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

Diesel knock is controlled by the ignition delay period, the gap between fuel injection and the start of combustion. Any factor that shortens or lengthens that gap changes how badly the engine knocks.

1. Injection timing: correct. If fuel is injected too early into cooler, lower pressure air, or too late, the delay period changes and more fuel can accumulate before ignition, increasing knock.
2. Cetane number of the fuel: correct. Cetane number is a direct measure of a fuel's self ignition quality, a low cetane fuel takes longer to ignite, builds up more unburned fuel, and knocks harder.
3. Spark timing: wrong. Diesel combustion is compression ignition with no spark plug at all, so there is no spark timing to adjust; this option belongs to petrol engines, not Diesel engines.
4. Compression ratio: correct. Raising the compression ratio raises the air temperature the fuel is injected into, shortening the delay period and reducing the chance of knock.

Since spark timing simply does not exist in a Diesel engine, the remaining three, injection timing, cetane number and compression ratio, are the correct answer: A, B and D.