Describe the phenomenon of vibration in marine diesel engines with suitable sketches. Explain the terms:

Transverse (lateral) vibration is the side-to-side (bending) oscillation of the engine or its components perpendicular to the axis. In a marine diesel engine, transverse vibration can occur in the crankshaft (bending), the engine structure (the bedplate and the entablature swaying), and the shafting. It is caused by the unbalanced forces and moments of the reciprocating and rotating masses, and by the gas-pressure forces. If the frequency of the exciting force coincides with the natural frequency of the structure (resonance), the amplitude becomes large, causing excessive vibration, noise, and stress. It is controlled by balancing, by the engine bracing (side/top bracing), and by the engine mounting.

Torsional vibration is the twisting oscillation of the crankshaft about its longitudinal axis. It arises because the crankshaft has torsional elasticity and the rotating masses (flywheel, propeller, crank throws) have inertia. The periodic torque from the cylinders excites the shaft, which twists and untwists at its natural torsional frequency. If the exciting frequency coincides with the natural frequency (resonance), the amplitude becomes large, causing high torsional stress and possible fatigue failure of the crankshaft. It is controlled by a torsional vibration damper/detuner and by avoiding the critical (barred) speed range.

Resonance is the condition when the frequency of the exciting force (e.g. the firing frequency of the engine) coincides with the natural frequency of the system (e.g. the crankshaft or the engine structure). At resonance, the amplitude of the vibration becomes very large (the system absorbs energy from the excitation), causing high stresses, excessive vibration, noise, and possible damage. Resonance must be avoided in the operating speed range (by design, by a damper, or by a barred speed range).

Vibration dampers (e.g. torsional vibration dampers, axial vibration dampers) are fitted to control the vibration. They consist of a mass (inertia ring) connected to the vibrating component (e.g. the crankshaft) by a rubber or viscous element. As the component vibrates, the mass tends to remain stationary (due to its inertia); the relative motion between the mass and the component is resisted by the rubber/fluid, which dissipates the vibrational energy as heat. This reduces the amplitude of the vibration and the stress on the component, preventing resonance damage. The damper is tuned to the natural frequency of the system to absorb the critical frequency.