Learn more about Hybrid vehicle</div> </div>
- For the common automotive term of hybrid vehichle see: Petroleum electric hybrid vehicle
A hybrid vehicle (HV) is a vehicle that uses two distinct power sources such as :
- an on-board rechargeable energy storage system (RESS) and a fueled power source for vehicle propulsion
- Human powered bicycle with battery assist
The term hybrid when used in relation with cars also has other uses. Prior to its modern meaning of hybrid propulsion, the word hybrid was used in the United States to mean a vehicle of mixed national origin; generally, a European car fitted with American mechanical components. This meaning has fallen out of use. In the import scene, hybrid was often used to describe an engine swap. Some have also referred to flexible-fuel vehicles as hybrids because they can use a mixture of different fuels — typically gasoline and ethanol alcohol fuel.
 Two-wheeled vehicles
Mopeds and Power-assisted bicycles can be considered hybrid vehicles in a sense, because power is delivered both via a conventional or electric motor and the rider's muscles (see also Electric bicycle).
There are many ways to create an electric-internal combustion 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.
One potentially interesting 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 levels 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.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.
 Fuel cells
Some fuel cell-powered vehicles currently in development use some hybrid-like technology to store auxiliary energy. Like diesels above and steam power outlined below, fuel cells are best at delivering a fairly constant flow of electricity, so having a secondary system is helpful. In some cases, batteries have been replaced with ultracapacitors, which can store and retrieve energy quickly, but are inappropriate for long-term electrical storage.
 Turbines and steam engines
From the 1950s to the 1970s Chrysler created several turbine-powered vehicles, though only small numbers were produced; they had complex drivetrains and achieved relatively slow starting speeds, with effects reminiscent of "turbo lag", but demonstrated that turbines could be used for automobiles (see Chrysler Turbine engines). Also, in late 1990s General Motors made a gas turbine series hybrid prototype based on now-cancelled EV1 electric vehicle. At present, no current or announced mass-market car is driven by a gas turbine or a steam engine, but hybrid technology could bring back gas turbines and the steam-powered car.
Both gas turbines and steam turbines (see below) are lighter than reciprocating steam and internal combustion engines, respectively, and more efficient than the corresponding reciprocating types when operating at their optimum power output. On the other hand, they have very limited optimum power output ranges, and must be used with electric drive or some other sort of transmission. Operation of turbines outside of their optimum power output ranges drastically reduces their efficiency. This is not prohibitive for a ship or aircraft that is mostly operated at very constant power output, or for a power plant containing many turbines that can be put on-line or off-line as needed to match load, but has resulted in near-eradication of turbine engines from land vehicles.
In the early 20th century, cars made by the Stanley Steamer Company with reciprocating steam engines did compete successfully with the internal combustion engine. Reciprocating steam engines have a much larger range of operating speeds than do internal combustion engines, including the ability to produce full torque at stall, thus eliminating the need for a transmission; however, they have not been able to compete with internal combustion for land vehicles for several reasons:
- Lower thermal efficiency possible with today's materials — a heat engine such as an internal combustion engine or steam has efficiency limited by its Carnot cycle temperature differential. A steam engine must transfer combustion heat through the material of the boiler, which therefore must be able to withstand the heat, while an internal combustion engine can bypass this limitation by having the piston and cylinder materials always remain at much less than the combustion temperature (at the cost of some loss of efficiency due to unwanted cooling of the combustion gas).
- Longer warm-up time and slow throttle response — this is no great problem for trains and ships which are restricted from quick acceleration by their huge mass and which also generally have predictable demand for power, but is a challenging issue for automobiles, trucks, and buses
- More complex controls — the driver of a Stanley Steamer had to keep a close eye on several pressure and temperature gauges while driving (on the other hand, with modern computers, much of this could be handled automatically)
Gas turbine (or other internal combustion engine), steam turbine, and hybrid technology could be combined to alleviate the disadvantages of gas turbines and steam engines while retaining most of their advantages. In combined cycle power plants, gas turbines drive generators, and their exhaust is used to generate steam for steam turbines, thus recovering some of the energy from the heat of the exhaust that would otherwise be wasted. This principle can be used in vehicles, and is currently in use in ships as COGAS or COGES, although the only public proposal for such technology in an automobile uses a conventional internal combustion engine for this purpose instead of a gas turbine (a configuration that has also seen use on ships). A combined cycle gas turbine/steam turbine (or internal combustion engine/steam turbine) set could be combined with hybrid technology to allow the combined cycle system to operate at its most efficient power output. The energy storage system would store energy from the combined cycle system when its output exceeds propulsion requirements and provide energy to the propulsion system when propulsion requirements exceed the combined cycle system output, including combined cycle system startup. The energy storage system would need to have an especially high capacity to work well with a combined cycle system, since the combined cycle system would operate inefficiently during startup and shutdown; therefore, the energy storage system would need to support long intervals between combined cycle startup and shutdown.
 Hybrid fuel
In addition to vehicles that use two or more different devices for creating motive power, some also consider vehicles that use distinct energy input types (fuels) to be hybrids, although to avoid confusion with hybrids as described above, these are better described as dual mode vehicles:
- Some electric trolleybuses can switch between an onboard diesel engine and overhead electrical power depending on conditions (see dual mode bus. In principle, this could be combined with a battery subsystem to create a true plug-in hybrid trolleybus, although as of 2006, no such design seems to have been announced.
- Flexible-fuel vehicles can use a mixture of input fuels in one tank — typically gasoline and ethanol, though diesel-biodiesel vehicles would also qualify. Liquified petroleum gas and natural gas are very different to each other and cannot be used in the same tanks, so it would be impossible to build an (LPG-NG) flexible fuel system.
- Some vehicles have been modified to use another fuel source if it is available, such as cars modified to run on autogas (LPG) and diesels modified to run on waste vegetable oil that has not been processed into biodiesel.
- Power-assist mechanisms for bicycles and other human-powered vehicles are also included.
 Human power
Motorized bicycles use human pedal power and an attached motor. Some bicycle conversion kits aided popularisation of "hybrid" vehicle bicycles that used electric hub motors (such as Bionx<ref>Template:Cite web</ref> and Wilderness Energy<ref>Template:Cite web</ref>), internal combustion engines (such as the 1940s "Pixie" bicycle motor), and pedal power. Such machines include electric bicycles and mopeds, which may often be simultaneously propelled by human and engine power. More sophisticated constructions are three wheeled and provide at least a windscreen (ZAP EPOD, TWIKE).
 See also
 External links
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