What can 'knock'?

• A. Incorrect fuel mixture
• B. Over advanced timing of the fuel pump
• C. Over advanced timing of the injector
• D. Improper scavenging

Answer: (C)

Knock happens when combustion starts too early in the cycle, causing a rapid, high-pressure rise that sends shock waves through the cylinder. In diesel engines, the exact moment fuel is injected determines when burning begins. If injection timing is advanced, fuel enters the cylinder earlier in the compression stroke, so ignition occurs sooner than intended and the pressure peak occurs at an improper point in the cycle, producing the knocking sound. The injector is the component that sets that timing, so over advancing the injector timing directly leads to knock. An incorrect fuel mixture can cause rough running but isn’t the primary cause of knock, scavenging issues affect breathing rather than ignition timing, and while advancing the fuel pump timing could also promote early ignition, the injector timing is the most direct and correct reference for this knock-causing scenario.