Diesel knock is caused by which of the following phenomena?

• A. Too rapid combustion of the charge causing rapid and violent fluctuations of pressure within the cylinder
• B. Inadequate lubrication
• C. Insufficient air intake
• D. Incorrect spark timing

Answer: (A)

Diesel knock comes from the way energy is released in a diesel engine. When fuel is injected into the hot, compressed air, it should burn in a controlled manner as the piston is moving. If the fuel ignites too rapidly, the pressure inside the cylinder spikes quickly and violently. That abrupt rise in pressure sends shock-like waves through the cylinder, which you hear as a knock.

This is why the option describing too rapid combustion and the resulting rapid, violent pressure fluctuations best captures the cause. Inadequate lubrication affects wear and friction, not the rate of combustion. Insufficient air intake can cause power loss or rough running, but not the specific rapid pressure rise that defines knock. And diesel engines rely on compression ignition, not spark timing, so incorrect spark timing isn’t the cause in diesels.