Fuel quality in CI engines — the cetane number of a Diesel fuel is an index of which property relevant to compression-ignition performance and cold starting?

Introduction / Context:
Compression-ignition (Diesel) engines rely on auto-ignition of the injected fuel in hot, high-pressure air. The fuel's chemical composition influences ignition delay, combustion smoothness, cold-start behavior, and emissions. The cetane number is the standardized metric used to rate this ignition behavior.

Given Data / Assumptions:

• Fuel injected near the end of compression must ignite promptly after a short delay.
• Higher cetane number corresponds to shorter ignition delay and smoother pressure rise.
• Octane number (for SI engines) measures resistance to auto-ignition, the opposite concept.

Concept / Approach:

Cetane number is determined by comparing a test fuel's ignition quality with blends of cetane (n-hexadecane) and heptamethylnonane in a standardized engine. Higher values indicate better (quicker) ignition quality, improving cold start, idle stability, and noise, and reducing white smoke. It is not a measure of volatility (distillation curve) or viscosity (flow properties); while ignition delay is related, the index represents overall ignition quality rather than a time unit.

Step-by-Step Solution:

Verification / Alternative check:

Standards (ASTM D613/EN ISO 5165) define cetane number via ignition delay comparisons in a CFR engine, confirming the property measured.

Why Other Options Are Wrong:

Volatility — influences vaporization, not the chemical readiness to ignite.
Viscosity — affects injection/atomization, not inherent ignition tendency.
Delay “peirod” — ignition delay is related but cetane number is the index of ignition quality, not a delay value itself.

Common Pitfalls:

Confusing octane and cetane scales; assuming high cetane always yields higher power—timing and air handling also govern performance.

Final Answer:

ignition quality