Fuel properties check in petroleum engineering: For straight-run petrol (gasoline) as compared to methyl/ethyl alcohol fuels, identify which property is lower for petrol. Consider typical commercial grades and standard test conditions.

Introduction / Context:
Straight-run petrol (gasoline) and lower alcohols (methanol and ethanol) differ markedly in several standard fuel properties used in engine and refinery practice. This question probes recognition of the property for which petrol shows a lower value when compared with methyl/ethyl alcohol, under normal commercial ranges and test methods.

Given Data / Assumptions:

• Fuelling context: spark-ignition (SI) engines under standard test conditions.
• Alcohols considered: methanol and ethanol as neat fuels reference.
• “Straight-run petrol” refers to naphtha fraction directly from crude distillation without reforming.

Concept / Approach:
Octane number measures resistance to knock. Alcohols typically exhibit very high octane ratings relative to straight-run petrol. Calorific value compares energy per unit mass or volume; gasoline generally exceeds alcohols here. Specific gravity and autoignition behavior vary, but the most consistent, widely taught contrast is octane number: petrol's is lower.

Step-by-Step Solution:

Verification / Alternative check:
Engine literature and refinery texts consistently teach alcohols as high-octane oxygenated fuels; straight-run naphtha requires reforming/isomerization to raise octane. This cross-checks the choice.

Why Other Options Are Wrong:

Common Pitfalls:
Learners sometimes conflate "more energy" with "higher octane." Octane is knock resistance, not energy content; alcohols are high-octane despite lower heating value.

Final Answer:
Lower octane number