Which of the following best describes the detonation phenomenon?

• A. rapid rise in cylinder pressure
• B. excessive cylinder head temperature
• C. decrease in engine power
• D. erratic ignition timing

Answer: (A)

Detonation is a rapid, explosive rise in cylinder pressure caused when the unburned portion of the air–fuel mixture ahead of the flame front auto-ignites after the spark starts combustion. This creates a second flame front and a pressure wave that spikes cylinder pressure abruptly, often heard as a knock and can stress and damage engine components. That sudden pressure surge is what the option describes.

Excessive cylinder head temperature can raise the tendency for detonation but isn’t the detonation event itself. A decrease in engine power can result from detonation damage or efficiency loss, but it’s not the defining phenomenon. Erratic ignition timing relates to when combustion starts, not the sudden pressure spike that characterizes detonation.