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Hybrid vehicles most commonly use engines and electric batteries to power electric motors. Modern mass-produced hybrids prolong the charge on their batteries by capturing kinetic energy via regenerative braking. Many hybrids shut down the combustion engine at idle, and re-start the combustion engine when needed. As well, when cruising or in other situations where just light thrust is needed, "full" hybrids can use the combustion engine to generate electricity by spinning an electrical generator (often a second electric motor) to either recharge the battery or directly feed power to an electric motor that drives the vehicle. This contrasts with all electric cars which use batteries charged by an external source such as the grid, or a range extending trailer. Nearly all hybrids still require gasoline and diesel as their sole fuel source though other fuels such as ethanol or plant based oils have also seen occasional use.

Types of hybrid vehicle

There are many ways to create an electric-internal combustion (ICE) hybrid. The variety of electric-ICE designs can be differentiated by the structure of the powertrain, the degree of hybridization and the mode of operation. The main categories are series hybrids and parallel hybrids, with combined hybrids having common characteristics of series and parallel designs.

Hybrids other than electric-internal combustion exist, for example hydraulic and pneumatic hybrids, where compressed fluids and compressed air, respectively, are used for energy storage with regenerative braking.

Engines and Fuel Sources

Gasoline engines are used in most hybrid 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, hybrids 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

One type of hybrid vehicle combination uses 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 performance in a car of over 100 mpg US (2.35 litres/100 km). Most diesel vehicles can use 100% pure biofuels (biodiesel), so they can use but do not need petroleum at all; if diesel-electric hybrids were in use, this benefit would likely also apply.

Diesel-electric hybrids with parallel drivetrains like the Prius may have a substantial cost disadvantage to other options. Diesel engines are generally more expensive than gasoline equivalents, due to the demands for higher compression (although this also makes diesels more durable). If this "diesel premium" is added to any additional expense for the hybrid, the diesel-electric combination may make the payback period for such vehicles even longer and less feasible for many consumers. In addition, the higher torque of diesel engines may obviate one of the advantages of the electric motors. As with regular diesel engines, diesel-electric hybrids may be more appropriate for high-mileage, intensive-use applications, such as buses, trucks, and delivery vehicles, and less appropriate for passenger vehicles. In addition, regular diesel vehicles may get similar mileage to gasoline-electric hybrids, for a smaller premium, and the marginal benefit of "hybridization" may not be viable.

Diesels are not widely used for passenger cars in the United States, as US diesel fuel has long been considered very "dirty", with relatively high levels of sulfur and other contaminants in comparison to the Eurodiesel fuel in Europe, where greater restrictions have been in place for many years. Despite the dirtier fuel at the pump, the US has tough restrictions on exhaust, and it has been difficult for car manufacturers to meet emissions requirements as higher sulfur levels are damaging to catalytic converters and other emission control systems. However, ultra-low sulfur diesel was mandated and became widely available in the U.S. in October 2006 for highway vehicles, which will allow the use of newer emissions control systems.

Diesel-electric motors are common for use as locomotives, but using a serial hybrid design. In locomotives, the diesel engine is used to generate electricity for the electric drivetrain. This configuration allows the internal combustion engine to be operated at more efficient operating parameters, while removing the need for a separate transmission for the ICE unit and allowing the efficient delivery of torque from the electric motors. Such a system may need a smaller diesel engine and allow for better emissions controls, since the operating range of the diesel engine would be optimized for electric generation rather than power delivery through the mechanical transmission and wheels. There have been studies of this type of diesel-electric hybrid, but there are no confirmed attempts to commercialize such a vehicle for passenger use.

PSA Peugeot Citroën has unveiled two demonstrator vehicles featuring a diesel-electric hybrid powertrain: the Peugeot 307 and Citroën C4 Hybride HDi (PDF). VW made a prototype diesel-electric hybrid car that achieved 2 litres/100 km (118 mpg US) 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.

So far, production diesel-electric engines have mostly just appeared in mass transit buses. Current manufacturers of diesel-electric hybrid buses include New Flyer Industries, Gillig, Orion Bus Industries, and North American Bus Industries. In 2008, NovaBus will add a diesel-electric hybrid option as well.

Benefits

Benefits of the hybrid design include:

• Current hybrid vehicles reduce petroleum consumption (compared to otherwise similar ICE vehicles) primarily by using three mechanisms: a) Reducing wasted energy during idle/low output, generally by turning the internal combustion engine 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 internal combustion engine.
• Hybrids may also make more aggressive use of other fuel-saving techniques, such as reduced weight; these are not advantages of the hybrid design, but engineering choices made for various reasons, including marketing to consumers conscious of these issues.
• Trade-offs include higher weight for electric motors and batteries, which may reduce fuel efficiency at highway speeds compared to otherwise equivalent ICE vehicles, or even result in lower fuel efficiency at highway speeds than in urban use; for this reason, hybrids may be considered to be particularly well suited to urban applications.
• The internal-combustion engine in a hybrid vehicle 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 a hybrid vehicle, the combustion engine 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.
• Like many electric cars, but in contrast to conventional vehicles, braking in a hybrid is controlled in part by the electric motor which can recapture part of the kinetic energy of the car to partially recharge the batteries. This is called regenerative braking and contributes to the higher efficiency of hybrid cars. In a conventional vehicle, braking is done by mechanical brakes, and the kinetic energy of the car is wasted as heat.
• Hybrids' greater fuel economy has implication for reduced petroleum consumption and vehicle air pollution emissions worldwide
• Reduced wear on the gasoline engine, particularly from idling with no load.
• Reduced wear on brakes from the regenerative braking system use.
• Reduced noise emissions resulting from substantial use of electric motor at low speeds, leading to roadway noise reduction and beneficial noise health effects. 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. Composite driving tests indicate total air pollution of carbon monoxide and reactive hydrocarbons are 80 to 90 percent cleaner for hybrid versus conventional vehicles. Pollution reduction in urban environments may be particularly significant due to elimination of idle-at-rest.
• Increased driving range without refueling or recharging, compared with electric vehicles and perhaps even compared with internal-combustion vehicles. Limitations in range have been a problem for traditional electric vehicles. Hybrids may have substantially longer "operating hours" per unit of petroleum in certain conditions than the mileage-rated fuel efficiency figures may indicate, due to the reduction of idle-at-rest.