Fuel quality for compression-ignition (Diesel) engines Which hydrocarbon type provides the best ignition quality (highest cetane) for CI engines?

Introduction / Context:
Diesel engines rely on auto-ignition. The fuel’s ignition quality is measured by cetane number. Understanding which molecular structures promote quick ignition aids in blending and troubleshooting.

Given Data / Assumptions:

• Cetane number correlates inversely with ignition delay.
• Fuel families include paraffins, naphthenes, aromatics, and olefins.
• Combustion conditions typical of road Diesels.

Concept / Approach:
Straight-chain (normal) paraffins have high cetane numbers and ignite readily. Aromatics and highly branched paraffins generally have lower cetane and ignite more slowly, increasing ignition delay and noise.

Step-by-Step Solution:
Identify molecular structure that favours short ignition delay.Normal paraffins (e.g., n-cetane) are the reference for high cetane quality.Therefore select straight chain paraffins.

Verification / Alternative check:
Standardized tests (CFR engine) show normal paraffins score higher cetane than aromatics and branched isomers.

Why Other Options Are Wrong:
Aromatics and polyaromatics tend to low cetane and higher soot.

Branched paraffins reduce cetane compared with their normal isomers.

Olefins’ behavior is variable and not as favourable as straight paraffins.

Common Pitfalls:
Confusing octane (spark-ignition knock resistance) with cetane (compression-ignition quality); additives can raise cetane but do not change base chemistry.

Final Answer:
straight chain paraffins