HEVs typically consist of an electrical storage device, such as a battery, flywheel, or an ultracapacitor. They also combine this energy storage source with a mechanical device, like an internal-combustion engine (ICE), gas turbine, or a fuel cell. This combination reduces both fuel consumption and tailpipe emissions. In general, hybrids capture energy lost during braking and return it to the on-board battery. This process is termed regenerative braking.
The nature of HEV configuration enables several important advantages over pure electric vehicles (EVs). Because the HEV engine shares the workload with the electric motor, it can be constructed smaller. This reduction in size engenders weight reductions, leading to greater fuel economy. Also, HEV engines can be optimized to operate within a specific speed range characterized by better fuel economy and reduced emissions. This allows HEVs to eliminate the higher emissions and poor fuel economy associated with conventional ICE vehicles.
While all hybrid-electric vehicles require a hybrid power unit (HPU), there are a number of options open to automobile manufacturers. The most commonly employed HPU is the combustion engine. Optional HPU technologies include the compression-ignition/direct-injection (CIDI) engine, the spark-ignition/ direct-injection (SIDI) engine, the stirling engine, and the gas turbine engine.
Providing greater driving range than systems that use only batteries, HEVs utilize liquid fuels, including gasoline, diesel, biodiesel, methanol, and ethanol, or gaseous fuels, including natural gas and liquefied petroleum gas, in addition to battery power. Although the advantages of dual power result in the increased range of HEVs over pure EVs, the increased complexity of the hybrid vehicle configuration somewhat offsets these benefits. Two such drawbacks are additional cost and increased emissions from the nonelectric portion of the fuel.
While the environmental benefits of an HEV depend on the design of the power system, emission levels are lower than those of typical combustion vehicles even in the worst cases. The reduced emissions of HEVs result from the nature of the HEV genset (engine/generator system). Either the HEV genset is switched off, thereby producing zero emissions, or it operates at a predetermined output where it produces the lowest emissions and achieves the best fuel economy per unit of output. In general, the hybrid genset isn't throttled for variable output, as is the engine of conventional vehicles. Because it's technically easier to control combustion-engine emissions when the engine runs continuously at a constant output, HEVs offer more effective emissions control.
One of the most prominent advantages of the HEV over the battery-electric vehicle (BEV) is the inherent bidirectionality of the HEV energy/work loop. The HEV powertrain converts stored energy into vehicle motion. Moreover, it converts vehicle motion back into stored energy through the employment of regenerative braking. Regenerative braking provides vast benefits. An estimated 60% of the total energy consumed in urban driving is spent overcoming the effects of inertia. Theoretically, up to half of this lost energy may be reclaimed by an HEV upon deceleration.
The optimal integration of subsystems united by a comprehensive control structure is the most promising approach to increasing energy efficiency in HEVs. A control strategy is integral to monitoring and balancing the energy flow throughout the vehicle. The proposed future control system will be a self-adaptive, integrated propulsion system that utilizes batteries (or supercapacitors) as an energy reservoir for load levelling. This approach differs from the traditional structure in which batteries merely supply the total vehicle motive power. To a great extent, current research is aimed at developing the most effective control strategy, the most efficient bias between subsystems, and the correct choice of subsystems to minimize hardware, mass, and manufacturing costs.