Fuel quality indices — which scale represents diesel ignition quality? Select the correct property index used to rate the ignition quality and ignition delay characteristics of diesel fuel.

Introduction / Context:
Different fuels are rated using different standardized scales relevant to their intended combustion modes. Petrol (spark-ignition) fuels use octane ratings to indicate resistance to knock, while diesel (compression-ignition) fuels use cetane numbers to indicate ignition quality (shortness of ignition delay) under compression.

Given Data / Assumptions:

• Diesel engines initiate combustion by auto-ignition due to high temperature from compression.
• Short ignition delay supports smooth pressure rise and reduced noise/emissions.
• Scale definitions: octane for SI knock resistance; cetane for CI ignition delay; calorific value measures energy content, not ignition behavior.

Concept / Approach:
Cetane number is defined by comparing a diesel fuel’s ignition delay to that of reference blends of cetane (n-hexadecane, high ignitability) and heptamethylnonane (low ignitability). Higher cetane implies shorter delay and better ignition quality, desirable for cold starts, smooth operation, and lower unburned hydrocarbon emissions.

Step-by-Step Solution:

Verification / Alternative check:
Engine test methods (e.g., CFR diesel engine) and modern ignition quality testers quantify cetane, confirming its role as the standard diesel ignition metric.

Why Other Options Are Wrong:

• Octane number: Opposite purpose; high octane resists auto-ignition, which is undesirable in diesel start-of-combustion.
• Calorific value: Does not indicate how readily the fuel ignites, only energy per unit mass or volume.
• None: A well-established metric exists: cetane number.

Common Pitfalls:
Assuming a higher energy content guarantees better diesel performance; ignition quality is separate from energy content and crucial for combustion phasing.

Final Answer:
cetane number