Octane number interpretation for SI fuels A petrol fuel with an octane rating of 75 has the same knocking tendency as which reference mixture by volume in the CFR test engine?

Introduction / Context:
The octane scale for spark-ignition fuels is defined using two primary reference fuels: iso-octane, which resists knock well, and n-heptane, which knocks readily. The octane number quantifies a fuel’s knock resistance by equating it to a blend of these two references.

Given Data / Assumptions:

• Octane number of the test fuel is 75.
• Reference fuels: iso-octane (100) and n-heptane (0).
• Mixtures are expressed in percent by volume for the CFR test engine comparison.

Concept / Approach:
An octane rating of 75 indicates the fuel behaves, in terms of knock tendency, like a blend containing 75% iso-octane and 25% n-heptane under standard test conditions. That is the operational definition of the research octane number scale.

Step-by-Step Solution:
Map rating to blend: Octane 75 → 75 parts iso-octane + 25 parts n-heptane (by volume).Exclude distractors that swap the percentages or replace the references with unrelated fuels.Select the mixture that matches the definition.

Verification / Alternative check:
Standard fuel testing protocols (RON/MON) rely on CFR engine comparisons to these defined blends.

Why Other Options Are Wrong:

• Swapped percentages invert the rating (would imply Octane 25).
• Petrol–diesel mixtures are not reference pairs for octane testing.
• Aromatic or alcohol mixtures may alter knock resistance but are not the scale’s endpoints.

Common Pitfalls:
Confusing octane number (knock resistance) with calorific value or flash point; they are independent properties.

Final Answer:
75% iso-octane and 25% n-heptane