Air–Fuel Ratio for Petrol – Stoichiometric (Chemically Correct) Mixture Select the approximate stoichiometric air–fuel ratio by mass for a conventional petrol (gasoline) engine mixture used for complete combustion.

Introduction / Context:
The air–fuel ratio (AFR) at which exactly enough oxygen is supplied to burn all the fuel without excess air or fuel is called stoichiometric. For petrol engines, this AFR is a key target for three-way catalysts and closed-loop lambda control to minimize emissions while maintaining good drivability.

Given Data / Assumptions:

• Petrol approximated as a representative hydrocarbon (e.g., iso-octane C8H18).
• Complete combustion to CO2 and H2O assumed.
• Mass-based AFR values are considered.

Concept / Approach:

For iso-octane as a surrogate, the balanced oxidation is C8H18 + 12.5 O2 → 8 CO2 + 9 H2O. Air contains about 21% O2 by volume (≈23% by mass), so the mass of air needed is roughly 14.7 times the mass of fuel. Rounded to a practical figure used in calibration and textbooks, the stoichiometric AFR for petrol is about 15:1 by mass.

Step-by-Step Solution:

Verification / Alternative check:

Engine calibrations use lambda = 1 at AFR ≈ 14.7:1. Emissions control and stoichiometric operation reinforce this canonical value.

Why Other Options Are Wrong:

10:1 is rich (used for power enrichment). 20:1 and 25:1 are lean burn values associated with fuel economy but higher NOx without special strategies. 12.5:1 is a common rich power AFR for knock avoidance and cooling but not stoichiometric.

Common Pitfalls:

Mixing up volume-based and mass-based ratios; assuming one exact value for all petrol blends—the 14.7–15:1 figure is an accepted practical average.

Final Answer:

15:1