Effects of variable inertia on the damped torsional vibrations of diesel engine systems

In the past the effects of ignoring the variable inertia characteristics of reciprocating engines on the accuracy of torsional-vibration calculations were considered to be negligible. The associated secondary resonances and regions of instability tended to be dismissed by most engineers as interesting but of no importance. The situation changed in recent years since there was evidence of the existence of the secondary resonance effects which could have contributed to a number of otherwise inexplicable crankshaft failures in large slow speed marine engines.The cyclic variation of the polar moment of inertia of the reciprocating parts during each revolution causes a periodic variation of frequency and corresponding amplitude of vibration of reciprocating engine systems. It also causes an increase in the speed range over which resonance effects are experienced. In the present paper a study of the effects of damping on the motion of the variable inertia system has been carried out. When the variable inertia effect is allowed for, the equation of motion including the effects of damping is non-linear. For small displacements, the equation can be linearized to predict important characteristics of the motion.Computer methods making use of a numerical analysis process, namely the modified Euler's equations, have been applied in the investigations. The waveform responses are studied at different speeds of engine rotation. The effects of damping and of variations in the ratio of the average reciprocating inertia to average total inertia on the regions of instability have been investigated.