Military trucks, transit buses, taxis and compact cars have different performance requirements. So you might think that designing their electrical systems would require different approaches, particularly when the vehicles are hybrid electrics (HEVs) with their complex power systems. However, these different types of HEVs have much in common, electrically speaking, according to Vincent P. Socci, chief engineer of On-Target Technology Development, in Endicott, NY.
In his paper, “System Design Considerations for Vehicle-based Mobile Electric Power Applications,” Socci spells out strategies for developing vehicle power systems architecture, power conversion, distribution, power management, and tests that can be applied in all types of hybrid designs. In the paper, the author draws on his experience in designing vehicle power systems for hybrid military trucks and commercial buses. (Socci's paper was presented in the Automotive Applications session at the recent Power Electronics Technology conference in Baltimore.)
“There are common problems regardless of platform, whether it's a hybrid military vehicle, HEV bus or Toyota Prius,” says Socci. “They have different concerns, but some of the solutions are similar.” Socci points out that there are similarities in electrical protection and interlocks across vehicle platforms. Even power sources are similar, whether the system uses a small ICE or a large diesel engine. Other issues such as grounding and shielding, and concerns about component size and weight, can also cut across different vehicle platforms.
Socci notes that there are two factors that influence the design of HEV power systems. One is the selection of energy storage components, which requires trade offs of the components' strengths and weaknesses.
For example, Li-ion batteries have excellent energy storage characteristics, but have safety issues that must be accounted for. Lead acid batteries are proven, safe, cheap and easy to maintain, but have low energy density. Ultracapacitors are long-lasting components that can be quickly charged and discharged, but are relatively pricey and offer limited long-term energy storage.
A second factor is the way the flow of energy and power is managed through the vehicle. Control algorithms tailored to the application determine how the electrical energy is stored and distributed among power sources and loads during various operating conditions.
These cornerstones of power system design are influenced by the design goals of the type of vehicle. Socci notes that with commercial HEVs, the main objective is high fuel efficiency, with cost another important consideration, and vehicle performance less of a factor. Contrast that with a military truck hybrid, which puts a higher premium on performance and reliability, and less on fuel efficiency.
“In commercial automotive, it's about efficiently getting energy to the wheels,” says Socci, “but that's just one part of the design in military vehicles where the goal is meeting the mission requirements once the vehicle arrives at its destination.” As Socci explains, military HEVs need fuel efficiency, but the main driver is demand for auxiliary power to run communications and other equipment.
Having auxiliary power in the vehicle may mean that military personnel no longer need to tow a generator or that equipment can be powered on the go. It also may provide backup power for equipment that required a UPS. Aside from those uses, a military hybrid has the ability to shut off its engine, run off batteries, and drive in an ultraquiet “stealth mode.”
In military hybrids, the power system components will need to be sized for higher worst-case loading conditions than would be demanded in commercial hybrids, where load shedding may be more acceptable.
“The military applications turn out to be more robust and higher performance,” says Socci who attributed those requirements to the military's broad user base, which translates into greater demands for electrical power. Then there's the mission-critical nature of the application. If a battery bank fails, you can't allow the vehicle to be stranded in a war zone.
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