knocking in engine

Knocking in engine

Knocking in engine is the phenomenon of an IC engine occurring due to uncontrolled combustion of fuel in the cylinder which produces sharp sound.

There are two types of knocking:

  1. Mechanical knocking

2. Fuel knocking

Mechanical knocking:

Mechanical knocking is a type of pinging sound that occurs in an IC engine when there is a problem with the engine’s mechanical components or physical contact between two engine components.

Mechanical knocking

Mechanical knocking occurs due to

  1. Worn out engine bearings

2. Broken or loose engine components

3. Malfunctioning engine accessories

Fuel knocking:

Fuel knocking is a type of knocking sound that occurs in an IC engine when either fuel is burnt prematurely or delay in burning before or after the correct position of the pump.   

There are two types of fuel knocking:

  1. Spark Ignition engine knocking
  2. Compressed Ignition Engine knocking

Spark Ignition engine knocking:

SI engine knocking occurs when air and fuel mixture in the combustion chamber detonates prematurely due to excessive heat or pressure in the cylinder. This can cause damage to the engine and decrease its efficiency.

SI engine knocking

Compressed Ignition Engine knocking:

CI engine knocking occurs when the fuel ignites too late, which can cause a knocking sound and reduce engine power. This type of knocking is often caused by fuel quality issues or incorrect injection timing.

CI engine knocking

What are the reasons for fuel knocking in the ship?

Reasons for fuel knocking are:

  1. Engine speed: If the supply of fuel to the combustion chamber is more than you need to increase the rpm of the piston to avoid knocking.

For example suppose an engine requires X amount of fuel and by chance you send X+Y amount of fuel. You will have to increase the engine speed. So that fuel is corresponding to the engine speed then knocking will stop.

engine speed

2. Load: If load is more than fuel consumption will be required more and if load is less fuel consumption will be less in the engine otherwise there can be a chance of knocking.


3. Compression ratio: It is a design thing that involves dimension of bore and length of piston stroke.

Compression ratio

4. Ignition delay: If fuel is not burnt at the correct time and ignition is delayed.

 Ignition delay

5. Injection timing: Incorrect injection timing can cause the air and fuel mixture in the combustion chamber to ignite at the wrong time.

Injection timing

6. Cetane & Octane number: For Diesel engine cetane number is required as it resembles the quality of fuel, higher the cetane number better it will give less ignition delay.

For a Petrol engine the higher octane number means better anti-knocking properties.

Cetane & Octane number


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