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Class 4 (4th Engineer) MEKM 📅 Oct 2024

Exam Question

A very sudden rise in pressure during combustion accompanied by a metallic hammer like sound is called knocking of the unit or detonation and usually ocours (16)

(a) Near the end of the pressure rise after ignition of fuel charge

(b) Just before the fuel charge is about to ignite

(c) Immediately after the unit reaches peak pressure and before ignition of fuel

(d) At any stage of the combustion process.

State the correct answer and explain why the other reasons given are not justified

Reference Answer

The correct option is **(a) Near the end of the pressure rise after ignition of fuel charge**.
### **Detailed Explanation of Diesel Knock (Detonation)**
Knocking, also known as detonation, in a Compression Ignition (CI) engine is a phenomenon characterized by an abnormally rapid rate of pressure rise following the ignition of the fuel charge. This creates pressure waves within the cylinder, which strike the cylinder walls, liner, and piston crown, producing a distinct, sharp metallic hammering sound.
The combustion process in a CI engine can be divided into four distinct phases. The root cause of knocking lies in the first two phases:

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