Question

Octane number of iso-octane is :
(A) 50
(B) 25
(C) 100
(D) 150

Answer 1

Iso-octane, also known as 2,2,4-trimethylpentane is an isomer of octane and an example of a branched hydrocarbon chain. It is a five-carbon chain molecule with three methyl groups at various points in the chain.

Complete step by step solution:
-The standard measure of the performance of an engine or aviation gasoline is known as an octane rating or octane number.
-In other words, the ability of a fuel to resist knocking when ignited in a mixture with air in the cylinder of an internal combustion engine is known as Octane number or Antiknock rating.
-In an internal combustion engine, the sharp sound caused by premature combustion of the part of the compressed air-fuel mixture in the cylinder is known as knocking.
-Branched hydrocarbon chains are more preferred than straight-chain hydrocarbons in petrol as straight-chain hydrocarbons cause ‘knocking’ which is an undesirable small explosion causing power loss. Hydrocarbons with branched-chain burns smoothly, this ability of a fuel to burn smoothly is the quality of petrol and is indicated by its octane number.
-The octane number is simply the percentage by volume of iso-octane in the iso-octane-heptane mixture that matches with the test fuel in a standard test engine.
-The higher the octane number, the more are the compressions in the fuel which it can withstand before detonating and hence higher the performance.
-The octane number is calculated by comparing the knock intensity of the fuel at standard conditions with the blend of two reference fuels, iso-octane which resists knocking, and heptanes which knock readily.
-A fuel with octane number as 0 is considered a poor fuel, while a fuel with octane number 100 is considered a good fuel. Iso-octane has the octane number of 100 as it shows minimal knock and heptanes have octane number as 0 as it shows bad knock.

So, the correct answer is option (C).

Note: Octane number matters as in a spark-ignition engine, using fuel with too low octane rating can lead to pre-ignition and engine knock which can cause engine damage. Chemically, compressing the air-fuel mixture may cause fuel to detonate before the flame front from the spark plug reaches it.