What could a Toyota Prius possibly have in common with a Cadillac Escalade, Chevy Silverado or Ford Fusion? Each is available with a hybrid powertrain, a propulsion system made famous by the Prius. While consumers benefit from the improved fuel economy, hybrids give automakers a way to meet ever-tightening Corporate Average Fuel Economy (CAFE) requirements and proposed limits on carbon dioxide (CO2) emissions.But what exactly is a hybrid, and how do they improve fuel economy? Are all hybrids laid out essentially the same under the hood, and do they all provide similar fuel savings?

Defining Hybrids

A vehicle is a hybrid if it utilizes more than one form of onboard energy to achieve propulsion. In practice, that means a hybrid will have a traditional internal combustion engine and a fuel tank, as well as one or more electric motors and a battery pack.

Hybrid cars are sometimes mistakenly confused with electric vehicles. Hybrids are most often gasoline-burning machines that utilize their electric bits to collect and reuse energy that normally goes to waste in standard cars. Theoretically, diesel-electric hybrids would be even more fuel-efficient, but hybrid systems and diesel engines both represent extra cost. Installing both in the same vehicle would be prohibitively expensive.

Hybrid Glossary

Below are the terms most often used when referring to hybrid vehicles.

Motor-generator (MG for short): The more accurate term for the electric motor. It provides supplemental acceleration "oomph" when operating as a motor by drawing electricity from the battery. Several hybrids have two, and a few models employ three.

Start-stop: Present on all hybrids, the engine's traditional starter motor is absent because the MG takes on that function, too. Hybrid control software shuts the engine off while stopped at traffic signals and automatically restarts it again with the MG when the driver releases the brake pedal. Eliminating the fuel waste of an idling gas engine causes overall mpg to climb significantly and tailpipe emissions to drop, especially in town. An idling engine consumes fuel at the rate of zero miles per gallon.

Regenerative braking: An important function of the MG is to generate electricity to recharge the battery as it absorbs a portion of the vehicle's momentum when slowing or coasting downhill. Normal cars waste all of their excess momentum as heat in the brakes. Regenerative braking is insufficient to stop a car quickly, so conventional hydraulic brakes are still necessary.

Electric drive: Operating the vehicle on electric power alone, possible if the hybrid system has enough electrical capacity. The maximum speed and distance over which electric-only operation is possible varies from essentially zero to a handful of miles, and has everything to do with the weight and aerodynamics of the vehicle, the strength of the MG and more than anything else, the capacity of the battery.