In this paper, a new hydraulic variable valve actuation system is proposed. Using this system, the engine valve opening and closing timings and lift are flexibly controlled with two rotary spool valves actuated by the engine crankshaft. High degree of flexibility with less control complexity and high repeatability are the advantages of this system over other camless valvetrains; however, in this system, there is a trade-off between its robustness and power consumption. A numerical model of the system is developed to study the system functionality at different operating conditions. To validate the developed model, the simulation results for a random operating condition are compared with those from the experiments. A sensitivity analysis is done to study the effects of variations in different design parameters on system robustness and power consumption. The results prove that increasing engine valve return-spring stiffness and actuator piston area will reduce the mechanism sensitivity to engine cycle-to-cycle variations; however, this results in poor energy efficiency. Therefore, a neat energy recovery strategy is developed to recuperate a portion of the energy used to compress the engine valve return-spring during valve opening interval. The results show that more than 90% of the extra energy wasted for the sake of system robustness could be regenerated through the proposed energy recovery system.

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