Combustion in SI engines — if the air–fuel mixture is described as “too rich” in a spark-ignition engine, which approximate overall air–fuel ratio (by mass) best represents this rich condition?

Introduction / Context:
For gasoline (petrol), the stoichiometric air–fuel ratio (AFR) is about 14.7:1 by mass. Mixtures richer than stoichiometric have less air per unit fuel (numerically smaller AFR), while lean mixtures have more air (numerically larger AFR). Recognizing indicative AFR values is important for diagnostics, tuning, emissions, and fuel economy discussions.

Given Data / Assumptions:

• SI engine operating on gasoline under typical conditions.
• “Too rich” implies well below stoichiometric, far on the fuel-rich side.
• Options span common AFR figures across rich/lean ranges.

Concept / Approach:

At 10:1 AFR, the charge contains substantially more fuel than required for complete combustion with the available oxygen. Such mixtures tend to produce black smoke, high CO/HC emissions, poor fuel economy, and can foul spark plugs and catalytic converters. While 13:1 is somewhat rich and 15–17:1 are around stoichiometric to lean, 10:1 clearly indicates an excessively rich condition consistent with the phrase “too rich.”

Step-by-Step Solution:

Verification / Alternative check:

Dynamometer tuning data show rich power AFRs near 12.5–13.0:1 for many engines; values near 10:1 are richer than typical power targets and are diagnostic of faults (e.g., leaking injectors, failed sensors).

Why Other Options Are Wrong:

17:1 — lean; 15:1 — slightly lean of stoichiometric; 13:1 — rich but not “too rich” in the diagnostic sense.

Common Pitfalls:

Confusing AFR direction (smaller number = richer); assuming lambda display equals AFR without fuel composition correction (ethanol blends change stoichiometric AFR).

Final Answer:

10:1