Fuel rating for diesel – reference fuels In the cetane scale used for compression-ignition fuels, the straight-chain paraffin n-hexadecane (cetane, C16H34) is assigned what cetane number?

Introduction / Context:
Diesel fuel ignition quality is rated by the cetane number, analogous to the octane number for petrol but measuring a very different property: readiness to auto-ignite (short ignition delay) under compression-ignition conditions.

Given Data / Assumptions:

• Cetane scale is defined by two reference fuels.
• Test conditions follow standardized CI engine procedures.
• Ignition delay is the key metric mapped to cetane number.

Concept / Approach:
The cetane scale historically uses n-hexadecane (cetane, C16H34) as the high-ignition-quality reference with a cetane number of 100, and alpha-methylnaphthalene (or, in modern practice, isocetane) as the low-ignition-quality reference near 0. Real fuels are compared by matching ignition delay to mixtures of the reference fuels.

Step-by-Step Solution:
Identify reference: C16H34 is cetane → high ignition quality.Map to scale: by definition, cetane = 100 CN.Therefore, the assigned number for cetane is 100.

Verification / Alternative check:
Standard fuel testing literature and engine handbooks list cetane (n-hexadecane) at CN = 100, establishing the upper reference point of the scale.

Why Other Options Are Wrong:

• 0 corresponds to the low-ignition-quality reference.
• 50 or 120 are not the defined reference values for cetane.
• 10 is arbitrary and incorrect.

Common Pitfalls:
Confusing octane and cetane concepts: high octane resists auto-ignition (good for SI), whereas high cetane favors easy auto-ignition (good for CI).

Final Answer:
100