Meaning of cetane number: The cetane number of a diesel fuel is fundamentally a measure of its tendency toward which ignition behaviour?

Introduction:
Cetane number (CN) quantifies diesel fuel ignition quality under compression ignition. Higher CN corresponds to shorter ignition delay, enabling smoother combustion, easier cold starting, and lower noise in diesel engines.

Given Data / Assumptions:

• Reference scale: n-heptane (CN = 100) ignites very readily; isocetane/α-methylnaphthalene historically represent low-CN behaviour.
• Engine-based or derived methods (e.g., CFR engine, DCN) exist.

Concept / Approach:
Ignition delay is the time between fuel injection and the onset of combustion. Fuels with higher CN autoignite more promptly; lower CN fuels delay longer, causing rough operation. CN does not directly measure viscosity, smoke point, oxidation stability, or the distillation curve, though these properties influence other aspects of performance.

Step-by-Step Solution:

Verification / Alternative check:
Standards (e.g., ASTM D613, EN 15195) define CN via engine/ignition delay response relative to a reference blend scale.

Why Other Options Are Wrong:

• Smoke point/Viscosity/Oxidation stability/Distillation: Important fuel specs but not what CN measures.

Common Pitfalls:
Confusing gasoline octane (knock resistance) with diesel cetane (ignitability); they are opposite in implication.

Final Answer:
Ignition delay