| As being restricted by the energy density of its energy storage component--hydraulic accumulator, the hydraulic drive train on hydraulic hybrid vehicles, after the engine working area is transfered to its sweat-line (the output of engine matches the load where the engine works with high efficiency), needs compensate for the insufficient of torque by using extra shaft speed, or compensate for the insufficient of shaft speed by using extra torque, in order to avoid over-reliance on the accumulator. Which means that hydraulic drive train on hydraulic hybrid vehicles requires the hydraulic pump/motor which converts extra shaft speed to torque or converts extra torque to shaft speed still has a high efficiency in the extreme cpnditions with normal speed low-torque or low-speed normal torque. In2004a company named ARTEMIS INTELLIGENT POWER in Scotland developed DDP(Digital Displacement Pump) based on a radial piston pump and solenoid high speed on-off valves, which made "digital displacement" very hot in hydraulic field. Researchers all over the world think that digital displacement motor would be developed soon with overall efficiency over80%in conditions of10%rated shaft speed or10%rated torque. Since then9years has passed, no one company has eliminated the cavitation problem caused by the throttle effect when high speed on-off valves reverse unbalanced which can damage the motor piston seriously, and the progress of study is very slow.The author’s supervisor, the founder of "switch mode hydraulic power supply technology", doesn’t agree to compensate for the internal defect of hydraulic component with electrical technique, since the movement of hydraulic components cannot be controlled by electrical elements at the exact point of pressure in balance. Cavitation is inevitable as long as the pressure difference is3times higher than the downstream pressure, resulting in a larger pressure shock, significantly reducing the reliability and durability of the components, the components’ lifetime is very short. During the visit in Minnesota in the United States in2008, the author and his supervisor put forward seat valve controlled variable axial piston motor. And during the following5years, the Author together with his supervisor applied the new principle to bent axis fix displacement piston motor successfully, and analyzed the characteristic of the high speed on-off seat valve controlled variable displacement mechanism in-depth on eliminating pressure shock while reversing, reducing throttling losses and leakage losses, as well as the ability to provide regenerative braking condition, opened up a shortcut for improving the reliability, durability and serviceability of variable displacement motor.The paper originally created the concept of "seat valve controlled variable displacement axial piston motor", applied the idea of "pure hydro-mechanical, low pressure difference, active off, passive on" in switch mode hydraulic power supply and the electronically controlled driving mechanism on radial digital variable displacement piston pump to the new variable displacement mechanism, put forward a new structure of seat valve controlled variable displacement axial piston motor:control a single piston of the bent axis piston motor by a high pressure distributing valve and a low pressure distributing valve (pure hydro-mechanical controlled high speed on-off valves), the high pressure distributing valve controls the flow path between the piston chamber and the high pressure port, the iow pressure distributing valve controls the flow path between the piston chamber and the low pressure port. The distributing system is made up of cone main valves controlled by spool pilot valves. All the high pressure pilot valves are mounted radially in a valve block just like the pistons in a radial piston pump while the valve block rotates synchronously with the cylinder block, and all the high pressure pilot valves are controlled by a high pressure pilot valves’ drive ring. The angle position as where the high pressure pilot valve reverses is determined by the displacement of the high pressure pilot valves’ drive ring, pistons whose angle position is beyond as will suck oil from tank port, the rest suction stroke of this piston is named virtual displacement during which the instantaneous torque produced by this piston is zero. Then the average torque can be adjusted by changing αs and variable displacement is achieved All the low pressure pilot valves are controlled by low pressure valves’ drive ring whose displacement determines the angle position αD where the low pressure pilot valve reverses, if the angle position of piston is beyond ao, the oil in the piston chamber will flow back to high pressure port instead of tank port, mechanical energy is converted into hydraulic energy, the piston works in pump condition, and producing regenerative braking torque corresponding to ap. In motor condition, the low pressure valves’drive ring is kept in its lowest position which means that there is no oil flows back to high pressure port during piston drain stroke, and vice versa. Put forward "orthogonal eccentric law" to set the initial position of the high pressure main valve, high pressure pilot, low pressure main valve and low pressure pilot valve correctly.The paper consists of eight chapters.Chapter1is a summary of the full text. Firstly the research background and significance of the dissertation are described. The hydraulic drive train for hydraulic hybrid vehicle is very demanding on hydraulic components, conventional digital variable displacement technique is general weak in reliability, durability, and maintainability, currently at home and abroad the processing to apply digital variable displacement technique to every single piston is not going well, since the pressure shock while the distributing valve reverses can’t be eliminated, the throttle losses and leakage losses are still large, the overall efficiency of regenerative braking condition is still very low. The author’s supervisor