Tuesday, October 30, 2007

Plug-in hybrid

A plug-in hybrid electric vehicle (PHEV) is a hybrid vehicle with batteries that can be recharged by connecting a plug to an electric power source. With an internal combustion engine and batteries for power, they share the characteristics of conventional hybrid electric vehicles and battery electric vehicles. Most PHEVs on the road today are passenger cars, but there are now PHEV versions of certain commercial passenger vans, utility trucks, school buses, scooters, and military vehicles as well. PHEVs are sometimes called grid-connected hybrids, gas-optional hybrids, or GO-HEVs.

The cost for electricity to power plug-in hybrids for all-electric operation in the U.S. state of California has been estimated as less than one fourth the cost of gasoline. Compared to conventional vehicles, PHEVs can help reduce air pollution and dependence on petroleum, and lessen greenhouse gas emissions that cause global warming. Plug-in hybrids use no fossil fuel during their all-electric range if their batteries are charged from renewable energy sources. Other benefits include improved national energy security, fewer fill-ups at the filling station, the convenience of home recharging, opportunities to provide emergency backup power in the home, and vehicle to grid applications.

As of September 2007, plug-in hybrid passenger vehicles are not yet in production. However, Toyota, General Motors, Ford, and Chinese automaker BYD Auto have announced their intention to introduce production PHEV automobiles. Toyota obtained permission in July 2007 to sell their plug-in Prius in Japan, BYD Auto expects to introduce their PHEV-60 sedan in the second half of 2008, and General Motors expects to introduce plug-ins in 2009 or 2010. Conversions of production model hybrid vehicles are available from conversion kits and conversion services. Most PHEVs on the road in the U.S. are conversions of 2004 or later Toyota Prius hybrid cars, which extend their electric-only range and add plug-in charging.

PHEVs are based on the same three basic powertrain architectures as conventional hybrids:

Series hybrids use an internal combustion engine (ICE) to turn a generator, which in turn supplies current to an electric motor, which then rotates the vehicle’s drive wheels. A battery or capacitor pack, or a combination of the two, can be used as a buffer of sorts to store excess charge. Examples of series hybrids include the Renault Kangoo Elect'Road, Toyota's Japan-only Coaster light-duty passenger bus, DaimlerChrysler's hybrid Orion bus, the Chevrolet Volt concept car, and many diesel-electric locomotives. With an appropriate balance of components this type can operate over a substantial distance with its full range of power without engaging the ICE. As is the case for other architectures, series hybrids can operate without plugging in as long as there is liquid fuel in the tank.

Parallel hybrids, such as Honda's Insight, Civic, and Accord hybrids, can simultaneously transmit power to their drive wheels from two distinct sources—for example, an internal-combustion engine and a battery-powered electric drive. Although most parallel hybrids incorporate an electric motor between the vehicle's engine and transmission, a parallel hybrid can also use its engine to drive one of the vehicle's axles, while its electric motor drives the other axle. The Audi Duo plug-in hybrid concept car is an example of this type of parallel hybrid architecture. Parallel hybrids can be programmed to use the electric motor to substitute for the ICE at lower power demands and to substantially increase the power available to a smaller ICE than would normally be used, either mode substantially increasing fuel economy compared to a simple ICE vehicle.

Series-parallel hybrids have the flexibility to operate in either series or parallel mode. Hybrid powertrains currently used by Ford, Lexus, Nissan, and Toyota, which some refer to as “series-parallel with power-split,” can operate in both series and parallel mode at the same time. As of 2007, most plug-in hybrid conversions of conventional hybrids utilize this architecture.

Modes of operation
Regardless of its architecture, a plug-in hybrid may be capable of charge-depleting and charge-sustaining modes. Combinations of these two modes are termed blended mode or mixed-mode. These vehicles can be designed to drive for an extended range in all-electric mode, either at low speeds only or at all speeds. These modes manage the vehicle's battery discharge strategy, and their use has a direct effect on the size and type of battery required:

Charge-depleting mode allows a fully charged PHEV to operate exclusively (or depending on the vehicle, almost exclusively, except during hard acceleration) on electric power alone until its battery state of charge is depleted to a predetermined level, at which time the vehicle's internal combustion engine or fuel cell will be engaged. This period is the vehicle's all-electric range. This is the only mode that a battery electric vehicle can operate in, thus their limited range.[26]

Charge-sustaining mode is used by production hybrid vehicles (HEV) today, and combines the operation of the vehicle's two power sources in such a manner that the vehicle is operating as efficiently as possible without allowing the battery state of charge to move beyond some predetermined narrow band. Over the course of a trip in a HEV the state of charge may fluctuate but will have no net change. The battery in a HEV can thus be thought of as an energy accumulator rather than a fuel storage device. Once a plug-in hybrid has exhausted its all-electric range in charge-depleting mode, it can switch into charge-sustaining mode automatically.

