Fuels and knocking in spark-ignition (SI) engines Among common reference fuels used to characterize knock resistance, which fuel detonates (knocks) most easily under engine conditions?

Introduction / Context:
In SI (petrol) engines, abnormal combustion called knock or detonation occurs when part of the unburned end-gas auto-ignites before the flame front arrives. The susceptibility of a fuel to this phenomenon is expressed with the octane rating scale, defined using two reference fuels: iso-octane and n-heptane.

Given Data / Assumptions:

• Comparing typical fuels used as knock references.
• Standard SI engine compression and temperature conditions.
• No fuel additives or special blending unless noted.

Concept / Approach:
On the octane scale, iso-octane is assigned an octane number of 100 due to its high knock resistance, while n-heptane is assigned 0 because it knocks very readily. A real fuel’s octane number indicates the percentage of iso-octane in a mixture with n-heptane that would have equivalent knocking behavior.

Step-by-Step Solution:

Verification / Alternative check:
Textbook octane definitions and CFR engine tests consistently rate n-heptane as the poor anti-knock reference. Alcohols generally have high octane ratings and resist knock.

Why Other Options Are Wrong:

• Iso-octane: the high anti-knock standard (100), resists detonation.
• Benzene: higher octane than n-heptane; not the easiest to detonate in SI conditions.
• Alcohol: alcohol fuels (e.g., ethanol, methanol) usually have very high octane numbers, resisting knock.
• Toluene: commonly used as a high-octane blending component.

Common Pitfalls:
Confusing cetane (diesel ignition quality) with octane (SI knock resistance). High cetane means easy auto-ignition in diesels, but SI engines desire high octane (hard to auto-ignite).

Final Answer: