Spark-ignition engines – stoichiometric mixture strength What is the approximate stoichiometric air–fuel ratio by mass for petrol (gasoline) used in spark-ignition engines?

Introduction / Context:
The stoichiometric (chemically correct) mixture provides exactly enough oxygen to fully combust the fuel without excess oxygen or fuel. For petrol (gasoline), this ratio is a cornerstone of engine calibration, emissions control, and lambda-sensor feedback strategies.

Given Data / Assumptions:

• Air is treated as 21% O2 and 79% N2 by volume (approximately).
• Gasoline is approximated by a representative hydrocarbon such as iso-octane, C8H18, for stoichiometric calculation.
• Complete combustion to CO2 and H2O is assumed.

Concept / Approach:
Balancing the reaction for a surrogate like C8H18 leads to a required oxygen amount that, when converted to air, yields a mass ratio near 14.7:1. Variations in actual gasoline composition and measurement conventions produce values around 14.5–15.0. Hence, 14.6:1 is a standard rounded figure used in many texts and engine maps.

Step-by-Step Solution:
Write ideal reaction: C8H18 + 12.5 O2 → 8 CO2 + 9 H2O.Convert O2 to air using air/O2 ≈ 100/21 by volume and corresponding mass ratio.Compute mass of air required per unit mass of fuel → ≈ 14.7:1; practical rounded value ≈ 14.6:1.

Verification / Alternative check:
Engine controllers target λ = 1 (stoichiometric) for three-way catalyst operation, corresponding to ≈ 14.7:1 AFR; textbooks and datasheets commonly quote 14.6–14.7:1.

Why Other Options Are Wrong:

• 18.5:1, 20.4:1, and 22.6:1 are lean mixtures (excess air), not stoichiometric.
• 10.0:1 is a rich mixture used for high load or cold start, not chemically correct.

Common Pitfalls:
Confusing volume ratios with mass ratios; for control and energy calculations, mass-based AFR is standard.

Final Answer:
14.6:1