MCQ Detailed View

The octane number is a measure of the resistance of a fuel to knocking during combustion in internal combustion engines. Knocking occurs when fuel-air mixtures ignite prematurely, leading to inefficient engine performance and possible engine damage.

In fuel chemistry, the octane scale is defined using two standard reference hydrocarbons:

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  n-Heptane is assigned an octane number of 0, representing a fuel that knocks very easily.

  
  


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  Iso-octane (2,2,4-trimethylpentane) is assigned an octane number of 100, representing a fuel that resists knocking.

  
  

The octane number of a fuel is determined by comparing its knocking characteristics to mixtures of n-heptane and iso-octane. For example, a fuel with the same knocking tendency as a mixture of 90% iso-octane and 10% n-heptane is assigned an octane number of 90.

n-Heptane (C₇H₁₆) is a straight-chain alkane, which burns rapidly and detonates easily under compression. Its low resistance to knocking makes it the baseline for zero on the octane scale. Branched alkanes, such as iso-octane or methyl-substituted octanes, have higher resistance to autoignition due to the stability of their molecular structure, which makes them ideal for higher octane ratings.

Understanding the octane number is essential in engine design and fuel formulation. Higher octane fuels allow engines to operate at higher compression ratios, improving efficiency and power without causing knocking. Fuels with a high proportion of straight-chain alkanes, like n-heptane, have lower octane numbers and are more prone to knocking.

Thus, the octane number of zero is specifically assigned to n-heptane, which serves as the reference standard for the lower end of the octane scale in fuel chemistry.