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
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.
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.
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
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
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
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.