Question

(a) Define the term Torsional Vibration with respect to an engine crankshaft, stating the effect that high levels can have on an engine crankshaft. (6)

(b) Explain how engine deterioration influences the risk of Torsional Vibration, stating what can be done to minimise that risk. (5)

(c) Explain TWO possible reasons for the activation of a Torsional Vibration alarm after an engine has been started if there had been no previous history of such an alarm and if no maintenance had been undertaken on the engine whilst it was stopped. (5)

Answer 1

Part (a)

Torsional vibration is caused by forces applied to the crankpin by the connecting rod, which vary according to the angle of thrust exerted by the connecting rod and the cylinder firing pressure. It occurs during the firing and compression strokes.

This stress is cyclic, meaning the crankshaft twists and untwists along its length. In direct-drive engines, torsional vibration can be exacerbated by an unbalanced engine cylinder or propeller shaft, potentially caused by a damaged propeller.

An increase in torsional vibrations results in higher torsional stress, which adds to the existing stress levels. This increase in stress can lead to the generation and growth of cracks in high-stress areas of the crankshaft. If left unaddressed for an extended period, this condition can lead to the crankshaft breaking.

Part (b)

As the engine deteriorates over time, the materials weaken due to fatigue. Fatigue occurs when a material becomes "tired" and fails at a stress level below its nominal strength. Torsional vibration is a cyclic stress that causes the crankshaft to twist and untwist repeatedly.

If the engine is overloaded, it exerts a high amount of stress on the crankshaft, leading to cracks and eventual failure. To minimize this risk:

Keep the engine cylinders balanced to ensure even loading on the crankshaft.
Operate the engine within the limits prescribed by the manufacturer, referencing performance results such as from sea trials.
Regularly check engine performance to analyze the engine's condition and ensure it remains within operational limits.

Part (c)

Two possible reasons for Torsional vibration alarm activation after engine start:

The engine's rotational speed might have coincidentally passed through a critical speed, where the excitation frequency matches a natural frequency of the crankshaft system. This resonance amplifies the vibrations, triggering the alarm.
If one or more cylinders are unbalanced (e.g., due to improper combustion or issues with the fuel system), uneven forces can generate excessive torsional vibration.
An imbalance in the engine's cylinders could generate irregular firing torques, leading to increased torsional vibrations and activating the alarm system. This could be due to unforeseen internal component failure or a previously undetected manufacturing defect.
Slight misalignment in the crankshaft's main bearings could induce high bending stresses and increase torsional vibrations
Maneuvering in shallow water can increase propeller load and generate additional cyclic stresses on the crankshaft, resulting in torsional vibration.
In rough seas, cyclic loading on the propeller shaft caused by wave action can transmit additional torsional stresses to the crankshaft, activating the alarm.

✓ Verified Model Answer (Text Solution)

Torsional vibration is caused by forces applied to the crankpin by the connecting rod, which vary according to the angle of thrust exerted by the connecting rod and the cylinder firing pressure. It occurs during the firing and compression strokes.

As the engine deteriorates over time, the materials weaken due to fatigue. Fatigue occurs when a material becomes "tired" and fails at a stress level below its nominal strength. Torsional vibration is a cyclic stress that causes the crankshaft to twist and untwist repeatedly.

Two possible reasons for Torsional vibration alarm activation after engine start: