In a compression-ignition (diesel) engine, diesel knock is primarily caused by:
Options:
(A) An excessively long ignition delay period leading to excessive accumulation of injected fuel that burns explosively upon ignition
(B) An excessively high cetane number of the fuel
(C) Hot-spot pre-ignition from a fouled spark plug
(D) Extremely advanced fuel atomization with very short delay period
Answer Key:
A
Step-by-Step Solution & Explanation
If the ignition delay period is too long, a large quantity of fuel is injected and vaporized before ignition begins. When auto-ignition occurs, this entire accumulated fuel mass burns almost instantaneously, causing an extremely high rate of pressure rise ($dp/d\theta > 8 - 10\text{ bar/deg}$) and audible diesel knocking.