Ignition quality reference scale (diesel): On the historical cetane number scale for compression-ignition fuels, the reference value assigned to alpha-methylnaphthalene is taken as what?

Introduction / Context: Cetane number quantifies diesel fuel ignition quality. The scale originally used two reference hydrocarbons: n-cetane (high ignition quality) and alpha-methylnaphthalene (poor ignition quality). Understanding the endpoints of this scale is foundational in fuel testing.

Given Data / Assumptions:

• CFR engine test methods define cetane number.
• Original references: n-cetane (assigned 100) and alpha-methylnaphthalene (assigned 0).
• Modern practice often uses heptamethylnonane (isocetane) as a 15 reference, but the 0-point legacy still appears in exams.

Concept / Approach: The conventional historical assignment places alpha-methylnaphthalene at the zero point due to its very poor auto-ignition tendency in CI engines, thereby setting the lower bound of the cetane scale. This provides a linear reference between poor and excellent ignition fuels for calibration and comparison.

Step-by-Step Solution:

Verification / Alternative check: Standards literature notes the later substitution of isocetane (15) for handling reasons, but the original 0 assignment for alpha-methylnaphthalene remains the canonical teaching point.

Why Other Options Are Wrong:

Common Pitfalls: Confusing the updated reference pair (isocetane 15) with the historical zero point; the question explicitly names alpha-methylnaphthalene.

Final Answer: 0