Characteristics of Bending Stress with Whirling at the Rear End of a Crankshaft in an Inline 4-Cylinder High Speed Diesel Engine

As engines become lighter and achieve higher output to meet carbon dioxide emissions targets, it becomes more challenging to design a crankshaft that is both lighter and capable of handling higher loads. Therefore, it is necessary to understand the characteristics of forces imposed on the crankshaft, and the mechanisms by which stresses are created in the crankshaft. This paper describes the characteristics of bending stresses measured on the rearmost crank pin fillet of a crankshaft. Two basic crankshaft resonant modes are described. Forward crankshaft whirl then has the effect of increasing the system natural frequencies by the stiffening effect, while reverse whirl reduces the system natural frequencies by the softening effect. The effect of whirl grows with increasing engine speed. This results in what appears to be four crankshaft natural frequencies rather than two. The four resonances appear at all non-zero engine speeds. The influence of flywheel mass on the stresses and natural frequencies is also described. It is shown that the bending stress in the crank fillet is proportional to the radial force acting on the crank pin. It is also shown that the direction of whirl affects the amplitude of stress imposed by the radial crank pin force, and that the effect of whirl becomes larger as the flywheel inertia is increased. Because increasing engine speed causes more whirl force and moment, engine speed has an influence on bending stress amplitude. Finally, the paper explains why the ratio of crank stress amplitude to radial force varies as a function of the rotational direction of whirl.