Question

Which of the following factors affect(s) knocking in a Diesel engine?

• A. Injection timing
• B. Cetane number of the fuel
• C. Spark timing
• D. Compression ratio

Hint:

Remember a Diesel engine ignites fuel by compression heat alone, so ask which factors change that ignition delay.

Answer 1

The Correct Option is A, B, D

Step 1: Recall how knock happens in a Diesel engine.
Diesel knock comes from an ignition delay: fuel is injected into hot compressed air but does not burn instantly, so fuel builds up before combustion starts, and when it finally ignites the sudden pressure rise causes the knocking noise. Anything that changes this delay or the air state at injection changes knock.

Step 2: Check injection timing, cetane number and compression ratio.
Injection timing sets when fuel enters the hot air, so injecting too early or too late lengthens the delay and worsens knock. Cetane number measures how readily the fuel self ignites, a higher cetane number means a shorter delay and less knock, so it directly affects knock. Compression ratio sets the air temperature and pressure at the end of compression, a higher ratio gives hotter air, shorter delay and less knock. All three factors change the ignition delay.

Step 3: Rule out spark timing.
A Diesel engine has no spark plug and no external ignition source, fuel ignites purely from the heat of compression. Spark timing is a control variable for petrol (SI) engines, not for a compression ignition Diesel engine, so it cannot affect Diesel knock.

Final Answer:
Injection timing, cetane number and compression ratio affect Diesel knock; spark timing does not apply to a Diesel engine. \[ \boxed{\text{A, B and D}} \]