NOVELTY - The hybrid vehicle has a prime mover for supplying power for driving the vehicle drive train, a motor/generator (29) coupled to the drive train for receiving electrical energy to selectively supply supplemental power to the drive train or to generate electrical energy, and an energy storage system for storing energy in the form of a compressed fluid and converting it to electrical energy to supply electrical energy to the motor/generator to supplement the engine power or to receive electrical energy generated by the motor/generator and compress the fluid to store the energy represented by the generated electrical energy.    USE - In hybrid vehicles, where energy is stored by compressing a compressible fluid or gas and converting this energy into electrical energy to drive an electric motor/generator to boost the engine output torque.    ADVANTAGE - Provides a safe, inexpensive and reliable means of energy storage for use in a hybrid vehicle. Is conveniently and inexpensively adaptable into the drive train of currently produced vehicles with internal combustion engines, or fuel cell powered vehicles. Can respond and interact with the vehicle's existing electronic engine control system.    DETAILED DESCRIPTION - The energy storage system has a second motor/generator and a hydraulic pump connected to be driven by the second motor/generator when it receives electrical energy and compresses the fluid, and to drive the second motor/generator to generate electrical energy in response to energy from the compressed fluid. The compressible fluid is gas, and an intensifier is connected between the hydraulic motor/pump and the pressure vessel. The engine may be an internal combustion engine, or an electric motor connected to a fuel cell.    An INDEPENDENT CLAIM is given for a hybrid vehicle.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of the energy storage system.    compressed gas (23)    bladder (24)    intensifier (27)    polarity control (30)    coupling (32)    reservoir (33)