A hybrid car is one that uses more than one method of power generation to move the wheels. Most hybrid cars use a dual-source system consisting of an internal-combustion engine (ICE) of some type combined with batteries or other high-density energy storage media. The primary purpose of the storage system in a hybrid is to even out the power consumption curve of the engine, as the constant up- and down-ramping of power in traffic is the primary cause of operating inefficiency. By allowing the ICE to operate at a near-constant speed, the energy use efficiency is maximized, increasing the miles-per-gallon performance.

Other hybrid combinations include fuel cell/flywheel, turbine/supercapacitor, and compressed air/heat engine. The key is to pair a long-running, reasonably efficient, replacable-source power plant (the turbine, ICE, fuel cell, etc) with an efficient energy storage system (flywheel, battery, air tank, supercapacitor, etc) to allow the one to charge the other and/or assist with the power delivered to the wheels.