Use of Lightweight Materials in 21st Century Army Trucks (2003)

The main drivers for making improvements in vehicle propulsion systems are these: reducing the weight of vehicle bodies, increasing power density, improving fuel economy, complying with stringent emissions regulations, reducing noise and signature, and increasing driver comfort and safety. Material- and process-improvement programs that target increased horsepower of diesel engines and alternative energy powertrains, as well as the evaluation of hybrid electric and fuel-cell power systems, are key to the realization of the needs of future Army tactical trucks. The use of these alternative energy sources also opens opportunities for inserting lightweight structural materials and new processing technologies into new truck designs.

The hybrid electric powertrain offers perhaps the greatest potential for tactical vehicle redesign in the near future.^1,2 This technology would be most beneficial in light and medium trucks with variable driving schedules, high speeds, and vehicle loading that varies widely between fully loaded and empty. The hybrid electric powertrain consists of an internal combustion engine coupled with electric motors and an energy storage system or battery. Operating energy is provided by the engine, by the electric motor, or by both. The battery is charged when the engine provides excess power, when the vehicle decelerates, or when the brakes are applied. When the vehicle requires additional power for passing, for grades during acceleration, or for

climbing hills, the battery adds power by channeling electrical energy to the motors.

The hybrid electric powertrain provides solutions for two of the most important sources of inefficiency in engine-based transportation. The first source of inefficiency is the need to use an oversized engine for the average duty cycle of the application. Currently, the size and power of an engine are both determined by the most extreme operating conditions, such as maximum torque and highest driving speed. However, because light or medium trucks make frequent stops and are often driven unloaded, they use only part of their engine power. The mechanical or friction losses from having an oversized engine can be quite large, of the same order of magnitude as the driving power. The second source of inefficiency is the transient operation of the internal combustion engine caused by the drive-wheel speed and the traction effort required. As a result of driving schedule, terrain configuration, vehicle load, driver technique, and other factors, the engine coupled with the driveline operates in a transient mode with a variable efficiency far below maximum.

By using a hybrid electric powertrain, the engine can be made the right size for the needs of the average application. It will operate at or close to a specific speed and load point at which the fuel efficiency is highest. A 3- to 4-liter diesel engine in a hybrid system will probably be able to replace a 7- to 9-liter diesel engine in a standard system. In addition, advanced turbocharging can be used to boost the engine torque and power, further reducing engine size and weight. In a smaller engine, the mechanical or friction losses are lower.

Two additional advantages make hybrid electric powertrains desirable. First, auxiliary systems and accessories can be decoupled from the engine, permitting their use on demand, reducing accessory losses, and increasing overall efficiency. Because the position of these accessories is no longer tied to the engine shaft, more compact packaging of the engine under the hood is possible, as are opportunities for improved cabin and hood design. Such designs could improve road visibility and stealth features, among other things. Second, because the electric traction motor is designed to function as a generator during deceleration, a portion of the kinetic energy of the vehicle is converted back into electrical energy. The vehicle is slowed down by this process, so the friction brakes can be downsized.

The optimum configuration of a hybrid electric powertrain depends on the specific application, the efficiency and performance requirements, manufacturing cost, serviceability, market differentiation, and customer acceptance of the new technology. Light-duty passenger cars with hybrid electric powertrains have already been commercialized (e.g., the Toyota Prius and the Honda Insight), albeit in small numbers. Although heavy-duty vehicles with hybrid electric powertrain systems are not yet in production, there have been several demonstrations of the system in urban transit buses. These demonstrations have highlighted several shortcomings. Most of the demonstrations to date have used commercially available components, rather than components designed and optimized for use in hybrid electric powertrains. In low volume, the precision manufacturing of these mostly electrical components cannot be achieved at reasonable cost. Moreover, systems engineering and integrated manufacturing technologies cannot be applied unless commercial capabilities and economies of scale are leveraged.

Several critical technologies require additional research to support innovative systems: electric motors and generators, electrical energy storage systems, power electronic products, electrical safety, regenerative braking, and purpose-built engines. Electric motors and generators are typical for series or parallel hybrid systems and their corresponding couplings and gear sets. Issues that need to be addressed with regard to these components include those of improving specific power, reducing weight and cost, and leveraging modern manufacturing and automated production.

