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Hybrid Vehicle Cruises for Battle

April 2004
By Robert K. Ackerman
E-mail About the Author

The next Marine Corps carrier could be positively electric.

The U.S. Marine Corps has built a hybrid-electric land vehicle that could be a harbinger of future battlefield mobility. The combination diesel/battery vehicle would have greater range than existing four-wheeled infantry carriers and would be able to operate quietly without any visible thermal fingerprint.

Instead of direct drive from an internal combustion engine, electrical cables carry the drive to the wheels. Independent wheel motors provide a better and faster ride over rugged terrain. The technology can be scaled up to power larger wheeled vehicles that could enjoy improved range and performance over existing versions.

Known as the reconnaissance, surveillance and targeting vehicle, or RST-V, the technology demonstrator was funded jointly by the Office of Naval Research and the Defense Advanced Research Projects Agency (DARPA). The program was designed to determine the feasibility as well as the advantages and disadvantages of a hybrid-electric tactical vehicle. The U.S. Army, which also is working on hybrid-electric vehicles, is looking at the Marines’ effort for its Future Combat Systems.

The vehicle consists largely of commercial off-the-shelf technologies. The main engine is the same one offered by Daimler-Chrysler in its European minivans. The RST-V’s electric wheel motors are used in buses in Germany. It uses an air suspension system common to large tractor trailers. Batteries are adapted from commercial technologies.

General Dynamics Land Systems of Muskegon, Michigan, built the RST-V under contract over the past four years. The goals of the program, which began in 1997, were to develop a highly mobile, off-road tactical vehicle that could fit in the back of a V-22 Osprey; to employ a hybrid-electric system and learn from the technologies; and to invest in integrated survivability features in the vehicle, says Jeffrey Bradel, manager of Marine Corps maneuver in Marine Corps science and technology, Office of Naval Research.

Bradel explains that a hybrid-electric land vehicle system offers several advantages over today’s conventional engines. First, a vehicle’s acoustic signature is decreased dramatically with an electric drive. Switching off the internal combustion engine and powering the vehicle exclusively off the battery pack provides a degree of stealth performance that could prove vital close to enemy forces. In the same vein, the vehicle’s thermal signature is virtually eliminated when it operates on battery power.

Of course, a hybrid vehicle provides a dramatic improvement in fuel economy, which reduces the strain on logistics support. This provides a corresponding increase in range. A hybrid system also accommodates an auxiliary power unit on board to provide electricity to payloads, which reduces the need for towed generators.

Another asset of the hybrid drive is that it provides maximum torque at 0 revolutions per minute (RPMs). A conventional turbocharged engine requires waiting for the turbocharger to spin up before achieving maximum torque, but the electric drive presents that at 0 RPM. This provides advantages in dash speed, for example. Bradel characterizes the RST-V’s 0-to-30-mile-per-hour times as “really good.”

Bradel offers that one unconventional advantage is that the vehicle’s engine can be optimized for a greater range than most conventional counterparts. Existing internal combustion engines tend to be sized for peak power needs. For the military, those needs include being able to climb a 60-percent grade that leads to a high torque and power demand. Acceleration also weighs in, with the result being that the military must build vehicles with large engines that would be tapped to their full capability only 1 percent of the time. The rest of the time, the vehicle would be underperforming, but it still would be consuming copious amounts of fuel.

But, the hybrid-electric vehicle is sized for an average power requirement. Its engine is considerably smaller than would be needed were it configured to serve peak performance requirements. The accompanying battery pack provides any necessary surge capability. The result is better overall fuel economy.

The system has two major power sources. First, its internal combustion engine is a 2.5-liter 4-cylinder Detroit Diesel. Coupled to the engine’s flywheel is a permanent magnet generator built by Magnet Motors in Germany. This engine is rated at 100 kilowatts, with the generator capable of 110 kilowatts.

The engine does not provide direct drive to the wheels. Instead, it generates electricity that powers four individual motors that turn the wheels. The RST-V features in-hub wheel motors that are coordinated by a computer receiving data from embedded speed and torque sensors. This provides much more flexibility in internal vehicle layout, Bradel points out.

The hybrid approach saves a lot of weight and space by eschewing drive shafts, differentials and half shafts. And, the individual wheel drive allows engineers to design the suspension in ways that allow the vehicle to be folded up for transport inside a helicopter. The computer-controlled wheel system may permit individual traction control where different speed commands are sent to each wheel if, for example, one side of the vehicle were in a muddy rut. This would require only new software code, Bradel says.

