Interpreting octane number from a reference blend A fuel blend containing 65% iso-octane and 35% n-heptane by volume will have which standard rating?

Introduction / Context:
Octane number defines a gasoline's knock resistance under specified test conditions by comparing it to blends of iso-octane (assigned 100) and n-heptane (assigned 0). Understanding this scale helps interpret fuel ratings and relate them to engine anti-knock performance.

Given Data / Assumptions:

• Reference fuels: iso-octane (ON = 100) and n-heptane (ON = 0).
• Linear scale for primary reference fuels under standardized tests.
• Blend composition: 65% iso-octane, 35% n-heptane (by volume).

Concept / Approach:

By definition, the octane number of a gasoline equals the percentage of iso-octane in a reference blend that gives the same knock characteristics as the test fuel under the CFR engine method. Therefore, a 65/35 iso-octane/n-heptane blend has an octane number of 65.

Step-by-Step Solution:

Verification / Alternative check:

Standard ASTM methods (e.g., RON/MON) use these primary reference fuels; the simple linear blend definition is the basis of the octane scale.

Why Other Options Are Wrong:

Cetane number applies to diesel fuels and is inverse in significance (higher cetane → shorter ignition delay). Octane 35 would correspond to a 35/65 blend, not 65/35.

Common Pitfalls:

Confusing octane with cetane; one measures knock resistance in SI engines, the other ignition quality in CI engines.

Final Answer:

octane number 65