The redesigned Renault Kangoo Elect'road operates in blended mode, using engine and battery power simultaneously.Blended mode is a type of charge-depleting mode normally employed by vehicles which do not have enough electric power to sustain high speeds without the help of the internal combustion portion of the powertrain. A blended control strategy typically takes more miles to use stored grid electricity than a charge-depleting strategy.[28] The Renault Kangoo and some Toyota Prius conversions are examples of vehicles that use this mode of operation. The Electri'cité and Elect'road versions of the Kangoo were charge-depleting battery electric vehicles: the Elect'road had a modest internal-combustion engine (ICE) which extended its range somewhat. 2004 and later model Toyota Prius conversions can only run without using the ICE at speeds of less than about 42 mph (68 km/h) due to the limits dictated by the vehicle's powertrain control software. However, at faster speeds electric power can still be used to displace gasoline thus improving the fuel economy in blended mode, generally doubling the fuel efficiency.

Mixed mode describes a trip in which a combination of the above modes are utilized.[29] For example, a PHEV-20 Prius conversion may begin a trip with 5 miles (8 km) of low speed charge-depleting, then get onto a freeway and operate in blended mode for 20 miles (32 km), using 10 miles (16 km) worth of all-electric range at twice the fuel economy. Finally the driver might exit the freeway and drive for another 5 miles (8 km) without the internal combustion engine until the full 20 miles (32 km) of all-electric range are exhausted. At this point the vehicle can revert back to a charge sustaining-mode for another 10 miles (16 km) until the final destination is reached. Such a trip would be considered a mixed mode, as multiple modes are employed in one trip. This contrasts with a charge-depleting trip which would be driven within the limits of a PHEV's all-electric range. Conversely, the portion of a trip which extends beyond the all-electric range of a PHEV would be driven primarily in charge-sustaining mode, like a conventional hybrid.

Further information: Battery electric vehicle
PHEVs typically require deeper battery charging and discharging cycles than conventional hybrids. Because the number of full cycles influences battery lifetime, battery life may be less than in traditional HEVs which do not deplete their batteries as deeply. However, some authors argue that PHEVs will soon become standard in the automobile industry. Design issues and trade-offs concerning battery life (they would last ten years[citation needed]), capacity, heat dissipation, weight, costs, and safety need to be solved. Advanced battery technology is under development, promising greater energy densities by both mass and volume, and battery life expectancy is expected to increase.

The cathodes of some early 2007 lithium-ion batteries are made from lithium-cobalt metal oxide. This material is expensive, and cells made with it can release oxygen if its cell is overcharged. If the cobalt is replaced with iron phosphates, the cells will not burn or release oxygen under any charge. The price premium for early 2007 conventional hybrids is about US$5000, some US$3000 of which is for their NiMH battery packs. At early 2007 gasoline and electricity prices, that would break even after six to ten years of operation. The conventional hybrid premium could fall to US$2000 in five years, with US$1200 or more of that being cost of lithium-ion batteries, providing a three-year payback. The payback period may be longer for plug-in hybrids, because of their larger, more expensive batteries.

Nickel-metal hydride and lithium-ion batteries can be recycled; Toyota, for example, has a recycling program in place under which dealers are paid a US$200 credit for each battery returned. However, plug-in hybrids typically use larger battery packs than comparable conventional hybrids, and thus require larger resource flows. Recently PG&E has suggested that utilities would purchase used batteries for backup and load levelling purposes. They state that while these used batteries may be no longer usable in vehicles, their residual capacity still has significant value.

Électricité de France and Toyota are installing recharging points for PHEVs in France, on roads, streets and parking lots.. EDF is also partnering with Elektromotive, Ltd.[38] to install 250 new charging points over the next six months in London and elsewhere in the UK. Recharging points also can be installed for specific uses, as in taxi stands.

Conversions of production hybrids

15 lead-acid batteries, PFC charger, and regulators installed into WhiteBird, a PHEV-10 conversion of a Toyota PriusFor more details on this topic, see Electric vehicle conversion.
Conversion of an existing production hybrid to a plug-in hybrid typically involves increasing the capacity of the vehicle's battery pack and adding an onboard AC-to-DC charger. Ideally, the vehicle's powertrain software would be reprogrammed to make full use of the battery pack's additional energy storage capacity and power output.

Many early plug-in hybrid electric vehicle conversions have been based on the 2004 or later model year Toyota Prius. Some of the systems have involved replacement of the vehicle's original Ni-MH battery pack and its electronic control unit. Others, such as Hymotion as well as builders of the CalCars Prius+, and the PiPrius, piggyback an additional battery back onto the OEM battery pack, this is also referred to as Battery Range Extender Modules (BREMs). This has been referred to as a "hybrid battery pack configuration" within the electric vehicle conversion community. Early lead-acid battery conversions by CalCars demonstrated 10 miles (15 km) of EV-only and 20 miles (30 km) of double mileage blended mode range.

EDrive Systems use Valence Technology Li-ion batteries and have a claimed 40 to 50 miles (64 to 80 km) of electric range. Other companies offering plug-in conversions or kits for the Toyota Prius include Hymotion, Hybrids Plus, and Manzanita Micro.

The EAA-PHEV project was conceived in October of 2005 to accelerate efforts to document existing HEVs and their potential for conversion into PHEVs. It includes a Conversion Interest page. The Electric Auto Association-PHEV "Do-It-Yourself" Open Source community's primary focus is to provide general information to curious parties and detailed conversion instruction to help guide experienced EV Converters through the process, including public conversions, lasting about two hours per car. Many members of organizations such as CalCars and the EAA as well as companies like Hybrids Plus, Hybrid Interfaces of Canada, and Manzanita Micro participate in the development of the project.

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