Spark-ignition engine fundamentals: What is the term for ignition of the air–fuel charge by a hot spot inside the cylinder occurring before the timed spark event?

Introduction / Context:
This question checks your understanding of abnormal combustion phenomena in spark-ignition (petrol) engines. Two terms are often confused: pre-ignition and detonation (knock). Distinguishing them is essential for correct diagnosis, calibration, and engine protection.

Given Data / Assumptions:

• A spark-ignition engine uses a spark plug to ignite a premixed air–fuel charge at a scheduled crank angle.
• Hot spots can exist on valves, spark plug electrodes, carbon deposits, or sharp edges.
• We assume normal mixture strength and no forced-induction unless stated otherwise.

Concept / Approach:
Pre-ignition is ignition that starts before the intended spark because a local surface is hot enough to ignite the mixture. Detonation is a violent pressure rise that occurs after normal spark ignition when the unburned end-gas auto-ignites spontaneously, producing pressure waves (knock). Ignition delay refers to the time between spark discharge and the start of flame kernel growth, a separate concept.

Step-by-Step Solution:

Verification / Alternative check:
Symptoms of pre-ignition include abnormally advanced pressure rise, severe overheating, and potential piston damage. Unlike knock, which often lessens when the throttle is eased, pre-ignition can persist and escalate with heat, validating the definition.

Why Other Options Are Wrong:

• Detonation: Occurs after spark because end-gas auto-ignites; it is not the initial cause of ignition.
• Ignition delay: A time interval after the spark, not an early ignition event.
• Auto-ignition: Generic term for spontaneous ignition; here the specific pre-spark, hot-surface event is called pre-ignition.

Common Pitfalls:
Confusing knock noise with pre-ignition. Knock can sometimes be audible; pre-ignition may silently overheat components. Both are harmful, but mechanisms and remedies differ.

Final Answer:
pre-ignition