A hybrid electric vehicle (HEV) is a vehicle which combines a conventional propulsion system with an on-board rechargeable energy storage system (RESS) to achieve better fuel economy than a conventional vehicle without being hampered by range from a charging unit like an electric vehicle. The different propulsion power systems may have common subsystems or components.

Regular HEVs most commonly use an internal combustion engine (ICE) and electric batteries to power electric motors. Modern mass produced HEVs prolong the charge on their batteries by capturing kinetic energy via regenerative braking, and some HEVs can use the combustion engine to generate electricity by spinning an electrical generator (often a motor-generator) to either recharge the battery or directly feed power to an electric motor that drives the vehicle. This contrasts with battery electric vehicles which use batteries charged by an external source. Many HEVs reduce idle emissions by shutting down the ICE at idle and restarting it when needed. An HEV's engine is smaller and may be run at various speeds, providing more efficiency.

Engines and fuel sources Gasoline engines are used in most hybrid electric designs, and will likely remain dominant for the foreseeable future. While petroleum-derived gasoline is the primary fuel, it is possible to mix in varying levels of ethanol created from renewable energy sources. Like most modern ICE-powered vehicles, HEVs can typically use up to about 15% bioethanol. Manufacturers may move to flexible fuel engines, which would increase allowable ratios, but no plans are in place at present. Diesel-electric HEVs use a diesel engine for power generation. Diesels have advantages when delivering constant power for long periods of time, suffering less wear while operating at higher efficiency. The diesel engine's high torque, combined with hybrid technology, may offer substantially improved mileage. Most diesel vehicles can use 100% pure biofuels (biodiesel), so they can use but do not need petroleum at all for fuel (although mixes of biofuel and petroleum are more common, and petroleum may be needed for lubrication). If diesel-electric HEVs were in use, this benefit would likely also apply. Diesel-electric hybrid drivetrains have begun to appear in commercial vehicles (particularly buses); as of 2007, no light duty diesel-electric hybrid passenger cars are currently available, although prototypes exist. Peugeot is expected to produce a diesel-electric hybrid version of its 308 in late 2008 for the European market. PSA Peugeot Citroën has unveiled two demonstrator vehicles featuring a diesel-electric hybrid drivetrain: the Peugeot 307 and Citroën C4 Hybride HDi.Volkswagen made a prototype diesel-electric hybrid car that achieved 2 L/100 km (118 mpg–U.S. / 141 mpg–imp) fuel economy, but has yet to sell a hybrid vehicle. General Motors has been testing the Opel Astra Diesel Hybrid. There have been no concrete dates suggested for these vehicles, but press statements have suggested production vehicles would not appear before 2009.

Benefits of the hybrid electric design include:

Fuel consumption

Current HEVs reduce petroleum consumption (compared to otherwise similar conventional vehicles) primarily by using three mechanisms: a) Reducing wasted energy during idle/low output, generally by turning the ICE off;
b) Recapturing waste energy (i.e. regenerative braking);
c) reducing the size and power of the ICE engine, and hence inefficiencies from under-utilization, by using the better torque response of electric motors to compensate for the loss in peak power output from the smaller ICE. Any combination of these three primary hybrid technologies may be used for different fuel usage, power, emissions, weight and cost profiles. The ICE in an HEV is smaller, lighter, and more efficient than the one in a conventional vehicle, because the combustion engine can be sized for slightly above average power demand rather than peak power demand. A standard combustion engine is required to operate over a range of speed and power, yet its highest efficiency is in a narrow range of operation; in an HEV, the ICE operates within its range of highest efficiency. The power curve of electric motors is better suited to variable speeds and can provide substantially greater torque at low speeds compared with internal-combustion engines. The greater fuel economy of HEVs has implication for reduced petroleum consumption and vehicle air pollution emissions worldwide

Durability

Reduced wear on the gasoline engine, particularly from idling with no load. Reduced wear on brakes from the regenerative braking system use.

Environmental impact

Reduced noise emissions resulting from substantial use of electric motor at low speeds, leading to roadway noise reduction and beneficial noise health effects(although road noise from tires and wind, the loudest noises at highway speeds fron the interior of most vehicles, are not affected by the hybrid design alone). Note, however, that this is not always an advantage; for example, people who are blind or visually-impaired, and who rely on vehicle-noise while crossing streets, find it more difficult to do safely. Reduced air pollution emissions due to lower fuel consumption, leading to improved human health with regard to respiratory and other illness. Pollution reduction in urban environments may be particularly significant due to elimination of idle-at-rest.