would agree is [an acceptable] solution.”

One desirable option, said Samborn, would involve an advanced chemistry battery—with some sort of ultra-capacitor, along with batteries. Energy management, he said, should be “transparent to the user,” so the operator just turns the key and drives, without worrying about batteries wearing down.

Current hybrid systems, Samborn said, are not sophisticated enough in that arena.

Hybrid Trucks
The hybrid drive that powers the Class 8 Volvo is made by BAE Systems Controls, in Johnson City, N.Y. The company plans to begin deliveries of 125 hybrid-electric buses to the New York City mass transit authority in June.

Steve Cortese, manager of power and drive systems business development at BAE Systems Controls, noted that the Volvo tractor-trailer is a “parallel hybrid, as opposed to the FMTV truck, which is a series hybrid.” Parallel systems are optimized for highway operation, while the FMTV was optimized for off-road and all-wheel drive operation.

For off-road driving, he explained, the technology needs a higher degree of ruggedness.

The FMTV, for example, must be able to traverse relatively deep waters. It has to be “all-wheel,” meaning the hybrid drive has to be able to drive both the front and the rear axles.

As with any hybrid-electric system, Cortese said, the system takes care of managing the charging and recharging of the battery.

Fuel efficiency is a significant benefit these vehicles offer, he said, but it all depends on the driving pattern. “We have to be careful about standardizing fuel efficiency, because it’s a very non-standard thing to measure.”

Advocates of hybrid vehicles, for example, advertise a 25 percent to 50 percent reduction in fuel consumption. But the only way to compare fuel efficiencies is to standardize a driving cycle and drive different vehicles on the same cycle, Cortese explained. A standard driving cycle was used to determine that New York City buses will consume 50 percent less fuel than their conventional predecessors. The cycle included a series of accelerations and decelerations, lasting 30 seconds each. For a UPS delivery truck, BAE Systems estimated a 25 percent improvement in fuel efficiency. The more stop-and-go, the higher the level of fuel economy that can be achieved.

Military vehicles should become less costly to equip with hybrid drives, once the technology becomes more commercialized. “We currently only have commercialized one product, to fit the buses,” said Cortese. “We are adapting that product for demonstration programs in other types of vehicles, such as military trucks.”

BAE Systems first developed a hybrid-electric FMTV for the NAC in 1998. During the October 2000 annual convention of the U.S. Army in Washington, D.C., the company unveiled a second-generation 5-ton FMTV hybrid truck.

The second-generation has more advanced power electronics and higher horsepower, Cortese said. A test program for this vehicle is being designed by the NAC and the FMTV manufacturer, Stewart & Stevenson Tactical Vehicle Systems. “We would like to begin testing early in 2001,” he said.

Cortese declined to provide a price tag for a military hybrid truck. The buses purchased by New York City were about $400,000 each, compared to $250,000 for a conventional bus. A comparable cost premium could be inferred for military vehicles.

“For the technology to be commercially viable in the long run, that kind of cost premium cannot be maintained,” Cortese said. “The ultimate goal is to get hybrid-electric propulsion to be cost-effective enough over conventional propulsion.”

Beyond truck programs, there will be opportunities to mature the technology for combat vehicles, said Cortese. A case in point is the Army’s Future Combat System (FCS) program, which seeks to develop a light armored vehicle by 2012. “We see hybrid-electric propulsion as an integral piece of FCS,” said Cortese. “On the commercial side, the goal is for the cost to be low enough so it can be introduced into the medium-truck market,” which is “cost-sensitive.”

It is too early to pinpoint a reasonable price tag, said Cortese. “We have to understand what the Army’s value equation is for this new technology.”

In addition to fuel savings, hybrid systems offer an extra source of electric power generation in the field, Cortese noted.

According to Samborn, this makes a hybrid-electric system “a great application for reducing the use of generators on the battlefield.” That could eliminate the need to bring trailers carrying generator sets. “If the military adopts hybrid vehicles, they will get better performance in terms of torque, but they also will have on-board power generation,” he said.

The engine/generator powerpack in the hybrid-electric FMTV can furnish up to 200 kW of power.

Most likely, said Cortese, the Army will choose to buy a mix of vehicles. Conventional propulsion would be used for standard cargo hauling. But for missions that require power generation, hybrids would be preferable.

In some ways, military customers are much less conservative and more forward-thinking than their commercial counterparts, said Cortese. “They are starting to make research dollars available to adapt the technology for military applications.” However, “they seem very conservative on the procurement side. ... They don’t want to buy large quantities of hybrid electric trucks until they are convinced that the technology is really ready and mature enough for the rigors of military deployment.”

Haris believes that the Army would benefit from the use of hybrid drives in vehicles that have to pull heavy loads, such as the heavy equipment trucks, known as HEMTTs. An improvement in performance during start-off, when the truck needs to pull heavy loads, would be significant, he said. Electric motors have a greater torque capability at zero speed. “As you try to get the motors moving, the hybrid can help,” Haris said.

NAC officials are hopeful that their success in hybrid-electric truck prototypes can expand into combat vehicles as well. “Several projects are ongoing,” said Samborn. The most promising systems are those that can be retrofitted into existing vehicles.

“A good example is the M113” armored personnel carrier, he said. A hybrid-electric M113 test vehicle was developed by United Defense LP, the M113 original manufacturer. The company said the technology could be available for production within five years.

United Defense also is participating in other hybrid-electric vehicle projects. One is the Combat Hybrid Power System (CHPS) program, sponsored by the U.S. Army and the Defense Advanced Research Projects Agency. CHPS is seeking advanced components for 15-ton combat vehicles. The company is working on a hybrid-electric prototype for a 25-ton Bradley infantry vehicle, which currently is in testing. Another effort is a 30-ton amphibious vehicle with an advanced propulsion system, using a 750 horsepower rotary engine driving a wound field generator. Induction motors provide traction on land as well as propulsion in water.

According to United Defense, the powertrain in the hybrid M113 will produce 500 horsepower in acceleration—versus 275 horsepower in the conventional vehicle. When stationary, the vehicle can generate approximately 200 kW of electricity.

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