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Figure 1 Plug-in hybrid vehicle and its components.
Figure 1 Plug-in hybrid vehicle and its components.

Almost all hybrid vehicles operating today utilize an auxiliary power unit (APU), such as a small spark ignition engine, a diesel, or a gas turbine to complement the batteries in providing power directly, or by acting as a generator to charge the batteries. The APU can be fueled by gasoline, diesel fuel, or any number of alternative fuels and is operated at relatively steady-state optimum condition to produce very low emissions and high efficiency (Figure 1).

Hybrid vehicles are commonly classified as either series or parallel. The main distinctions between the two configurations are in the way the APU transfers power to the wheel and whether batteries become fully discharged (charge-depleting) or retain the charge by continuous charging (charge-sustaining).

In a series configuration, all motive power comes from the electric motor powered by batteries. The gasoline engine drives a generator to produce electricity, which either supplies power to an electric motor or charges the battery. With series hybrids, there is no mechanical connection between the engine and the wheels; power is transferred electrically to an electric motor that drives the wheels. Because electric motors generate torque that matches requirements at different speeds, series hybrids have a simpler transmissions or no transmission at all (Figure 2a). As a result, modern hybrid-electric drive systems are much lighter, weighing about one third to one half the weight of batteries required for a purely battery-operated electric car.

Figure 2 Series and parallel hybrids.
Figure 2 Series and parallel hybrids.

In the simplest configuration, the vehicle operates as a pure electric vehicle until batteries are depleted to their preset thresholds, at which time the internal combustion engine begins to recharge them. Since the engine does not have to meet changing power demands directly, it can be set to operate in a narrow speed range where it has the highest efficiency, the lowest emissions, or a combination of both. Alternatively, control strategies can be devised where a small IC engine operates continuously at its optimal point so as to keep the battery at or near its full charge. One major advantage of a series hybrid is that it can attain a long range with an engine only a fraction of the size of the conventional engine and with a battery pack weighing far less than those of pure electric vehicles.

In a parallel configuration, the engine, the electric motor, or both supply power to the wheels (Figure 2b). Parallel hybrids are primarily used in electric-only mode for short trips and city driving, whereas long trips and highway cruising are carried out by engine-only operation. The electric motor can be used to help overcome hill climbs, to accelerate quickly, and in instances when the engine cannot single handedly meet the power demand. Because the engine, electric motor, or both must deliver the power, a clutch/transmission assembly is necessary in parallel hybrids.


Air Hybrid Vehicles

Instead of storing the energy as electric energy in a battery in an electric car, air hybrid vehicles work by storing the energy needed to propel the engine in an onboard tank of compressed air. Like a conventional engine, a piston compresses air in a cylinder. As the piston reaches the top, a small amount of compressed air is released into the expansion chamber to create a low pressure, low temperature pressure wave that drives the piston to power the engine. When vehicle brakes, the engine is used as an air compressor to absorb the braking energy and store it into the air tank. The engine is essentially shut off when it stops behind a traffic light. As the car accelerates, more and more air is allowed into the cylinder until the compressed air is depleted. The tank can be refilled in a refill station or by a compressor operated by a conventional internal combustion engine.

Advantages and Disadvantages of Hybrid Vehicles

Hybrid vehicles have advantages over both electric and conventional vehicles by combining features and enhancing performance to a degree not possible using either propulsive system, and without their limitations. For example, ICE performance is a strong function of engine speed, designed to operate optimally at cruising speeds around 90-100 km/hr (roughly 50-60 miles per hour); efficiency drops rapidly at both very high and low speeds. On the other hand, electric motors have their highest efficiency at low speeds, generate no on-board emissions, have favorable torque characteristics, and can utilize regenerative braking. These characteristics make conventional engines ideal for freeway driving and constant speed operation and electric vehicles best suited for city driving and transient conditions.

Another advantage of the hybrid system is that individual components can be sized to fit different driving conditions. Since the engine is not directly coupled to the wheels, engine size can be selected to run near its optimal conditions at all times. In most designs, the engine is sized to meet only the cruising demands, whereas power for acceleration and hill climbing is offered by batteries and other storage devices. The major disadvantages of hybrid vehicles are additional complexity of dealing with two power systems and potentially higher capital cost.


(1) Toossi Reza, "Energy and the Environment:Sources, technologies, and impacts", Verve Publishers, 2005

Further Reading

Tillman, D., Fuels of Opportunity: Characteristics and Uses In Combustion Systems, Academic Press, 2004.

Sorensen, K., Hydrogen and Fuel Cells: Emerging Technologies and Applications, Academic Press, 2005.

Dhameia, S., Electric Vehicle Battery Systems, Academic Press, 2001.

Hussain, I., Electric and Hybrid Vehicles: Design Fundamentals, CRC Press, LLC. 2003.

Jefferson, C.M., and Barnard, R. H., Hybrid Vehicle Propulsion, WIT Press, 2002.

Spelberg, D. The Hydrogen Energy Transition: Moving Toward the Post Petroleum Age in Transportation, Academic Press, 2004.

Fuel, Direct Science Elsevier Publishing Company, Fuel focuses on primary research work in the science and technology of fuel and energy fuel science.

Transportation Research Part C: Emerging Technologies, Direct Science Elsevier Publishing Company; this journal focuses on scholarly research on development, application, and implications in the fields of transportation, control systems, and telecommunications, among others.

Fuel Cells Bulletin, Direct Science Elsevier Publishing Company, Fuel Cells Bulletin is the leading source of technical and business news for the fuel cells sector.

International Journal of Hydrogen Energy, Direct Science Elsevier Publishing Company, Quarterly journal covering various aspects of hydrogen energy, including production, storage, transmission, and utilization, as well as economical and environmental aspects.

External Links

US Department of Transportation (

US Department of Energy (

US Environmental Protection Agency (

National Energy Renewable Laboratory Hybrid Electric &Fuel Cell Vehicles (

FreedomCar (