Diesel ignition quality trend: Among the hydrocarbon families listed, which generally shows the lowest cetane number (poorest ignition quality) in compression-ignition engines?

Introduction:
Cetane number measures readiness to autoignite under diesel engine conditions. Molecular structure has a strong effect: straight chains ignite readily, while ringed and highly branched structures resist autoignition.

Given Data / Assumptions:

• Families considered: n-paraffins, isoparaffins, naphthenes, aromatics, and olefins.
• We compare typical diesel-range components.

Concept / Approach:
General ordering for cetane: n-paraffins > isoparaffins ≈ naphthenes > olefins > aromatics (lowest). Aromatics have stable ring structures and lower hydrogen-to-carbon ratio, leading to longer ignition delays and lower cetane numbers.

Step-by-Step Solution:

Verification / Alternative check:
Fuel blending practice limits aromatic content in diesel to maintain acceptable cetane number and emissions performance.

Why Other Options Are Wrong:

• n-Paraffins: Highest cetane among common families.
• Isoparaffins/Naphthenes: Intermediate values.
• Olefins: Typically above aromatics but below paraffins/naphthenes.

Common Pitfalls:
Confusing gasoline octane (aromatics high) with diesel cetane (aromatics low).

Final Answer:
Aromatics