Which term describes the rapid fluctuations of cylinder pressure caused by too rapid combustion in a diesel engine?

• A. Pre-ignition
• B. Knock
• C. Misfire
• D. Detonation

Answer: (B)

The main idea is rapid pressure rise causing pressure waves inside the cylinder. When a diesel engine injects fuel into the hot, highly compressed air, combustion normally progresses in a controlled way. If the rate of heat release is too fast, the pressure in the cylinder shoots up rapidly and creates pressure oscillations that you can hear or feel as knocking. This phenomenon is known as knock (often called diesel knock in diesel engines). It reflects an abnormal, too-rapid combustion process that the engine components have to withstand.

Pre-ignition would be ignition happening before the intended timing due to hot spots, which isn’t the situation described here since diesel ignition is controlled by injection timing, not an external spark. Misfire means the cylinder fails to ignite (little or no pressure rise). Detonation is a term more associated with explosive burning of the unburned charge in spark-ignition engines and isn’t the standard description for this diesel scenario. So the described rapid pressure fluctuations due to too-rapid combustion fit the idea of knock.