Question

The knock in diesel engine occurs due to:
A. Instantaneous and rapid burning of the first part of the charge
B. Instantaneous auto ignition of last part of charge
C. Reduction of delay period
D. None of the mentioned

Answer 1

Diesel motors are widely used as handheld drives, as mechanical engines and as steam generators. In locomotives, heavy vehicles, vehicles, and numerous manufacturing applications, they see widespread use.

Complete answer:
Diesel engines run purely by compressing air. This raises the air temperature within the cylinder to such a high degree that it ignites spontaneously with atomized diesel fuel pumped into the combustion chamber. They were initially used by stationary steam engines as a more powerful alternative.
Knocking, in an internal-combustion engine, sharp sounds caused by premature combustion of part of the compressed air-fuel mixture in the cylinder. In a properly functioning engine, the charge burns with the flame front progressing smoothly from the point of ignition across the combustion chamber.
The diesel engine knock happens when the first component of the charge is instantaneous and easily burnt. If there is fuel build-up in the combustion chamber, the diesel engine knocks, which occurs when the pumped fuel fails to ignite, since there would be a significant delay time. And all the mixture burns concurrently at a single moment, allowing elevated peak friction and shock waves to propagate at greater speeds, leading to irreversible damage to the piston crown and the wall and head of the cylinder.
To avoid component breakage, which can result from extreme knock, the timing of the fuel being pumped into the diesel engine is crucial.

Hence, from the above discussion we can see that option A is correct.

Note: Knocking on diesel engines may be avoided by arranging the fuel injector in such a way that only a small amount of fuel should be injected at the start of the engine. If we boost the pressure of injection, which facilitates the atomization of fuel and avoids the knock.