How Do Plug-in Hybrid Electric Vehicles Work?

hcc02121A plug-in hybrid electric vehicle (PHEV) is a specially designed hybrid vehicle that uses both batteries and an internal combustion engine (ICE) to power the drive train. Typically, a PHEV can operate in all-electric mode until its batteries are exhausted. Once the battery pack has been drained, the ICE engages. Normally, the ICE does not power the drive train directly. Instead, it powers a generator that produces electricity to charge the batteries or provide power to the vehicle's electric motors. A PHEV can also use household current to charge its battery pack. Quick charge features provide a less-than-complete charge for the battery. A full charge may take 4-8 hours when household current is used.

For the most part, a PHEV uses the same technology that standard hybrid electric vehicles use, with a few exceptions. The battery pack on a PHEV is larger, and the hybrid system has been modified to accept a charge from household current. The hybrid drive train contains one or more electric motors, an internal combustion engine and a large battery bank. Electronic controls in the vehicle control the rate of battery charging and discharging, and also control the operation of the internal combustion engine.

A few manufacturers have introduced – or plan to introduce – diesel electric hybrid vehicles, mostly for the European markets. A diesel engine is more efficient, and offers better overall fuel economy than an internal combustion engine does, but diesel fuel can be significantly more expensive and does not "burn clean." In the past several years, significant improvements to the diesel engine have reduced the particulate emissions of vehicles that use the technology. "Clean" diesel options are becoming more common and are being adapted for use with hybrid electric vehicles. Currently, there are no plug-in diesel-electric hybrids in production.

Statistically, there are an insignificant number of plug-in hybrid electric vehicles in operation today. Most PHEVs in service today are custom modifications of standard hybrid vehicles. Conversion kits for certain vehicles have been created, and kits can range in cost from a few thousand dollars to about $10,000 for the most common conversions.

A large part of the conversion expense is related to the battery needs of a PHEV. Currently, most PHEVs use nickel-metal hydride battery packs. Toyota, which plans to bring a mass produced Prius PHEV to market in the 2010 model year, will initially use a nickel-metal hydride battery pack, but has designed the vehicle with conversion to a lithium-ion battery source at some point in the future. Toyota estimates that the all-electric range of the Prius PHEV will be less than 10 miles.

Chevrolet will bring the Volt to market in late 2010, and plans to use a lithium-ion battery pack with the vehicle. Chevrolet has said that the all-electric range of the Volt will be in the neighborhood of 40 miles. To accomplish this, the Volt uses a 600-V battery pack, which is about twice the size of the most robust conventional hybrid battery pack.

A PHEV never requires household current to charge. If the driver doesn't plug in a PHEV, the car will operate in its HEV mode at all times, relying on the internal combustion engine to supply power to its generator. In contrast, electric cars do require an external charging source for their batteries. Periodically, these vehicles must be charged to remain operational.

Unfortunately for consumers, EV manufacturers have not settled on connector standards. Also unsettled is the issue of current. Some vehicles charge on 220V, while other charge on 110V. Electric vehicles typically come with a number of adapters to enable the use of public charging stations. When an EV charges at home, an EV or PHEV may increase the daily household electrical consumption by 25% or more.

One of the major advantages of a PHEV is its exceptionally low fuel consumption. Over the course of a year, a PHEV may consume as little as one-seventh the amount of gasoline that a conventional vehicle does. For most drivers, the all-electric range of a fully charged PHEV will enable them to complete their daily commute without using any gasoline at all.

What are the major disadvantages of a PHEV? First, the cost of a PHEV is likely to be higher than that of a regular hybrid electric vehicle. Most of the cost differential can be attributed to the increased battery needs of the car. The increased battery requirements also mean increased cost when the battery pack requires replacement. Currently, manufacturers warranty their HEV battery packs for about 8-10 years. Insufficient data exist to know how long mass produced HEV and PHEV battery packs will last, but most hybrid vehicles have not experienced significant battery problems during what would be considered the normal life expectancy of the vehicle.

Industrialized nations have a well-established network of refineries, pipelines, storage facilities, transportation operations and retail sales outlets for gasoline. There is no such supporting infrastructure for PHEVs. Over time, if PHEVs are widely accepted by consumers, municipalities and private companies may invest in the construction of a charging infrastructure for PHEVs. Without a standardized charging system in place, the primary benefits of PHEVs will be limited to short-distance travel.

A third disadvantage of the PHEV is that the impact on the nation's electrical infrastructure is not yet clear. If PHEVs are widely accepted by consumers, the increased load on the electrical grids could be significant. Meeting the new demand for electricity could require the construction of new power plants, many of which would be coal-fired. Electricity produced by coal-fired power plants will merely shift the carbon emissions from the tailpipe to the smokestack, and will not produce significant reductions in greenhouse gas emissions. Achieving this goal through the use of PHEVs would also require the construction of clean power plants. These new plants would be fired by more expensive natural gas or would use nuclear power, a technology that has not yet proven its safety in the minds of the public.

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