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Getting Power to the Pavement
Published in Patrick Hossay, Automotive Innovation, 2019
Whatever gearset configuration is used, it is clear that sophisticated computer control and innovation have dramatically changed the possibilities. Initially, the planetary gearset was adopted for automatic transmissions because the goal of automated operation required it. At the time, it was easier to devise an automated hydraulic mechanism for engaging and disengaging hydraulic clutches than it was to automate the actuators in a layshaft gearbox. Until recently, these transmissions were controlled with a network of hydraulic fluid channels in a complex valve body that actuated the various clutching mechanisms. However much of these functions have now been integrated into a digital TCU that uses electronically controlled hydraulic solenoids, expanding design possibilities greatly. Computer control enables enhanced clutch-to-clutch actuation, with a clutch to one gear disengaged the instant a clutch for the other is engaged. At idle, the transmission can be automatically placed into neutral, helping manage temperature and improve fuel efficiency. Multiple variable displacement pumps can be used to produce just the hydraulic pressure needed, and significantly reduce associated loss. With pump loss accounting for nearly two-thirds of transmission power loss, this can improve efficiency significantly.8
Hydraulic Power Generation
Published in Qin Zhang, Basics of Hydraulic Systems, 2019
Based on their capability to change displacement, hydraulic pumps can be categorized into fixed-displacement pumps and variable-displacement pumps. Based on their configurations, hydraulic pumps can be categorized into a gear pump, a vane pump, or a piston pump. Normally, gear pumps are fixed-displacement pumps, while vane pumps and piston pumps have both fixed or variable-displacement designs. Different types of industry have a preference in choosing the design of the pumps. For example, machine tool manufacturers often select vane pumps because of their low noise and ability to deliver a variable flow at a constant pressure. Mobile equipment manufacturers commonly like to use piston pumps due to their high power-to-weight ratio, and agricultural equipment manufacturers prefer gear pumps for their low cost and robustness.
Hydraulics
Published in Don M. Pirro, Martin Webster, Ekkehard Daschner, Lubrication Fundamentals, 2017
Don M. Pirro, Martin Webster, Ekkehard Daschner
Although gear and vane type pumps can be designed to work with higher pressures, the life expectancy of piston pumps in severe service will be greater, making them cost-effective. The key advantages of variable displacement piston pumps include having infinitely adjustable flow along with being reversible. This means that the pump can act as a pump or a hydraulic motor if the direction of flow is changed. Circuits using these pumps can be simplified, as reversing valves are not needed. With reversing pumps, rapidly moving masses can be smoothly decelerated, reversed, and accelerated without system shocks.
Temporary and Permanent Viscosity Loss Correlated to Hydraulic System Performance
Published in Tribology Transactions, 2018
Paul Michael, Mercy Cheekolu, Pawan Panwar, Mark Devlin, Rob Davidson, Duval Johnson, Ashlie Martini
The effects of flow losses on the power requirements of the system were examined. The input power of the system was determined from pump torque and rotational frequency measurements. The output power of the system was determined from the motor torque and rotational frequency measurements. System power loss was determined from the difference between the output power of the motor and the input power of the pump. As shown in Fig. 14, the system required approximately 15% less input power at a fluid temperature of 50°C. Less power was required because the leakage flow at 50°C was lower. In a variable displacement pump, the compensator comes “off-stroke” when the flow demand of the hydraulic system is reduced. The resulting reduction in pump displacement decreased the input torque requirement of the pump because torque is a product of displacement and outlet pressure. Because system power outputs were similar for 50 and 80°C, reducing the input power to the pump yielded a net decrease in system losses. Pump efficiency (overall) is the ratio of pump output power to input power. Pump overall efficiency was relatively insensitive to the effects of fluid temperature on leakage flows. The fact that pump efficiency exhibited a negligible change when the input power shifted exemplifies one of the key performance benefits of variable displacement pumps. It also illustrates why “losses” tend to be a better indicator of the effects of fluid on system performance than efficiency.