Fuel quality metrics — which index expresses the ignition quality of petrol (gasoline)? Choose the correct property used to express the knock resistance (ignition quality) of petrol used in spark-ignition engines.

Introduction / Context:
Different fuels use different indices to characterize their combustion behavior. For petrol (gasoline) in spark-ignition engines, the key is resistance to auto-ignition (knock) under compression and high temperature prior to spark.

Given Data / Assumptions:

• Spark-ignition operation with timed spark.
• Comparison between octane number, cetane number, and calorific value.

Concept / Approach:
Octane number quantifies knock resistance of petrol: higher octane means greater resistance to auto-ignition, allowing higher compression ratio or more spark advance without knock. Cetane number applies to diesel fuels and measures ease of auto-ignition (short ignition delay). Calorific value indicates energy content but not knocking tendency directly.

Step-by-Step Solution:
Identify property relevant to SI knock: octane number.Differentiate from diesel: cetane number is for CI ignition quality.Energy content (calorific value) does not express knock resistance.Therefore the correct index for petrol is octane number.

Verification / Alternative check:
Fuel pumps and engine manuals specify minimum octane requirements (RON/MON/AKI) for petrol engines, confirming the industry standard.

Why Other Options Are Wrong:
“Cetane number” applies to diesel. “Calorific value” is important but not a measure of knock resistance. “All of these” is overbroad and incorrect.

Common Pitfalls:
Confusing octane (resisting knock) with cetane (promoting ready ignition). They measure opposite tendencies in different engine types.

Final Answer:
Octane number