Ever growing engine power has become an important problem in a crank shaft system. As having increased engine power, such troubles as torsional or bending vibrations have become the point at issue. In this paper, the modelling techniques about dynamic analysis of a crank shaft system are presented to analyze engine vibrations. Among the modelling techniques, two methods concerning about dynamic modelling of a crank shaft system are supposed so as to analyze vibrational analysis. One is a theoretical method that has been made by assuming the engine to be an ideal equivalent system (equivalent length, moment of inertia), i.e., the multidegree of freedom torsinal vibration model of undamped system. And the other is a new equivalent system of crankshaft that is supposed so as to formulate the vibrational models : torsinal and two kinds of lateral vibrations. In the latter, equations of motion are written in matrix form, transformed into engen-value problems and solved with a digital computer, not iteratively but directly. New equivalent system has a stiffness matrix that is derived from theoretical, not empirical equations for the calculation of stiffness values from the energetic review. Now, the results of theoretical analysis which is a calculation of natural frequencies and mode shapes, are figured and compared to experimental modal analysis.
