During which operating condition of an automobile engine is the carbon monoxide (CO) content in the exhaust gas generally at its maximum level?

Introduction / Context:
Carbon monoxide is a toxic gas produced by incomplete combustion of fuel in internal combustion engines. The amount of CO in exhaust gases depends on how efficiently the fuel air mixture burns under different operating conditions. The question asks you to identify when, in typical automobile operation, the carbon monoxide content of the exhaust is generally highest. This is important in environmental science, automotive engineering, and pollution control studies.

Given Data / Assumptions:

• The engine is a conventional petrol automobile engine.
• Operating conditions mentioned are acceleration, cruising, idle running, and deceleration.
• Carbon monoxide is formed when there is incomplete combustion, especially with rich mixtures.
• We consider typical tuning and behaviour of standard engines without advanced emission controls.

Concept / Approach:
Incomplete combustion, which produces more carbon monoxide, occurs when the fuel air mixture is richer than ideal, meaning there is more fuel relative to air than needed for complete combustion. At idle, the engine is running with the throttle nearly closed and airflow is low. Many engines tend to run on a slightly rich mixture at idle to maintain smooth operation and prevent stalling. The low airflow and rich mixture conditions promote incomplete combustion and higher CO formation. During steady cruising, mixtures are closer to stoichiometric or slightly lean for fuel economy, leading to lower CO. Acceleration can momentarily enrich the mixture, but the engine also receives more air and there is strong mixing. Deceleration may even cut off fuel in modern engines, reducing CO.

Step-by-Step Solution:
Step 1: Recall that high CO emissions are associated with rich fuel mixtures and incomplete combustion.Step 2: At idle running, the engine operates at low speed with the vehicle stationary, and the airflow through the intake is minimal.Step 3: To maintain stable combustion at idle, the carburettor or injection system often supplies a slightly rich mixture.Step 4: This combination of low air supply and relatively high fuel per unit air results in incomplete combustion and high CO formation.Step 5: Therefore, the exhaust gas typically has maximum carbon monoxide content during idle running.

Verification / Alternative check:
Emission testing data for older petrol engines and technical literature often show idle CO measurements as a key indicator of mixture richness and engine tuning. Regulations may set limits for CO levels at idle because they can be significantly higher than during normal cruising. Mechanics adjust the idle mixture screw to bring CO within acceptable limits. While acceleration can produce short bursts of higher CO, the sustained condition with consistently high CO is idle running. This practical evidence supports the selection of idle running as the condition with maximum CO content in the exhaust.

Why Other Options Are Wrong:
During rapid acceleration, option A, the mixture may briefly become richer, but the increased airflow and changing conditions make this a transient state, not typically where sustained maximum CO is measured. Steady cruising at constant speed, option B, is usually tuned for efficiency with near stoichiometric or slightly lean mixtures, resulting in lower CO. During deceleration, option D, the throttle is largely closed and fuel supply may be reduced, sometimes even cut off, lowering CO emissions. Only option C, idle running of the engine at standstill, matches the condition where engines often produce their highest sustained carbon monoxide levels.

Common Pitfalls:
Students may intuitively pick acceleration because it feels like the engine is working hardest and burning more fuel. However, emissions are not just about total fuel burned but about how completely it burns. Another pitfall is to assume that cruising produces maximum emissions because the vehicle is moving, but engines are usually most efficient in that state. To avoid such mistakes, focus on mixture richness and completeness of combustion rather than speed alone. Remember that idle operation with low airflow and slightly rich mixtures is typically associated with higher CO in exhaust gases.

Final Answer:
Idle running of the engine at standstill