Diesel fuel rating – low reference on the cetane scale Alpha-methylnaphthalene (C11H10) historically defines the low end of the cetane scale. Its assigned cetane number is:

Introduction / Context:
The cetane number rates a diesel fuel’s tendency to auto-ignite easily (short ignition delay). The scale is anchored by two reference fuels that set the endpoints of ignition quality for standardized testing.

Given Data / Assumptions:

• Reference high: n-hexadecane (cetane) = 100.
• Historical reference low: alpha-methylnaphthalene = 0 (modern low reference may use isocetane, CN 15, but the classic exam convention remains 0 for alpha-methylnaphthalene).
• Question targets the historical definition commonly used in textbooks.

Concept / Approach:
Alpha-methylnaphthalene exhibits very poor ignition quality with long ignition delay, establishing the zero point of the original cetane scale. Real diesel fuels are compared by matching their ignition delay to blends of the reference fuels.

Step-by-Step Solution:
Recall reference endpoints.Match material to endpoint: alpha-methylnaphthalene → low end.Assigned number: 0.

Verification / Alternative check:
Standards literature notes the historical use of alpha-methylnaphthalene (CN 0) and the later substitution of isocetane (CN 15) for practical reasons; exam problems typically retain CN 0 for alpha-methylnaphthalene.

Why Other Options Are Wrong:

• 100 is the value for cetane (n-hexadecane), the high reference fuel.
• 50, 120, or 15 are not the classic assigned value for alpha-methylnaphthalene in this context.

Common Pitfalls:
Confusing updated testing practices with the classical exam convention; the question explicitly names alpha-methylnaphthalene.

Final Answer:
0