Electrical-energy storage systems in hybrid electric powertrains capture energy from the generator, store energy during braking events, and return energy when required by the driver. Systems under consideration include electrochemical batteries, ultracapacitors, and electric flywheels. Because of the potential for commercializing them in the short term for light-duty vehicles, batteries have received more attention in the past through the Partnership for a New Generation of Vehicles. For heavy-duty hybrid electric systems, batteries must be developed that have high specific energy, improved life, and good cold-temperature performance. Ultracapacitors are capable of providing even higher energy density than batteries and could be used to provide primary energy during acceleration and hill climbing, as well as to recover braking energy. In addition, ultracapacitors can be used as a

secondary energy provider for load leveling power to chemical batteries. Research is needed to develop suitable materials for ultracapacitors that can be used in automotive applications. Electric flywheels have high power-handling capability and moderate energy density. They are efficient, durable, and have robust performance in various ambient temperatures. Although their initial cost is high, they are attractive on a life-cycle cost basis.

In the area of power electronic products for military and commercial applications, more research is needed in high-power transistors, which are not currently produced by any domestic manufacturer. Obstacles with respect to high-power transistors that need to be overcome by the developers of motors and inverters include high cost, excessive complexity, insufficient reliability; and the demands of harsh operating environments.

Electrical safety is an area in which hybrid electric powertrain technology needs R&D. The presence of higher voltage components and cabling requires the development of standard practices and protocols in categories ranging from functional to personnel. The identification, management, and mitigation of hazards will also be required.

Regenerative braking is an essential capability of the hybrid electric powertrain concept. Significant development is required, however, in order to maximize energy recovery, provide adequate storage, and minimize the dependence on the conventional braking system.

In order to obtain the full efficiency benefits of hybrid electric powertrain systems, engines must be built specifically for these systems. Only then can the engine be operated in such a way as to avoid inefficient points (i.e., low load and high speed) and remain close to peak efficiency most of the time. Alternative power sources such as gas turbines have been used in demonstration hybrid vehicles. Although these power sources have some merits due to the synergism between the turbine and the electrical generators, they represent a major departure from vehicular engines and cannot therefore leverage the advantages of high-volume production.

Fuel cells are electrochemical devices that convert energy from the chemical reaction of hydrogen and oxygen into electricity. Fuel cells are seen as the ultimate power source in the hydrogen-based energy scenario of the

future. This type of energy conversion has the greatest potential for combining high energy with low emissions. There are significant barriers to be overcome, however, before the technology can be used in consumer vehicles. These barriers include overall performance limits; cost; fuel availability, including onboard storage; and lack of infrastructure.³

By 2005, fuel-cell vehicles using pressurized hydrogen may be produced for cars and sport utility vehicles (SUVs). They will probably be limited to special fleet applications for which hydrogen can be made available. The production of hydrogen from natural gas involves considerable loss of energy and the generation of emissions. These factors must be included in the energy balance of a fuel cell if a systems approach is used. The use of methanol or conventional petroleum fuels can circumvent the difficulties of implementing a hydrogen fuel delivery infrastructure. However, the onboard fuel processor required in such a case reduces energy efficiency so much that the fuel cell is not superior to an internal combustion engine.

For military applications, fuel cells may not be a viable primary power alternative for many years to come. But as fuel-cell stacks of high efficiency are developed, they can be used as auxiliary power units (APUs) in tractor-trailer, vocational, or medium-duty trucks that have a lot of accessory equipment and long idle times. For such applications, the main engine running at idle is very inefficient, fuel consumption is high, and emissions are high. Auxiliary power units based on small internal combustion engines are heavy, bulky, and costly.⁴ An interesting alternative is the use of a proton exchange membrane power cell as an APU for a Class 8 truck. The Department of Transportation has a demonstrator unit that uses methanol and an onboard reformer to generate the means for powering a truck's accessories and refrigeration unit overnight. It is claimed that with such an APU, 1 gallon of methanol could replace 11 gallons of diesel fuel. The reason for these outstanding savings is that the idle operation of a diesel engine is very inefficient, while a small fuel cell operates at high efficiency.