Abstract: An energy storage system, for a hybrid vehicle including an internal combustion engine having an output shaft and a transmission operably coupled to the output shaft, includes a reservoir containing working fluid, a reversible pump/motor in fluid communication with the reservoir, a first clutch selectively drivably coupling the output shaft of the engine and the reversible pump/motor, a second clutch selectively drivably coupling an output shaft of the transmission and the reversible pump/motor, and an accumulator containing working fluid and gas. The accumulator is in fluid communication with the reversible pump/motor to deliver pressurized working fluid to the reversible pump/motor when operating as a motor to drive one of the respective output shafts of the engine and the transmission, and to receive pressurized working fluid discharged by the reversible pump/motor when operating as a pump driven by the one of the respective output shafts of the engine and the transmission.