선박용 디젤엔진의 크랭크축 비틀림진동에 의한 엔진 본체의 연성진동에 관한 연구

Abstract

In order to prevent the adverse effects on the ship and to protect the related equipment by the vibration, the vibration of main engine(s) which is the large 2-stroke diesel engine is checked at the design, shop test and/or sea trial. The concerned vibration consists of the transverse one of main engine itself and the torsional/axial one of the shafting system normally. The main engine vibration is mainly caused by the gas force of the fuel combustion in the cylinder inside and the inertia force by the reciprocating and rotating parts like the piston/piston rod, crosshead, connecting rod, crankshaft, etc. Especially, the transverse vibration of main engine body is generated by the guide force of crosshead during an up-and-down motion of crosshead and this guide force shakes the engine body in the transverse direction. The H, X and x-mode vibrations of engine body are known to be related with the guide force which is generated and transferred by the torque of the crankshaft generally.

The many measurement results of transverse vibration show that the vibration level and its frequency is related with the rotation velocity of crankshaft and the natural frequency of the engine structure body. The resonance of engine vibration has strong and direct connection with the natural frequency of engine body. However, some resonance is made at the rotating frequency which is not related with the natural frequency of engine body unexpectedly. When this resonance and its frequency is investigated in detail, the unexpected frequency is considered to be related with the natural frequency of torsional vibration of the shafting system and its resonance. As a result, the torsional vibration of the shafting system has a strong or weak influence on the transverse vibration of the engine body according to the level of torsional vibration.

Based on the measurement of the torsional and transverse vibration, its effect and relation between the torsional vibration of shafting system and the transverse vibration of the main engine is reviewed. And it turns out that this coupled vibration of engine with the torsional vibration is connected with the guide force by the vibratory torque of crankshaft. The vibratory torque is transferred to the crosshead between the upper end of connecting rod and the lower end of piston rod through the connecting rod from the rotating crankshaft. The vibratory guide force of the crosshead is made by the vibratory torque in addition to the major nominal torque. This vibratory guide force shakes the engine body with the vibratory torque frequency of the crankshaft.

The research object of this study is to review the phenomenon of the coupled vibration between the transverse vibration of the engine body and the torsional vibration of crankshaft and to find out the relation and its effect of the torsional vibration on the transverse vibration by the 3D FEM analysis of main engine body and crankshaft.

In order to increase the accuracy of 3D FEM vibration analysis for the 2 stroke diesel engine which is consistent of the many complicated components, the condensed modeling method instead of the simplified equipment modeling for various major components is proposed, which can give the detailed and high quality of vibration analysis for whole engine. The high quality and consistency of the each models and its assembly model is checked by the comparison of vibration characteristics like mass, stiffness and natural frequency between 3D CAD model and the shell based contended model using the mode analysis. The gas force of the fuel combustion and the inertia force of the moving components is calculated by the kinematic formula, dynamic analysis and cylinder pressure for the input data of the forced vibration analysis.

Also, the 1(one) point excitation method is introduced newly to solve the non-linear characteristics of the up-and-down motion of crosshead for modal transient analysis and its validity is confirmed.

When the forced vibration analysis of engine is done using the above gas and inertia force as the excitation force of the engine body and compared with the vibration measurement result, some resonances of measurement are not displayed on the analysis result. This phenomenon is supposed that the engine body vibration is to be coupled with the torsional vibration of crankshaft. In order to solve this problem, the torsional vibration analysis of the crankshaft is performed and the coupled excitation force is calculated by adding the vibratory torque to this excitation force of the engine body. The forced vibration analysis is recalculated using this modified excitation force. Then, the coupled effect of the transverse vibration of engine body with the torsional vibration of the crankshaft is verified since the omitted resonances of analysis are reflected in the reevaluation result.