How Plug-in Hybrid Cars Work

By: Tom Appel
In the next few years, Saturn plans on selling a plug-in version of its hybrid Vue Green Line midsize SUV. See more

Because of their promise of improved fuel economy and reduced tailpipe emissions, vehicles employing hybrid drivetrain technology have drawn the attention of both the media and automotive consumers. There is no question that the technology works, typically providing fuel economy gains of 25-40 percent when compared to similar vehicles with conventional drivetrains.

Looking for ways to further improve fuel-economy, manufacturers are exploring enhancements to the basic hybrid concept. One promising avenue is the plug-in hybrid car. In simplest terms, plug-in hybrid cars incorporate traditional hybrid technology but benefit additionally from plug-in charging.


While several manufacturers claim to be working on the technology, General Motors' Saturn division has promised to introduce a plug-in hybrid version of its Vue SUV by 2009.

Expanding further on plug-in hybrid technology are two vehicles still in the concept stage. The Chevrolet Volt, which was introduced at the 2007 Detroit Auto Show, and the Ford Edge with HySeries Drive, also introduced in 2007, combine elements of plug-in charging with other potentially fuel-conserving technologies.

Many hybrid vehicles conserve additional fuel by incorporating a shut-off system that shuts down the engine during stops. In these cars and trucks the electric motor alone may propel the vehicle for short distances.

The following article will explain exactly how plug-in hybrid cars work. It also will examine some of the benefits of plug-in hybrid cars.


Plug-in Hybrids Explained

Current hybrid vehicles have a gasoline engine that is assisted by an electric motor, which helps it save gas. A typical hybrid vehicle conserves fuel a few different ways. It recaptures energy lost through braking and decelerating. This recaptured energy is stored in a battery pack as electricity. As demand warrants, the stored electricity is used to power an electric motor that assists the vehicle's gasoline engine--usually during acceleration.

While a conventional hybrid vehicle may travel short distances in pure-electric mode, plug-in hybrids are designed to travel extended distances with little or no assistance from the gasoline engine. Even before the charge is depleted, the gasoline engine may be called on to provide additional power for recharging the battery, accelerating, passing, and merging.


While operating on the additional plug-in charge, a plug-in hybrid more-or-less works the opposite of a conventional hybrid, with the electric motor acting as the primary power source, and the gasoline engine providing supplemental motivation. In the case of the Vue, once the initial charge is depleted, it would operate just as conventional hybrid does, using the gas engine. The Chevrolet Volt Concept is designed to use only its electric motor, using gasoline only to aid in battery recharging, not driving.

To keep vehicle weight inline with a conventional hybrid, plug-in hybrids would have little or no additional battery capacity. As such, the distance a plug-in hybrid will travel in pure-electric mode will be relatively modest. Saturn has suggested distances as great 40 miles, and as low as 20; GM estimates that the Volt Concept could travel an average of 40 miles per charge.

In the next section, we'll detail the benefits of plug-in hybrid cars and take a sneak peak at pure plug-in concepts from Chevrolet and Ford.


The Benefits of Plug-In Hybrid Cars

The Chevrolet Volt Concept was designed to showcase GM's growing ability and ambition to create a plug-in electric vehicle that requires no gasoline.

So, what are the benefits of plug-in hybrid cars? Plug-in hybrid drivetrains may seem like a complicated solution to a simple problem, but in reality they address the two fundamental issues that have prevented pure electric vehicles from becoming economically viable: range and weight.

Though battery technology has improved dramatically in recent years, the potential range of a pure electric vehicle is still below the roughly 300-miles of travel a typical consumer expects from a tank of gas.


According to Saturn, with a pure-electric range of up to 40 miles, the Vue plug-in hybrid will accommodate the 80 percent of consumers who live within a 20-mile radius of where they work. For these drivers, the gasoline engine would only be used to provide extra power for acceleration, passing, and merging.

Because the drivetrain is engineered for maximum efficiency in daily short-range driving, a plug-in hybrid is not saddled with what Saturn estimates is 400-600 pounds of additional battery capacity.

Given that it should be possible to always drive a plug-in hybrid vehicle within the range of its plug-in battery capacity, and gently enough to avoid requiring power assistance from the engine, it is theoretically possible to never consume gasoline.

More likely, fuel consumption would decrease dramatically in routine commuting, though the reduced cost of gasoline is offset to some extent by increased electricity usage.


Pure Plug-in Hybrid Concepts

While Vue will be able to run on its gas engine after the battery's power is exhausted, the Chevrolet Volt Concept's motor will solely get its power from the electric battery. This concept car is most notable for what it doesn't have than for what it does. Not a hybrid in the conventional sense, the Volt's gasoline engine never powers the vehicle's wheels. Instead, in the event that the car's plug-in charge is depleted, a small gasoline engine is used to power a generator that supplies back-up electricity. As the gasoline engine never provides power directly to the drive wheels, Volt does not require a conventional transmission.

Relatively light and compact, Volt's three-cylinder engine weighs less than a long-range battery pack might, and consumes less space. The gas backup provides an added dimension of range as well as flexibility, providing immediate power when time for a full battery charge may not be available.


Though displayed only with the gasoline engine at the 2007 North American International Auto Show in Detroit, Chevrolet says that Volt's backup power could also be provided by a diesel engine or a hydrogen fuel cell.

While the stylized two-seat Volt is not a likely candidate for production, its gas-assisted electric drivetrain may someday find its way into Chevrolet's lineup.

While more conventional in appearance, the Ford Edge with HySeries Drive Concept showcases technology very similar to Volt. Introduced at the 2007 Washington Auto Show, the Edge HySeries features electric drive with power coming from either a plug-in charged battery, or a hydrogen fuel cell.

Unlike the Volt Concept, which uses another power source to supplement the plug-in charge, Edge HySeries uses plug-in charging as a backup for the fuel cell.

Fuel cells, like the one in HySeries, create electricity-using hydrogen as fuel. Because hydrogen-filling facilities are scarce, the plug-in electric backup power source adds a much-needed dimension of range and safety.

While Volt is purely a concept vehicle, the Edge HySeries is completely functional, though a similar regular-production vehicle is unlikely in the near future.

These concepts from Chevrolet and Ford look down the road at another generation, one working toward the end goal of fossil-fuel independence. In time, they may be able to bridge the gap between gasoline and electric powered cars; making the goal of an automotive landscape virtually independent of fossil fuels a reality.