Cam mechanisms are one of the most popular devices for generating intermittent motion and are widely used in many areas. The advantages of the cam mechanisms are its simplicity and its great versatility in synthesizing motion patterns. The cam motion including displacement, velocity and acceleration can be easily synthesized and controlled by the designer. Also special characteristics such as high load carrying, low shock, quiet operation, and etc., can be included from the beginning stage of cam design. In the early days, cam designers were concerned mainly with the problems of the cam kinematic profile and cam follower vibrations such as residual vibration, cam-follower system separation phenomena(jump and bounce), return spring surge phenomenon, and etc. When the cam-follower system is operated at relatively low speeds, kinematic design is enough to get an accurate follower motion. But with increasing operational speeds, the cam-follower system dynamics influence much on its dynamic motion. Thus, the dynamic characteristics of the cam-follower system tend to be considered in order to analyze the follower motion accurately.
In this work, an effective optimum cam profile design procedure for an OHV- type cam valve system is proposed. The modelling of an cam mechanism, cam profile functions, and the optimal design of the cam profile considering the dynamic characteristics of the cam mechanism is studied.
First, an OHV-type cam-valve system for an internal combustion engine is modeled as a four-degree-of-freedom lumped-parameter model. In order to improve the reliability of the model, the construction model is tuned by comparing the model analysis results with the measured valve motion.
Fourier and spline functions are chosen to represent cam profile mathematically and further to optimize analytically. And, especially, it is developed that spline functions can be effectively applied to not only cam profile optimum design but also the dynamic synthesis of cam profile.
Finally, in this thesis, a multicriteria optimization technique is used to design the cam profile considering the cam-follower system dynamic characteristics. When designing a cam profile for an high speed internal combustion engine, it is desirable to make the flow area be as large as they can be within the cam-event angle and its acceleration be as low as possible. But as we know, those features conflict each other. This problem is a good example to apply multi-criteria optimization techniques. Generally, the deterministic single solution does not exist in the problem of having multi-objective functions. Some compromises are inevitable to get a good design. In this work, as a compromising tool for the mutually conflicting objective functions, a trade-off curve, which is known as a Pareto-optimal solution set, is drawn by allocating the proper weighting factors in the space of the objective functions. From the trade-off curve, both the conflicting objective functions can be improved over the existing cam-valve system. In this work, the valve flow area of the test cam-valve system can be increased by 10.12% and its total squared valve acceleration can be reduced by 35% at the maximum. When the compromising weighting factors for the competing objective functions are determined, the corresponding cam profile can be easily designed by following the conventional methods.