has been opposed to make up the internal defect of hydraulic component with electrical technique since the movement of hydraulic components can’t be controlled by electric element at the exact point when the pressure difference across the valve is zero. Based on the theory of switch mode hydraulic power supply, the research goal and main contents of this dissertation-seat valve controlled variable displacement axial piston motor is put forward.The overall structure of seat valve controlled variable displacement axial piston motor is introduced in chapter2. The drive train of hydraulic hybrid vehicle requires efficient hydraulic components even in extreme poor work conditions (10%torque output at rated shaft speed or10%shaft speed output at rated torque). Then the author developed "pure hydro-mechanical digital variable displacement motor", whose distributor consists of a group of high pressure distributing valves and low pressure distributing valves, achieving digital variable displacement with regenerative braking condition by adjusting the displacement of high pressure drive ring and low pressure drive ring. Designed a.displacement adjusting mechanism with valve control based on "orthogonal eccentric law", by which the hydraulic motor achieves variable displacement and can set the initial position of the high pressure main valve, high pressure pilot, low pressure main valve and low pressure pilot valve correctly. Built a single piston model and a7pistons model based on AMESim, developed a prototype of seat valve controlled variable displacement axial piston motor, and built a test rig to test the prototype.In chapter3the valve controlled distributor for seat valve controlled variable displacement axial piston motor is introduced, which is the core technique for valve controlled variable displacement motor, and analyzed its static characteristics. Firstly analyzed the extreme demanding requirement for the flow rate, frequency response of the distributing valve of seat valve controlled variable displacement axial piston motor, and set their parameters. In a motor work cycle, the process of each piston is divided into6stages under the control of distributing valves, it is confirmed that the motor can achieve the characteristic of "variable displacement" and regenerative braking condition based on seat valve controlled variable displacement mechanism and "orthogonal eccentric law". At the end of this chapter the simulation results and experiment results verified the above mentioned concept.Similar with "high speed on-off fluid power, very few researchers and experts are optimistic about "digital displacement", since solenoid valve can’t be drived at the exact point where the pressure difference across the valve is zero. Cavitation is inevitable as long as the pressure difference is3times higher than the downstream pressure, resulting in a larger pressure shock, which will significantly reduce the reliability and durability of the components, and the component’s lifetime would be very short. In chapter4the pressure balance issue across the distributing valves is studied during all the6stages in a motor work cycle one by one, it is confirmed that all the main valves and pilot valves would move at the exact point where the pressure difference across the valve is zero. On this basis, analyzed the dynamic characteristic when the seat valve controlled variable displacement axial piston motor start, brake and reverse, and was verified by simulation results and experiment results at the end of this chapter.The second reason for which many researchers and experts are negative about "digital displacement" is that the throttle loss is considerable when the high speed on-off valves reverse, the loss is even more than the energy saved by applying digital displacement to the system, and steady and transient hydrodynamic forces are inevitable when the high speed on-off valves reverse, then high speed on-off valves need more time to reverse which causes more throttle loss. In chapter5dynamic characteristic of seat valve controlled variable displacement axial piston motor are studied, especially steady and transient hydrodynamic forces on distributing valves. On this basis, it is confirmed that the throttle losses of the all main valve, pilot valves during all6stages in a motor work cycle is very low by analyzing all the work stages, and was verified by simulation results and experiment results at the end of this chapter.The third reason for which many researchers and experts are negative about "digital displacement" is that there are too many parts in a digital displacement motor, and most of them rotate or slide relatively with each other in high speed conditions, and the unbalanced hydraulic force will change with the displacement of the motor which can cause large leakage. The author had finished the research work presented in-the past several chapters2years ago, but further research work are stalled by leakage problem. In chapter6, hydrostatic thrust bearing with central flow path and circular flat slit fed restrictor is introduced, which is developed to solve the leakage problem on portplate, reduced the leakage loss on all the moving surface in high speed and high pressure difference conditions, and then was proved by simulation results and experiment results.Hydraulic hybrid drive train requires the efficiency of the seat valve controlled variable displacement axial piston motor is the same in two directions, which means the efficiency of the motor in motor condition’ should be the same as in regenerative braking condition. The static and dynamic characteristic of the motor are analyzed in regenerative braking condition, it is confirmed that the performance of the motor is almost the same in motor condition and regenerative braking condition, and then was proved by simulation results and experiment results.In Chapter8there is the conclusion of the paper, and proposed directions for further research. |
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