Component class preference across fuels\nWhich component class is desirable in diesel and kerosene but generally undesirable in gasoline?

Introduction / Context:
Hydrocarbon classes influence ignition quality, smoke tendency, and octane rating. Diesel/kerosene require good ignition (high cetane) and clean burning, whereas gasoline requires knock resistance (high octane).

Given Data / Assumptions:

• Paraffins include normal and isoparaffins; cetane is best for normal paraffins.
• Aromatics raise octane but reduce cetane and raise smoke.

Concept / Approach:
In diesel and kerosene, higher normal paraffin content increases cetane number and improves ignition quality; kerosene smoke point also benefits from lower aromatics. In gasoline, paraffins (especially normal) lower octane relative to aromatics and isoparaffins; thus, paraffins are comparatively undesirable in gasoline blends targeting high octane. Therefore, paraffins are the class that is desirable in diesel/kerosene but undesirable in gasoline.

Step-by-Step Solution:

Verification / Alternative check:
Fuel property charts show normal paraffins: high cetane, low octane; aromatics: low cetane, high octane; consistent with this selection.

Why Other Options Are Wrong:

• Aromatics improve gasoline octane but harm diesel/kerosene properties.
• Mercaptans and acids are undesirable impurities in all fuels.
• Olefins cause gum and instability issues.

Common Pitfalls:
Failing to distinguish normal paraffins from isoparaffins; isoparaffins can aid octane more than normals but still lag aromatics for RON.

Final Answer:
Paraffins