The second key power element is the battery system. The RST-V employs a lithium-ion battery pack that can provide 10 kilowatt hours of power. Bradel explains that the program conducted a tradeoff analysis on battery technology and opted for high-end lithium-ion batteries instead of less-expensive, but heavier, lead-acid batteries. The vehicle can accommodate two lithium-ion battery packs for a total of 20 kilowatt hours of power.

This battery system could provide a burst of 80 kilowatts. Coupled with the diesel engine/generator’s 100 kilowatts, the hybrid system can generate a total of 180 kilowatts of peak power. This translates to roughly 240 horsepower.

When operating on battery power alone, the RST-V can move more than 20 miles across smooth level terrain. In off-road conditions, the vehicle draws more power for hill climbing. This increases power drain, so the RST-V probably has a battery range of only about 5 miles in rugged terrain.

The RST-V can handle rough terrain better than a conventional vehicle, Bradel states. The enablers are the independent in-hub wheel motors. Conventional wheels with differentials, CV joints and half shafts have limited range of motion—usually no more than 25 degrees. Wheel travel is the key determinant of a vehicle’s ability to cross off-road rough terrain, with a greater range of wheel travel increasing the speeds at which a vehicle can travel in these conditions. The RST-V has unlimited wheel travel, Bradel reports, and this provides a dramatic increase in off-road speed, more mobility and a smoother ride.

Controls on the dashboard give the driver a choice of modes for operation. These can include running just on the engine and generator alone, the battery pack alone or a combination of both for burst power. The vehicle’s default would be to operate from the engine and generator, and it could switch automatically to a combination. Override switches on the dashboard ensure that the power system does not change modes at an inopportune time, such as the engine coming back on to replenish the battery pack when the vehicle is operating in the battery-only stealth mode.

Bradel notes that the vehicle did not begin its program life as a reconnaissance platform. It was only after the feasibility of its hybrid design was proven that the vehicle was tested with a sensor suite to demonstrate its applicability to the battlefield. This was successful, and the hybrid Marine four-wheeler became the RST-V.

Program engineers outfitted it with a Marine Corps target location and designation handoff system (TLDHS). The goal was to evaluate how the system could collect data through a sensor and transmit its imagery through a satellite to an offsite receiver.

Tests showed that the onboard power system can supply electricity to the TLDHS for a long period of time, Bradel relates. Under battery power only, the vehicle can conduct reconnaissance with this sensor system in a silent mode. The system provides plenty of power to support sensor operation and related communications activities. The sensors can be powered up for several hours before recharging.

Some pleasant surprises emerged from the overall testing regiment. Bradel describes the success the individual wheel motors had in operation. These hub motors were one of the biggest concerns entering the testing, and each wheel station received considerable abuse during rough terrain testing. Yet, their durability and reliability proved their worth in testing. Not a single wheel motor assembly has failed, he warrants.

Maintaining electric performance in a combat environment is a challenge, especially with the vehicle’s volumetric restraints. Bradel attests that thermal management of a hybrid-electric system is, and will remain, a challenge. Experts had to change fans and upgrade coolant systems to keep temperatures manageable, he relates.

Bradel admits that the program does not yet have any hard data on how the RST-V would withstand a harsh environment. It has been tested in the Arizona desert in temperatures higher than 100 degrees Fahrenheit, but it has not been tested in extreme cold or in other difficult conditions.

The volumetric challenge was significant, he continues. Designing an electric vehicle can be pretty simple if size limitations are eliminated. However, the quick and easy design would produce a power system that could fill a room. Packaging a 180-kilowatt system that could fit in a vehicle designed to be transported by a V-22 was another matter, and this proved to be a huge challenge. Nearly every component was optimized for size and weight, Bradel says.

Developing the system’s control and power management also was a problem. Debugging took a lot of work and a long time, he allows.

While tapping commercial off-the-shelf systems, the program explored the state of the art in some technologies. Bradel relates that RST-V is taking advantage of new controller boxes and control systems as well as more exotic battery technologies. These battery technologies provide increased range and performance, but they are costly.

The RST-V technology lends itself to scaling up. The current configuration features identical wheel assemblies on all four corners. Adding two more identical wheel assemblies on the rear will produce a 6 x 6 vehicle, although it might need a larger engine for the heavier total weight, Bradel says. Contractor studies have shown the feasibility of building an 8 x 8 or even a 10 x 10 simply by stacking up more of these identical wheel assemblies. These configurations would require a larger engine and greater battery power to maintain RST-V performance standards.

The current phase of the RST-V contract ends next month. Officials are in the process of collating all of the test and evaluation data and analysis for presentation to the Marine Corps leadership. “The original program goals and objectives—design, build, fabricate, test and collect data on a hybrid-electric, high-mobility tactical vehicle—have been met,” Bradel declares. Now, any future for the RST-V will depend on decision making, he offers.