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Utilization of hydrostatic drives for power transmission, along with hydropneumatic accumulator energy storage can provide significant opportunities for improvements in vehicle design. These advantages are particularly relevant when considering advanced concept vehicles such as flying automobiles. Research and development in this technology has primarily focused on the fuel economy improvements that can be achieved in automobiles using a hydrostatic transmission and hydropneumatic accumulator energy storage. The accumulator permits the engine power to be uncoupled from the road load, thus enabling the engine to be operated at a more efficient point. By using wheel drive units that can operate as either motors (when driving) or pumps (when braking), regenerative braking can also be achieved, with the energy stored in the accumulator. In addition to improved fuel economy, several other significant design opportunities can be exploited.

Recent advances in hydrostatic transmission efficiency and accumulator technology make the hydropneumatic energy storage automobile appear quite attractive as a means of improving fuel economy. The system examined in this paper utilizes a conventional internal combustion engine, and two hydrostatic pump/motor units with an accumulator between them. The accumulator allows regenerative braking and permits the engine operation to be uncoupled from the road load. Detailed, second-by-second driving cycle simulations have been used to study the fuel economy possible with various combinations of component parameters. The design can provide excellent fuel economy with a moderate size accumulator.