Which condition can cause knocking in a gasoline engine?

Which condition can cause knocking in a gasoline engine?

• A. Too much fuel
• B. Low octane fuel
• C. Under advanced timing of the injector
• D. Over advanced timing of the injector

Answer: (D)

Knock happens when the air–fuel mix ignites too early in the cycle, creating pressure waves that distort the piston’s motion. If the injector timing is advanced beyond what the engine can safely handle, fuel is introduced earlier in the compression stroke. That early injection means combustion starts sooner, when the cylinder is under higher pressure and temperature, so the flame front and any auto-ignition tendencies fight the piston’s movement. The result is a high-pressure surge that causes the characteristic knocking sound and can stress engine components. Other factors like using low octane fuel reduce the fuel’s resistance to detonation, which can cause knock, and an overly rich or lean mixture can affect combustion as well, but the condition that directly leads to knocking in this context is injecting fuel earlier than intended.

Knock happens when the air–fuel mix ignites too early in the cycle, creating pressure waves that distort the piston’s motion. If the injector timing is advanced beyond what the engine can safely handle, fuel is introduced earlier in the compression stroke. That early injection means combustion starts sooner, when the cylinder is under higher pressure and temperature, so the flame front and any auto-ignition tendencies fight the piston’s movement. The result is a high-pressure surge that causes the characteristic knocking sound and can stress engine components.

Other factors like using low octane fuel reduce the fuel’s resistance to detonation, which can cause knock, and an overly rich or lean mixture can affect combustion as well, but the condition that directly leads to knocking in this context is injecting fuel earlier than intended.