The Transitory Vibrations for a Variable Compression Ratio Mechanism

摘要

This paper considers the case of a Variable Compression Ratio (VCR) mechanism for an internal combustion engine. The mechanism consists in the crank, the shaft, a triangular plate which links the previous two parts one to another and connected to a lever. One end of the lever has a vertical translation motion, the cause of its displacement being the practical conditions of driving. For constant conditions the lever acts as a constraint which states that an apex of the triangular plate must be situated on a circle. In this case one deals with a mechanism with one degree of freedom, this degree being given by the angle of rotation of the shaft. If the conditions change, then the end of the lever moves on the vertical direction, the mechanism having now two degrees of freedom: the angle of rotation of the shaft and the vertical displacement of the lever. When an optimal condition of functioning for the engine is obtained, then the vertical motion of the end of the lever stops and the mechanism becomes again a one degree of freedom one. In the cases of one degree of freedom mechanism the vibrations of the mechanism may be studied similar to the vibrations of a simple one degree of freedom system, the parameters of vibrations being different and depending on the position of the end of the lever. The main goal of this paper is to present the transitory vibrations for the two degrees of freedom system which occur during the vertical motion of the one end of the lever. The study is realized using a multibody approach and the numerical results are presented and discussed. The transitory vibrations start when the end of the lever starts its motion and they end after the lever stops its motion. The dependence of the length of the interval of time of transitory vibrations as function of the geometric dimensions of the elements and the law of motion of the end of the lever is also highlighted by the authors.