Air–fuel ratio at idle (spark-ignition engines) During idling of a petrol (gasoline) engine, which mixture strength is generally required for stable combustion and smooth running?

Introduction / Context:
Idle operation presents a challenging combustion environment: low air motion, low manifold pressure, and low in-cylinder temperatures. Carburetor idle circuits and modern ECU idle strategies enrich the mixture to maintain a stable flame and prevent misfire. Recognizing the appropriate mixture at idle is vital for troubleshooting rough idle and setting carburetors or ECUs.

Given Data / Assumptions:

• Spark-ignition engine operating at low speed and very light load (closed or nearly closed throttle).
• Conventional homogeneous-charge operation.
• Goal is smooth, reliable combustion without misfire.

Concept / Approach:

At idle, air velocity through the intake is low, fuel atomization and vaporization are poorer, and flame propagation can be inconsistent. A slightly rich mixture (more fuel than stoichiometric) improves flame speed and stability and reduces the chance of lean misfire. Historically, carburetors include dedicated idle passages calibrated to deliver this richer mixture; modern engines may use closed-loop control but still target a mixture richer than the nominal stoichiometric in many conditions during cold or unstable idle.

Step-by-Step Solution:

Verification / Alternative check:

Service literature for carbureted engines specifies idle mixture screws that typically enrich for highest stable idle speed. Modern ECU maps may use enrichment during cold idle and stabilization phases to prevent misfires and maintain stable rpm.

Why Other Options Are Wrong:

“Lean” or “ultra-lean” increases lean misfire risk at idle due to poor ignition quality.“Chemically correct” is a laboratory ideal and often too lean for stable idle under cold or low-turbulence conditions.“Oxygen-enriched” is not a practical mixture description for automotive idle control.

Common Pitfalls:

Confusing warm, well-calibrated closed-loop idling (near stoichiometric) with cold-start and idle stabilization phases that still use enrichment for stability.

Final Answer:

rich