Fuel Properties in Spark-Ignition Engines In automotive fuels for petrol (gasoline) engines, the ease with which the liquid fuel changes into vapour under normal conditions is termed what?

Introduction / Context:
For spark-ignition (petrol) engines, the fuel must readily form a combustible air–fuel mixture. The property that indicates how easily the liquid turns into vapour is crucial for cold starting, throttle response, and drivability. This question asks you to identify the correct term for that property.

Given Data / Assumptions:

• Engine type: petrol (gasoline) spark-ignition.
• Ambient conditions typical of vehicle operation.
• We are comparing standard fuel-property terms used in automotive engineering.

Concept / Approach:
Volatility describes a liquid’s tendency to evaporate. Petrol with appropriate volatility will vaporize adequately in the intake manifold and ports, assisting mixture preparation. Too low a volatility causes poor cold start and hesitation; too high a volatility can cause vapour lock and excessive evaporative emissions. Other listed terms (octane, cetane) measure entirely different fuel behaviors (anti-knock and ignition quality, respectively) rather than ease of vaporization.

Step-by-Step Solution:
Identify the needed property: ease of vaporization → volatility.Check alternatives: octane relates to knock resistance; cetane to autoignition quality in diesel; oxidation is chemical reaction with oxygen, not a volatility metric.Therefore, choose the option that directly describes evaporation tendency: volatility.

Verification / Alternative check:
Fuel datasheets report distillation curves (e.g., ASTM D86) and Reid vapour pressure. These are volatility indicators used by refiners to tune seasonal fuel blends.

Why Other Options Are Wrong:
Oxidation: relates to long-term stability and gum formation, not instant vaporization.Octane number: measures anti-knock in SI engines; unrelated to evaporation ease.Cetane number: measures diesel ignition quality; irrelevant for petrol.Vapour lock index: not a standard single-property name; vapour lock is a symptom influenced by volatility.

Common Pitfalls:
Confusing octane with “overall fuel quality”; it does not indicate starting or evaporation behavior.Assuming higher volatility is always better; excessively high volatility can cause vapour lock in hot conditions.

Final Answer:
volatility