Chevrolet Tahoe Hybrid (© General Motors)Click to enlarge picture

The two-wheel-drive version of the 2009 Chevrolet Tahoe Hybrid full-size SUV can seat seven, tow 6,200 pounds, and has a 21 mpg city fuel economy rating — the same as a 4-cylinder Toyota Camry.

In genetics, a hybrid is the result of combining elements from different species. When it comes to cars and trucks, a hybrid refers to a vehicle whose powertrain combines the aspects of different technologies (i.e. gasoline and electric) to improve efficiency and reduce emissions.

In automotive terms, hybridization is nothing new. Hybrid cars and trucks that combined an electric motor with a gasoline engine date back to the turn of the 20th century (see Back to the Past). Hybrid diesel-electric locomotives have been in operation for years, and diesel-electric buses began to appear in the 1970s.

These days, automotive choices that combine two or more sources of power are known as hybrid vehicles. The most common is the electric hybrid, which melds a gasoline engine with an electric setup (electric motor, charging system, and a battery/storage system).

Parallel vs. Series
Due to the inherent complexity of the technology involved, and the numerous options engineers have in employing hybrid systems, we're forced to speak in some general terms here. That said, the two most popular forms of hybrids available today are called "series" or "parallel" hybrids. Series hybrids are also called "mild" or "partial" hybrids, while parallel hybrids are often referred to as "full" hybrids.

In a series hybrid, an electric motor assists the gasoline engine when needed, such as during acceleration or times of heavy load, but it can't power the car on its own. The batteries that power the electric motor can be recharged by the engine, or during deceleration (called regenerative braking). The Saturn Vue Green Line and Honda Civic Hybrid are examples of series hybrids.

Parallel hybrids can use similar recharging scenarios, but can be powered by the gasoline engine only, the electric motor only, or both at the same time. The Toyota Prius, Ford Escape Hybrid, Nissan Altima Hybrid, and GM's new Tahoe/Yukon hybrid SUVs are examples of parallel hybrids.

How They Operate
In practice, like a regular car, a series-hybrid vehicle has an engine that is generally running all the time. An electric motor placed "in series" between the engine and the wheels helps motivate the car. In assisting the engine, the electric motor helps improve fuel economy and reduce emissions. Some series hybrids also provide a "start-stop" function, which turns off the engine completely when the car is stationary, then restarting it instantly when the brake pedal is released.

Unlike a series hybrid, a parallel hybrid can operate like a fully electric car up to speeds of roughly 20-30 mph. In situations such as stop-and-go traffic, or under light acceleration at low speeds, the vehicle's engine may not run at all, relying completely on electric propulsion. The results are vastly improved city mpg ratings and, while in full-electric mode, essentially zero emissions.