| The development of Automobile Engine Variable Valve Technology has been a rather long history, but the traditional camshaft-driven valve system has resulted into great energy consumption and high pollution. With the emerging of electro-hydraulic variable valve timing technology, the methods of automotive engine valve gas distribution and its scalability have been greatly improved, and the fuel economy as well as the power has been greatly magnified. But due to the restriction of the low operating frequency with the core control components, the electro-hydraulic variable valve timing technology has currently not been able to meet the requirements of the high rotation speed of automotive engine. And thus this paper put forward using high-speed On/Off valve to control the electro-hydraulic variable valve timing system with its focus concentrated on the design and development of its high-frequency electric-mechanical converter.In order to meet the high frequency requirement of the electro-hydraulic variable valve timing system, a spiral tube solenoid based on a PWM driven circuit with a small number of turns, great current and reversed polarity discharge was successfully developed by comparing the advantages and disadvantages of different electric-mechanical actuators. Based on Ansoft finite element simulation software, part of the critical structural parameters, such as the taper angle, the length of the armature and non-magnetic length which affect the performance of On/Off electromagnet in a great deal but could not be obtained through traditional empirical formula, were finally obtained.Based on comparison of the strengths and weaknesses of different valve spool structure, the traditional slide-valve spool structure was adopted to avoid the influence of high flow force resulted from high-frequency dynamic response condition. The spool quality was effectively reduced by structural improvements. An optimized spring stiffness recovery methods was chosen to recover valve spool instead of adopting hydraulic pressure force based on comparison of their advantages and disadvantages, which could effectively balance the opening and closing time of the valve spool. A high-speed On/Off valve model and its unloaded flow rate characteristics as well as pressure characteristics versus duty cycle ratio simulation models were established in AMESim software environment. The structural parameters of the valve were optimized by simulation analysis, and finally a no-load flow rate versus duty cycle ratio curve as well as pressure versus duty cycle ratio curve were obtained. In addition, the maximum stroke of 10mm was reached by the hydraulic cylinder in the simulation model of the electro-hydraulic variable valve timing system. In addition, the simulation results of both liquid pressure recovery method and spring recovery method were compared, results showed that spring recovery method has a better response characteristic, and the system pressure will not be restricted.An internal flow field simulation model of the high speed On/Off valve was established by the pre-processor Gambit of the Fluent software. Curves of the static hydro-dynamic force as well as the transient hydro-dynamic force versus different valve port openings were obtained. Besides, the static/total pressure distribution and the oil flow velocity maps were shown and analyzed to get an intuitive understanding of the turbulence, swirls and the caused noises.In order to examine the characteristics of the high-speed On/Off valve as well as the high-frequency electromagnet, test methods were designed to get the dynamic current rising curve and static force versus displacement descending curve of the high-frequency electromagnet, experimental results show that the high-frequency electromagnet could accomplish a 1.4mm stroke within 2.2ms and the maximum magnetic force output reached 295N. A high-speed On/Off valve flow characteristic test bench was established, and its no-load flow rate versus cyclic ratio curve was obtained, which agreed quiet well with the simulation results. Under the conditions of 133Hz excitation frequency, 5V voltage amplitude input, 35N/mm spring stiffness with 265N preload, 8.5MPa system pressure with a flow rate of 20L/min, the hydraulic cylinder with spring recovery spool could achieve a maximum stroke of 10mm, which is 25% higher than that of liquid pressure recovery method, which testified the effectiveness of the high-speed On/Off valve as well as its electric-mechanical converter, and the experimental results agreed with the simulation results to a large extent. In addition, the paper made a further research to the maximum valve lift under 200Hz signal input, and the experimental results showed that the maximum valve lift under the same external condition could achieve 5mm. Based on the comparison between simulation results and experimental results of the loaded electro-hydraulic variable valve timing system, the paper inferred that a maximum valve lift of 8mm, which could meet the frequency response and valve lift requirements of the electro-hydraulic variable valve with an 6000rpm engine speed, could be achieved when the system pressure was improved to 10MPa and the valve recovery spring stiffness be increased to 70N/mm. |
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