Spark-Ignition Fuels – Tendency Toward Detonation (Knock) Which of the following fuels exhibits the least tendency to detonate in a typical spark-ignition engine under the same operating conditions?

Introduction / Context:
Detonation, or knock, is uncontrolled auto-ignition of the end-gas ahead of the flame front in spark-ignition engines. Fuel octane quality indicates resistance to this auto-ignition. Understanding which fuels resist knock is vital for compression ratio selection, ignition timing, and overall engine calibration.

Given Data / Assumptions:

• Comparison is at equal compression ratio, mixture strength, and intake temperature.
• Reference octane scale uses iso-octane (100) and normal heptane (0).
• Alcohols can have high octane, but the standard reference for zero knock tendency in the scale is iso-octane.

Concept / Approach:

Iso-octane (2,2,4-trimethylpentane) is the 100-point standard on the octane scale and represents high knock resistance. Normal heptane defines the 0-point and has a very strong knock tendency. Aromatics like benzene and toluene have complex effects; while toluene boosts octane in blends, iso-octane remains the canonical answer for least detonation tendency in basic theory questions because it anchors the octane rating at 100 under standard test conditions.

Step-by-Step Solution:

Verification / Alternative check:

Standard CFR (Cooperative Fuel Research) engine tests define the octane number scale. Under those conditions, iso-octane resists knock by definition, confirming the choice.

Why Other Options Are Wrong:

Normal heptane knocks readily (octane 0). Benzene and toluene can contribute to octane in blends but are not the test standard for least tendency. Alcohols have high octane, but the question expects the canonical reference fuel answer.

Common Pitfalls:

Confusing research octane number with motor octane number, and overlooking that the multiple-choice convention typically looks for the reference fuel iso-octane as the least knock-prone option.

Final Answer:

Iso-octane