What is the effect of delay period on Knock in CI engines?

 

🔥 Effect of Delay Period on Knock in CI (Compression Ignition) Engines

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📌 What is Delay Period?

The delay period (also called ignition delay) in a CI engine is the time interval between:

• Start of fuel injection, and

• Start of combustion (auto-ignition) of the fuel

It is usually measured in crank angle degrees (CAD).

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⚠️ Effect of Delay Period on Knock:

In CI engines, diesel knock is a sharp metallic sound caused by sudden and uncontrolled combustion of accumulated fuel. This is directly related to the delay period.

🔄 When the Delay Period is Long:

• More fuel is injected into the cylinder before combustion starts.

• This leads to accumulation of fuel-air mixture.

• Once ignition begins, a large volume of fuel combusts rapidly and simultaneously.

• This causes high pressure rise rates → diesel knock.

🔄 When the Delay Period is Short:

• Less fuel is accumulated before combustion starts.

• The combustion process is more controlled and progressive.

• This results in smoother pressure rise and less knocking.

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📉 Summary of Effects:

Parameter	Long Delay Period	Short Delay Period
Fuel Accumulation	High	Low
Combustion Rate	Sudden, rapid	Controlled, gradual
Pressure Rise Rate	Very high	Moderate
Knock Intensity	Severe (more knock)	Mild (less knock)
Engine Noise	Higher	Lower

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✅ Conclusion:

A longer ignition delay in a CI engine leads to more severe knock due to the rapid and uncontrolled combustion of accumulated fuel. To minimize knock, engine designers aim to reduce the delay period using high injection pressures, good fuel atomization, proper injection timing, and fuels with high cetane number (better auto-ignition properties).