Geometric Parameter Design of a Multiple-Link Mechanism for Advantageous Compression Ratio and Displacement Characteristics

Variable compression ratio and variable displacement technologies are adopted in internal combustion engines because these features provide further degrees of freedom to optimize engine performance for various operating conditions. This paper focuses on a multiple-link mechanism that realizes variable compression ratio and displacement by varying the piston motion, specifically the Top Dead Center (TDC) and Bottom Dead Center (BDC) positions relative to the crankshaft. It is determined that a major requirement for the design of this mechanism is when the control action changes monotonically over its whole range, the compression ratio and the displacement should change in opposite directions monotonically. This paper presents an approach on how to achieve multiple-link mechanism geometric designs that fulfill this requirement. First, a necessary and sufficient condition, and a stronger sufficient condition are obtained on how the TDC and BDC positions should change with respect to the control action to fulfill the design requirement. Then Design of Experiments (DoE) methodology is used for creating sets of geometric designs of the mechanism, for which kinematics are calculated and checked against the conditions. A feasible design that satisfies the conditions is selected and detailed study on such characteristics as piston motion, stroke length, displacement, combustion chamber volume, and compression ratio etc. is performed. The design approach and obtained results serve as a basis for further analysis and optimization of the multiple-link mechanism.