Combustion in SI engines — meaning of auto-ignition In a spark-ignition petrol engine, auto-ignition refers to which phenomenon within the combustion chamber?
Correct Answer: ignition of part of the charge ahead of the flame front (end-gas) before the flame reaches it
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:
Spark initiates flame kernel near top dead center.Flame propagates; end-gas is compressed and heated.If end-gas reaches its auto-ignition conditions, spontaneous ignition occurs ahead of the flame.Rapid energy release generates pressure oscillations (knock).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:
ignition of part of the charge ahead of the flame front (end-gas) before the flame reaches it