Exam Question
Explain briefly how the different moving parts of a main engine are subjected to various stresses. Name the stresses and justify by stating how they originate. (16)
Class 4 (4th Engineer)
MEKM
📅 Nov 2024
Reference Answer
The complex operational cycle of a main engine, involving high pressures, extreme temperatures, and rapid accelerations/decelerations, subjects its various moving parts to a multitude of stresses. These stresses are dynamic and cyclic, demanding robust design, material selection, and precise manufacturing to ensure reliability and longevity. Understanding their origin is crucial for effective maintenance and fault diagnosis.
### 1. Piston Assembly (Piston Crown, Skirt, Rings, Piston Pin/Gudgeon Pin, Piston Rod)
* **Compressive Stress**: Originates from the immense gas pressure exerted on the piston crown during the combustion and expansion strokes. This pressure, reaching hundreds of bar, attempts to crush the piston.
* $$ \sigma_{\text{compressive}} = \frac{P_{\text{gas}}}{A_{\text{piston}}} $$
### 1. Piston Assembly (Piston Crown, Skirt, Rings, Piston Pin/Gudgeon Pin, Piston Rod)
* **Compressive Stress**: Originates from the immense gas pressure exerted on the piston crown during the combustion and expansion strokes. This pressure, reaching hundreds of bar, attempts to crush the piston.
* $$ \sigma_{\text{compressive}} = \frac{P_{\text{gas}}}{A_{\text{piston}}} $$
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