Development of a two-step variable valve actuation mechanism equipped with an electronic switching system

Paper #:
  • 2017-01-2412

Published:
  • 2017-10-08
Abstract:
A variable valve actuation(VVA) system that changes the valve lift profiles according to the rotational speed and load condition of the engine, increases the intake and exhaust efficiency and gives a lot of possibilities to improve engine performance. A two-step VVA system has a relatively simple structure and is a cost effective way to improve engine performance. However, most two-step VVA mechanisms include hydraulically controlled switching systems. The biggest problem of the hydraulic switching systems is that oil temperature and pressure affect the operability of the mechanism, which is a major obstacle to achieving the goals of a VVA system to reduce fuel consumption and improve engine performance. In this study, we developed an end pivot rocker arm type two-step VVA mechanism, in which single cam drives two valves. The mode conversion of the two-step variable mechanism is done by an electronic switching system instead of a conventional hydraulic system. The electronic switching system proposed in this study is driven by a dedicated solenoid and is not affected by the temperature and pressure of the engine oil. Therefore, not only the stability of the mode switching operation of the VVA mechanism can be secured but also the operation delay time can be kept short enough. In practice, several models of two-step VVA mechanisms were fabricated and the operability of the instrument and switching system was experimentally confirmed. Experiments were conducted to evaluate the operability in various conditions of the engine speed and oil temperature. The solenoid operation time and the valve displacement delay time were analyzed using the measured data. Experimental results show that the electronic switching system has a significantly shorter operating delay time than the hydraulic switching system. Unlike hydraulic type, it was not influenced by oil temperature and pressure. It was found that operation delay time varies depending on solenoid response and solenoid operating position. In order to minimize the variation of the operating delay time during the mode switching, the design factors were optimized and the effect on the dynamic performance of the mechanism was also analyzed. The two-step VVA mechanism developed in this study can also be used as a cylinder deactivation(CDA) system by assigning the lift of the low-speed cam to be zero. By attaching a roller to a portion of the rocker arm that is in contact with the base cam, the problem of pad wear, which is often caused in CDA mechanism, is also fundamentally solved.
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