Diesel antiknock (ignition improver) additive — identify the correct compound Which of the following is commonly used as an anti-knock (ignition quality improver) additive for compression-ignition (diesel) engines?

Introduction / Context:
Diesel knock arises from long ignition delay. Additives that shorten ignition delay improve ignition quality and reduce knock. Knowing which additive serves this role prevents confusion with petrol antiknock agents used in spark-ignition engines.

Given Data / Assumptions:

• Compression-ignition operation and diesel fuel context.
• Goal: reduce ignition delay and smooth combustion.

Concept / Approach:
Amyl nitrate (and related nitrate compounds like ethylhexyl nitrate) act as ignition improvers in diesel fuel, effectively raising cetane number by accelerating low-temperature pre-flame reactions. Tetraethyl lead is an SI antiknock additive (now largely phased out) and is inappropriate for CI fuels. Naphthenes are hydrocarbon types, not specific additives for delay reduction. Neat cetane (n-hexadecane) defines the high end of the cetane scale but is not used as an additive per se in commercial fuels.

Step-by-Step Solution:
Define the target: shorter ignition delay in CI → higher cetane behavior.Identify appropriate chemical class: nitrates/nitrites (e.g., amyl nitrate) as ignition improvers.Exclude SI antiknock agents and unrelated hydrocarbons.Hence, amyl nitrate is the correct choice.

Verification / Alternative check:
Fuel specifications and additive datasheets list nitrate-based cetane improvers as standard for diesel ignition quality enhancement.

Why Other Options Are Wrong:
Tetraethyl lead pertains to petrol octane improvement. Naphthene is a hydrocarbon family, not a specific improver. Cetane is a reference compound, not an additive in normal blending practice.

Common Pitfalls:
Assuming the same additive combats knock in both SI and CI engines; the phenomena and remedies differ.

Final Answer:
Amyl nitrate