CI Engine Fuels — Additives That Improve Ignition Quality (Anti-Knock) For compression-ignition (diesel) fuels, which additive is commonly used to improve ignition quality and reduce delay period, thereby enhancing anti-knock behavior?

Introduction / Context:
Diesel knock arises from long ignition delay leading to rapid pressure rise when accumulated premixed fuel ignites. Improving ignition quality shortens this delay. Additives and reference fuels are distinct concepts; recognizing which chemical acts as a cetane improver is critical.

Given Data / Assumptions:

• Engine type: compression-ignition (diesel), not spark-ignition.
• Goal: increase cetane number (improve ignition quality, reduce knock).
• Common additive families: nitrate and peroxide compounds.

Concept / Approach:
Amyl nitrate (and related alkyl nitrates) serve as ignition improvers for diesel fuels by decomposing readily and facilitating radical formation that accelerates ignition. Tetraethyl lead is an octane (SI) improver, not used for CI fuels. Hexadecane (cetane) is a reference compound defining 100 cetane number, but is not an additive used in ordinary blending. Trimethyl pentane is an SI reference (iso-octane).

Step-by-Step Solution:
Identify the engine: CI → needs higher cetane.Recall common diesel improvers: amyl nitrate, 2-ethylhexyl nitrate.Select amyl nitrate as the additive used to improve ignition quality.

Verification / Alternative check:
Fuel handbooks list alkyl nitrates as standard cetane improvers at small treat rates.

Why Other Options Are Wrong:
Tetraethyl lead/trimethyl pentane pertain to octane rating in SI engines.Hexadecane is a cetane reference component, not a typical additive for retail fuel.Ethanol is not used to improve cetane; it lowers cetane if blended in diesel.

Common Pitfalls:
Confusing octane (SI) with cetane (CI); they measure opposite tendencies regarding autoignition.

Final Answer:
amyl nitrate