Define the diesel index used to estimate ignition quality of diesel fuels by combining API gravity with aniline point; select the correct formula form and units as commonly specified.

Introduction / Context:
Ignition quality of diesel is characterized directly by cetane number or indirectly by indices that correlate with composition. The diesel index is an older but still referenced empirical correlation combining density (via API gravity) and solvency/aromaticity (via aniline point) to estimate ignition quality.

Given Data / Assumptions:

• API gravity is an inverse function of density (higher °API = lighter fuel).
• Aniline point reflects aromatic content/solvency (lower aniline point = higher aromatics).
• Diesel index formula uses specific units.

Concept / Approach:
The classic diesel index is defined as: Diesel Index = (°API) * (Aniline Point in °F) / 100. Lighter fuels (higher °API) and higher aniline points (more paraffinic character) increase the index, generally aligning with higher cetane quality. The index is a screening tool and does not replace measured cetane number.

Step-by-Step Solution:

Verification / Alternative check:
Textbook definitions and refinery laboratory methods document the Diesel Index formula exactly in this unit system.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing Diesel Index with Cetane Index; they are related screening metrics but not identical and use different inputs.

Final Answer:
(°API) x (Aniline Point, °F)/100