Automotive emissions behavior: During which operating condition of a spark-ignition engine is the carbon monoxide (CO) concentration in the exhaust typically at its maximum?

Introduction / Context:
CO formation in spark-ignition (SI) engines is strongly influenced by air–fuel ratio. Rich mixtures (fuel > stoichiometric) favor incomplete oxidation to CO, while leaner mixtures promote full conversion to CO2. Understanding CO behavior aids engine tuning and emission control strategies.

Given Data / Assumptions:

• Conventional SI engine without transient-perfect controls (conceptual behavior also holds with modern systems though mitigated).
• Operating modes: idle, acceleration, cruise, and deceleration.
• CO correlates with rich operation and low excess oxygen.

Concept / Approach:
During acceleration, throttle opening increases and enrichment strategies (or transient wall-wetting effects) momentarily provide rich mixtures to prevent hesitation/knock and to deliver torque. This rich spike increases CO formation. Idle CO can be elevated if mixture is slightly rich, but generally lower than acceleration spikes. Cruise uses near-stoichiometric or slightly lean mixtures, and deceleration often cuts fuel or runs very lean, minimizing CO.

Step-by-Step Solution:
1) Identify CO driver: rich mixtures with insufficient O2.2) Map operating modes: acceleration → richest; cruise → near stoichiometric; idle → moderate; decel → lean/fuel cut.3) Conclude maximum CO occurs during acceleration.

Verification / Alternative check:
Engine emission maps show transient CO peaks with throttle tip-in. Modern closed-loop systems and catalytic converters reduce the peak magnitude but the relative tendency remains.

Why Other Options Are Wrong:
Deceleration: Fuel cut/lean → minimal CO.Cruising: Near-stoichiometric/lean burn → low CO.Idle: Can be rich but generally lower CO than under load during acceleration.

Common Pitfalls:
Confusing hydrocarbon spikes at cold start with CO behavior during warm operation.Ignoring the catalytic converter’s role in post-combustion CO oxidation.

Final Answer:
Acceleration (rich mixture demand)