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 electric vehicles. This type of piston engine is used in hybrid electric vehicles which have begun to enter the market as of 2002.

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