Question

A fuel with high octane number has less tendency to knock, whereas furl with high cetane number has more tendency to knock in an automobile engine.

• A. True
• B. False
• C. Ambiguous
• D. None of these

Answer 1

The correct option is A True

Octane number is the percentage of isooctane in the mixture of iso-octane and n-heptane. Octane number is for gasoline (petrol) fuel. A gasoline with octane number of 80 means 80% of isooctane and 20% of n-heptane, i.e., branched - chain alkane is more, so knocking is less.

Branched-chain hydrocarbons burn more smoothly to form more stable, less reactive 3 radicals, whereas straight-chain hydrocarbons (e.g., n-heptane form less stable, more reactive 1 and 2 radicals).

In contrast, cetane number is used for diesel fuel. It is defined as the percentage of cetane in mixture of cetane and $\alpha$ -methyl naphthalene. A diesel fuel with cetane number 80 means 80% of cetane $\left(C_{16}{H}_{34}\right)$ (a straight-chain hydrocarbon) and 20% of $\alpha$ - methyl naphalene (aromatic compound). So, it will not burn smoothly and produce igh knocking. But straight-chain hydrocarbones, e.g., cetane, ignite spontaneously in comparison to aromatic compounds. So, cetane number determines the ignition of fuel. Higher cetane number means fuel will ignite faster but will produce higher knocking. Lower cetane number means fuel will ignite slowly and will produce lesser knocking (eating sound).