Exhaust emissions — When do hydrocarbons (HC) rise sharply? Identify the operating condition most associated with large quantities of unburned hydrocarbon emissions from a spark-ignition engine.

Introduction / Context:
Hydrocarbon (HC) emissions represent unburned or partially burned fuel. High HC output indicates poor combustion quality, misfire, or flame quench effects, and is regulated by emissions standards.

Given Data / Assumptions:

• Spark-ignition engine, port or direct injection.
• Focus on engine-out emissions (pre-catalyst).
• Normal ambient conditions.

Concept / Approach:
HC rises when combustion is incomplete. Causes include misfire (too lean or too rich), low in-cylinder temperature causing flame quench near cold walls, poor mixture preparation, or inadequate ignition energy. While “low temperature combustion” can contribute by quenching the flame near surfaces, the broader, more fundamental root is incomplete combustion, encompassing all of these failure modes.

Step-by-Step Solution:
Consider combustion outcomes: complete burn → CO2 and H2O; incomplete burn → CO and HC.Identify drivers of incomplete combustion: misfire, quench, wall wetting, poor atomization.Therefore, “incomplete combustion” best captures the condition for high HC emissions.

Verification / Alternative check:
Emission maps show HC spikes during cold start, idle instability, and misfire events when combustion completeness suffers.

Why Other Options Are Wrong:
High temperature combustion: tends to raise NOx, not HC.Low temperature alone is a contributing mechanism but is encompassed by incomplete combustion.High atmospheric temperature: minor influence compared to in-cylinder processes.Stoichiometric with active catalyst: leads to minimal tailpipe HC.

Common Pitfalls:
Confusing CO and HC: both rise under rich or misfire, but reasons differ.

Final Answer:
incomplete combustion