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The depletion of natural resources, air pollution, traffic congestion, and the rising price of fossil fuels are all issues driving communities and individuals to search for alternative means of transportation. Some of these options are hybrid-electric, hydrogen, and fuel-cell-based drive systems for buses and trucks, such as those produced by ISE (San Diego, CA) since 1996.
These drive systems are gaining favor throughout the automotive industry because they improve fuel economy and reduce harmful emissions. ISE specializes in production of series hybrid-electric drive systems, where the engine is completely decoupled from the driveline and is used only to generate electrical power.
Photo: ISE
This type of hybrid architecture is especially attractive for large vehicles that perform large amounts of stop-and-go driving, such as urban transit buses and delivery trucks. Conventional buses and trucks are highly inefficient and produce high levels of toxic emissions because they have large (typically diesel) engines that are constantly ramping up and down—the least efficient way to operate a power source.
In the ISE series hybrid system, a smaller engine is mated to a generator and operated at a constant, efficient speed and power output level. When vehicle power requirements temporarily increase—such as during acceleration or hill-climbing—additional power is drawn from an onboard energy storage system comprised of batteries and ultracapacitors. When vehicle power requirements are low, the energy storage system is recharged. Not only is engine efficiency increased, but also the vehicle is able to recapture energy whenever it slows down through regenerative braking.
The problem
ISE's Hybrid Electric Vehicle (HEV) technology aims to leverage the best characteristics of fuel-driven engines, electric motor drives, and energy storage components. Their hybrid architecture is designed with a combustion engine that functions as the primary power source, and an electric motor with a power storage system that functions as the secondary power source. Design engineers are able to size the combustion engine for cruising power requirements, thanks to the presence of the secondary power source that handles peak power demands for acceleration. Additionally, regenerative braking energy is captured by the secondary power system. That energy is applied for further acceleration or for the basic energy needs of supplementary electrical systems.
Drawbacks
Standard hybrid electric designs, which use only batteries to provide electrical power storage, have drawbacks. These deficiencies are multiple, and they create many design challenges for automotive engineers. Firstly, batteries have difficulty functioning in cold weather. Secondly, batteries require sophisticated charge equalization management. Thirdly, batteries have limited cycle life under extreme conditions, which results in high-cost replacement throughout the life of the vehicle.
A new battery has to be purchased and installed; the old battery has to be removed and disposed. Battery disposal can be problematic unless the manufacturer has a recycling program. All of this adds to the cost of a battery-based system, not to mention downtime of the vehicle.
Perhaps most importantly though, batteries are limited in their ability to capture and regenerate energy, or provide bursts of high power during short duration events, such as acceleration and braking. This high power limitation reduces the efficiency of the hybrid electric drive system design. Because most city buses are in a constant brake-acceleartion mode, the ability to capture and regenerate braking energy was central to ISE’s design.
The limiting factors of the battery proved a particular challenge in ISE engineers’ goal of designing highly efficient bus and truck systems that would overcome traditional inefficiencies while delivering the expected rugged, all-weather, stop-and-go traffic capability. To bring a truly alternative solution to market, ISE would need to design an electric power and storage system that overcomes the limitations of hybrid electric batteries as well as those of conventional trucks and buses.
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