Combustion in SI engines — meaning of auto-ignition In a spark-ignition petrol engine, auto-ignition refers to which phenomenon within the combustion chamber?

Introduction / Context:
Auto-ignition is central to knock (detonation) in spark-ignition engines. Recognizing its definition distinguishes normal flame propagation from abnormal combustion.

Given Data / Assumptions:

• SI engine with spark-initiated flame kernel.
• Homogeneous mixture in the combustion chamber.
• End-gas region subject to pressure and temperature rise during combustion.

Concept / Approach:
Normal combustion begins at the spark plug and a flame front travels across the chamber. Auto-ignition occurs when unburned end-gas spontaneously ignites due to high temperature/pressure and reaction kinetics before the flame front reaches it, often producing pressure waves perceived as knock.

Step-by-Step Solution:

Verification / Alternative check:
Knock sensors detect characteristic frequency vibrations; higher octane fuels resist auto-ignition by elevating knock resistance.

Why Other Options Are Wrong:

• “Automatic ignition at end of compression” is vague and not specific to end-gas auto-ignition.
• Spark-induced ignition is normal, not auto-ignition.
• Supplemental ignition event is not a standard engine process.
• ECU spark control is unrelated to spontaneous chemical ignition.

Common Pitfalls:
Confusing pre-ignition (hot spot triggered before scheduled spark) with knock (auto-ignition of end-gas after spark).

Final Answer: