Fuel detonation tendency (octane perspective) Among the following, which fuel detonates most readily (i.e., has the poorest knock resistance in an SI engine)?

Introduction / Context:Detonation (knock) in SI engines is inversely related to octane rating. The octane scale is anchored by two reference fuels: iso-octane and n-heptane, which set the extremes of knock resistance used for fuel testing and calibration.

Given Data / Assumptions:

• Octane number of iso-octane is defined as 100.
• Octane number of normal heptane is defined as 0.
• Comparative evaluation under standard test conditions.

Concept / Approach:

A fuel with a lower octane number detonates more readily under compression and heat. Since normal heptane has the lowest octane in the set, it will knock first. Iso-octane is highly resistant to knock. Aromatics like benzene/toluene and alcohols typically have relatively high octane indices compared with n-heptane.

Step-by-Step Solution:

Verification / Alternative check:

Standard ASTM methods (RON/MON) use blends of iso-octane and n-heptane to match knock behavior; n-heptane’s designation as 0 confirms its propensity to detonate readily.

Why Other Options Are Wrong:

Iso-octane, benzene/toluene, and alcohols have comparatively higher octane ratings; they resist knock better than n-heptane.

Common Pitfalls:

Assuming aromatics always promote knock due to soot formation; octane rating, not soot, dictates knock tendency in SI engines.

Final Answer:

Normal heptane