MCQ Detailed View

The quality of petrol (gasoline) is commonly measured by its octane number. The octane number is a scale that indicates the fuel’s ability to resist knocking during combustion in an internal combustion engine. Knocking occurs when the fuel-air mixture in an engine cylinder ignites prematurely, leading to reduced efficiency and possible engine damage. Fuels with higher octane numbers have greater resistance to knocking and provide smoother engine performance.

The octane scale is defined using two hydrocarbons as reference points: iso-octane (2,2,4-trimethylpentane) and n-heptane. Iso-octane is assigned an octane number of 100 because it resists knocking very well, while n-heptane is assigned a value of 0 because it knocks very easily. The octane number of a fuel is determined by comparing its performance in a test engine with mixtures of iso-octane and n-heptane. For example, if a petrol sample has the same knocking tendency as a mixture of 90% iso-octane and 10% n-heptane, its octane rating is 90.

The other options are not correct measures of fuel quality. Heptane number is not used for petrol quality, although n-heptane defines the lower end of the octane scale. Reforming is a refinery process that increases the octane rating of gasoline by converting low-octane hydrocarbons into branched and aromatic compounds. Refining refers to the general process of converting crude oil into useful products like petrol, diesel, and kerosene, but it does not directly define fuel quality.

Thus, the quality of petrol is judged by its octane number, which ensures efficient combustion, reduced knocking, and better performance in engines. Fuels with higher octane values are particularly important for high-compression and modern engines, where knocking resistance is critical.