What can "knock"?

What can "knock"?

• A. Under advanced timing of the injector
• B. Detonation due to high octane fuel
• C. Ignition timing is irrelevant
• D. Over advanced timing of the injector

Answer: (D)

Knock happens when combustion starts too early or too aggressively, producing a sharp pressure rise that the piston can’t smoothly absorb. When the injector timing is too far advanced, fuel begins to burn earlier in the compression stroke, pushing pressure up while the piston is still near top dead center. That premature, rapid combustion creates the knocking or pinging sound and can harm the engine. Using higher octane fuel actually helps resist premature ignition, so the idea that knock comes from high octane fuel isn’t correct. Ignition timing clearly matters—if it’s not set correctly, knock can occur. Retarding timing (making it less advanced) would reduce or prevent knock rather than cause it. So the condition that can cause knock is over advanced timing of the injector.

Knock happens when combustion starts too early or too aggressively, producing a sharp pressure rise that the piston can’t smoothly absorb. When the injector timing is too far advanced, fuel begins to burn earlier in the compression stroke, pushing pressure up while the piston is still near top dead center. That premature, rapid combustion creates the knocking or pinging sound and can harm the engine.

Using higher octane fuel actually helps resist premature ignition, so the idea that knock comes from high octane fuel isn’t correct. Ignition timing clearly matters—if it’s not set correctly, knock can occur. Retarding timing (making it less advanced) would reduce or prevent knock rather than cause it. So the condition that can cause knock is over advanced timing of the injector.