Exhaust Emissions — When is carbon monoxide (CO) highest? Identify the engine operating condition most likely to produce large quantities of CO in the exhaust.

Introduction / Context:
Carbon monoxide (CO) is a primary pollutant from spark-ignition engines. It forms when carbon in fuel is not fully oxidized to carbon dioxide (CO2). Understanding mixture effects on CO formation is fundamental to emissions control and closed-loop fuel management.

Given Data / Assumptions:

• SI engine without or before catalytic converter conversion.
• Comparison of rich vs. lean and temperature effects.
• Normal engine temperature, no severe misfire scenario.

Concept / Approach:
CO increases sharply with rich mixtures (lambda less than 1) because there is insufficient oxygen to fully oxidize carbon to CO2. Lean mixtures provide excess oxygen, typically reducing CO dramatically, though they may increase NOx if temperature is high. Extremely low temperature can increase unburned hydrocarbons (HC), but the dominant factor for CO is oxygen deficiency.

Step-by-Step Solution:
Relate CO formation to equivalence ratio: rich → high CO due to oxygen shortfall.Lean conditions → more O2 available → lower CO.Therefore, “insufficient air during combustion” (rich) yields high CO.

Verification / Alternative check:
Engine maps show CO rising rapidly at equivalence ratios above 1.0 and dropping as mixtures approach stoichiometric or lean.

Why Other Options Are Wrong:
Insufficient fuel (lean): tends to reduce CO but may increase NOx if hot.Low temperature combustion: affects HC more than CO.High temperature alone does not cause high CO if air is sufficient.Lean misfire: CO not dominant; HC spikes due to misfire.

Common Pitfalls:
Confusing CO (partial oxidation) with HC (unburned fuel) and NOx (temperature/oxygen dependent).

Final Answer:
insufficient air during combustion