Meaning of a higher octane number for SI fuel A higher octane number for spark-ignition (petrol) fuel primarily indicates that the fuel has:

Difficulty: Easy

Correct Answer: a longer ignition delay (greater resistance to auto-ignition)

Explanation:


Introduction / Context:
The octane number rates a gasoline’s knock resistance in spark-ignition engines. Knock arises from premature auto-ignition of the end-gas ahead of the flame front, producing damaging pressure oscillations. A higher octane rating means the fuel better resists auto-ignition under compression and temperature.



Given Data / Assumptions:

  • Standard test methods (RON/MON) assess knock resistance under controlled conditions.
  • Target attribute: resistance to auto-ignition, not energy content.
  • We compare qualitative implications of octane number.


Concept / Approach:
High-octane fuels lengthen the chemical induction time (ignition delay) of the end-gas under engine conditions, allowing the flame to consume the mixture before spontaneous ignition can occur. This enables higher compression ratios or more advanced spark timing without knock. Octane rating does not directly correlate with heating value, flash point, or volatility across fuels.



Step-by-Step Solution:
Define octane: measure of knock resistance.Relate to chemistry: longer ignition delay in end-gas → less tendency to auto-ignite.Implication: permits higher performance settings with reduced knock risk.



Verification / Alternative check:
Engine calibration practice advances spark and/or compression ratio with higher octane fuels, consistent with increased knock margin (longer delay to auto-ignition).



Why Other Options Are Wrong:

  • Heating value is largely independent of octane rating.
  • Flash point and volatility specifications are separate properties.
  • Sulfur content is an impurity limit, unrelated to octane rating.


Common Pitfalls:
Assuming “premium” fuel has more energy; performance gains are from knock resistance, not higher calorific value.



Final Answer:
a longer ignition delay (greater resistance to auto-ignition)

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