plug-in hybrid electric vehicle (PHEV)

A plug-in hybrid electric vehicle (PHEV) is a type of hybrid electric vehicle that combines a gasoline or diesel engine with an electric motor and a large battery that can be recharged by plugging into an electrical outlet or electric vehicle charging station. Conventional hybrid automobiles have an electric motor and battery, but derive all their power from gasoline or diesel.

Plug-in hybrids typically can run in at least two modes: "all-electric," in which the motor and battery provide all the car's energy; and "hybrid," in which both electricity and gasoline are employed. Some PHEVs can travel more than 70 miles on electricity alone.

Plug-in hybrids provide the fuel- and cost-efficiency of hybrid models along with the all-electric capabilities of battery-electric or fuel-cell vehicles. Some PHEVs can travel more than 70 miles on electricity alone and under typical driving conditions, store enough electricity to cut their gasoline use. PHEVs use approximately 30 to 60 percent less gasoline than conventional vehicles, potentially saving the owner hundreds of dollars a year, according to the U.S. Department of Energy (DoE).

How PHEVs Work

In addition to plugging into an outside electric power source, PHEV batteries can be charged by an internal combustion engine or regenerative braking. (During braking, the electric motor acts as a generator, using the energy to charge the battery.) The electric motor supplements the engine's power; as a result, smaller engines can be used, increasing the car's fuel efficiency without compromising performance.

PHEVs typically start up in all-electric mode and operate on electricity until their battery pack is depleted. Some models shift to hybrid mode when they reach highway cruising speed, generally above 60 or 70 miles per hour. Once the battery is empty, the engine takes over and the vehicle operates as a conventional, non-plug-in hybrid.

There are two basic plug-in hybrid configurations, according to the DoE:

In series plug-in hybrids, or extended-range electric vehicles, only the electric motor turns the wheels and the gasoline engine generates electricity. Series plug-ins can run on electricity alone until the battery runs down. The gasoline engine then generates electricity to power the electric motor. These vehicles might use no gasoline at all for short trips.

In parallel, or blended plug-in hybrids, both the engine and electric motor are connected to the wheels and propel the vehicle under most driving conditions. The electric-only operation usually occurs only at low speeds.

PHEV Costs and Benefits

Although the electric motor and battery help plug-in hybrids use less fuel and produce less pollution than conventional cars, even when in hybrid mode, fuel consumption depends on the distance driven between battery charges. If the vehicle is never plugged in to charge, fuel economy will be about the same as a similarly sized hybrid electric vehicle. If the vehicle is driven a shorter distance than its all-electric range and plugged in to charge between trips, it may use only electric power.

Plug-in hybrids have a significantly lower carbon footprint than their gas-only counterparts for two reasons: PHEVs can run on electricity from the power grid, and electricity is typically cleaner than gasoline or diesel fuel. Plug-in hybrids don't emit tailpipe pollution when driving on electricity; plus, having an electric motor and battery is fuel-efficient.

Although plug-in hybrids typically emit less greenhouse gas than conventional vehicles, the amount generated depends partly on how the electricity is produced. For example, nuclear and hydroelectric plants are cleaner than coal-fired power plants, according to the DoE.

Additionally, although using electricity is usually cheaper than using gasoline, fuel savings may not always offset the higher PHEV cost (approximately $4,000 to $8,000), according to the DoE. Fuel savings depend on the vehicle, how many miles it operates using electricity and fuel costs. Plug-in hybrid cars purchased in 2010 or later may be eligible for federal tax credits of up to $7,500; state and local incentives may also apply.

Re-charging a PHEV is time-consuming – several hours using a 120-volt outlet and 1 to 4 hours using a 240-volt charger. A "fast charge" to 80 percent capacity may take as little as 30 minutes, according to the DoE.