Fuel ignition quality — reference fuels for cetane number In the conventional cetane rating scale for diesel fuels, which pair of reference fuels defines the ends of the scale used to match ignition delay in a standardized engine?

Introduction / Context:
Cetane number is the ignition quality index for diesel fuels, analogous to octane number for petrol. It is defined by comparing ignition delay to that of blends of standard reference fuels in a test engine under specified conditions.

Given Data / Assumptions:

• Cetane number scale based on ignition delay time.
• Reference fuels establish fixed ignition characteristics for calibration.

Concept / Approach:
Traditionally, the high-ignition-quality reference is cetane (n-hexadecane) assigned a cetane number of 100, and the low-ignition-quality reference is alpha-methylnaphthalene assigned a cetane number of 0. A test fuel is matched to a blend of these references to obtain its cetane number. Although a different low-end reference (HMN) is used in some modern standards, the classic textbook pairing remains cetane with alpha-methylnaphthalene.

Step-by-Step Solution:
Identify high reference: cetane with very short ignition delay (good ignition quality).Identify low reference: alpha-methylnaphthalene with long ignition delay (poor ignition quality).Determine the blend that matches the test fuel ignition delay to read the cetane number.

Verification / Alternative check:
Diesel fuel test standards and classic engine-fuel texts describe the cetane scale using this pair, explaining the methodology of fuel matching in CFR engines.

Why Other Options Are Wrong:
Iso-octane and normal heptane are octane-rating references, not cetane. Tetraethyl lead is an antiknock additive for petrol and does not serve as a reference fuel for cetane ratings.

Common Pitfalls:
Confusing octane and cetane scales or assuming that a modern substitute for the low reference changes the conceptual definition.

Final Answer:
Cetane and alpha-methylnaphthalene