Hydraulic Drive Systems and Potential Use for Automobiles and Airplanes 975581
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. The drivetrain concept that has been most extensively studied would facilitate incorporation of additional attractive features such as antilock braking and traction control. The availability of hydraulic power also facilitates the incorporation of active suspension in the vehicle. Opportunities also exist for size and weight reductions, particularly by utilizing an auxiliary hydrostatic drive unit for accessories such as the air conditioning compressor and the alternator. By operating these units at a constant (relatively high) speed, they can be much smaller and lighter that those found in conventional vehicles. The electric motor which drives the radiator fan can be replaced with a more compact, lighter hydraulic unit, and the power steering pump can be eliminated altogether.
When considering even more advanced vehicle concepts, such as airplanes that could be operated as roadable vehicles, hydrostatic drive once again offers some unique advantages. The technical challenges associated with practical roadable aircraft include propulsion system weight and vehicle performance for both ground and air modes of operation. When compared to conventional drivetrains, the compact size of hydrostatic pump/motors and the relative ease of distributing the power permits significant design flexibility in locating the principle powertrain components. This design flexibility inevitably leads to easier achievement of a more aerodynamic design. Also, the flexibility of hydrostatic drive allows optimal load matching of the engine and propeller over a range of operating conditions, supporting minimal weight designs.