A gas-electric hybrid engine is a combination of a gasoline/petrol or diesel internal combustion engine with an electric motor to power a vehicle. Various designs exist for how the motor and engine interact with the drive train. Because the engine recharges the battery smaller batteries are required than in an electric vehicle.

A standard combustion engine is required to operate over a range of speed and power, yet its highest efficiency is in a narrow range of operation. Also, an engine designed for a reduced operating range can be more efficient than a standard engine. The battery storage and electric motor allows the engine to operate at its point of maximum efficiency, to be of a higher efficiency design, and to be smaller than non-hybrid applications.

The gas-electric hybrid uses less gasoline than conventional internal combustion engines and does not have the limitations in range that have been a problem for traditional battery electric vehicles. This type of piston engine is used in hybrid electric vehicles which have begun to enter the market as of 1999.

See the more complete discussion of this article in the article hybrid vehicle.