Fuel quality indices — understanding the Diesel Index (DI) Which statement correctly describes the Diesel Index (an alternative measure of diesel fuel ignition quality)?

Introduction / Context:
Diesel ignition quality is commonly represented by cetane number (engine test). However, in routine refinery control, empirical indices are useful. The Diesel Index (DI) provides a quick estimate from simple lab measurements without running a test engine.

Given Data / Assumptions:

• Diesel Index is a correlation-based figure, not a direct engine rating.
• Inputs include aniline point and API gravity (or specific gravity at 15.6°C).

Concept / Approach:
The aniline point reflects aromatic content/solvent power, which inversely influences ignition quality. API gravity stands in for density and composition. Combining these gives a calculated DI that correlates with cetane number, though it is not identical.

Step-by-Step Solution:
Identify the nature of DI: correlation, not engine-measured → rules out option (a).Recognize link to aniline point → contradicts (b).No fixed “+3” relation to cetane number → (c) false.Therefore, (d) correctly states that DI uses aniline point and API gravity (or SG) in a formula.

Verification / Alternative check:
Typical form: DI ∝ (aniline point in °F) * (API gravity) / 100 (exact constants vary by method), confirming it is a calculated index.

Why Other Options Are Wrong:

• (a) Confuses DI with cetane number testing.
• (b) Contradicts the definition of DI.
• (c) There is no universal constant offset relation.

Common Pitfalls:

• Using DI as a substitute for certification cetane tests; it is only an estimate.

Final Answer:
It is a calculated index that uses the fuel’s aniline point and API gravity (or specific gravity).