Analysis of the Impact of Variable and Non-Variable Inertia Torsional Vibration Characteristics of Marine Propulsion Plant Driven by Diesel Engine

Reciprocating engines are generally analyzed for torsional vibrations by taking into account number of rotors with constant inertias coupled together by torsional springs having no mass. However the reality is that a slider crank mechanism when considered as a vibrating system has a non-constant inertia. The equivalent inertia of the entire engine mechanism fluctuates for every crankshaft rotation. High fluctuations in inertia torques will further lead to a phenomenon called secondary resonance in marine propulsion diesel engines. This would lead to the failure of a crankshaft due to excessive stresses induced. This phenomenon cannot be comprehended by the normal practice of considering the average values of fluctuating inertias. The periodic fluctuations of moment of inertia of an engine in every crank rotation give rise to cyclic variation of frequencies and corresponding amplitudes. This also leads to an increase in the speed range over which the system experiences resonance effects. In this paper a typical marine propulsion engine has been analyzed for torsional vibrations for constant as well as non-constant inertia of the engine. The linear analysis constitutes of modal analysis, order analysis, harmonic analysis, stress analysis corresponding to critical speeds of the system. The same system was further analyzed for studying the effect of non-linear parameter i.e. the non-constant inertia on the critical speeds and mode shapes. An computer program was developed to carry out such an analysis. It was found that after considering the variable inertia the range of critical speed over which hazardous vibration stress would be induced was widened.