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heat whereas other fuels produce less heat. The efficiency of a fuel is expressed in
terms of its calorific value.

The amount of heat produced by the complete burning (or complete combustion)
of 1 kilogram of a fuel is called its calorific value.
The calorific value of a fuel is expressed in the unit of ‘kilojoules per kilogram’ or
kJ/kg. The calorific value of a fuel can be calculated by the given formula,
Heat produced on complete combustion of the fuel
Calorific value =
Total mass of the fuel combusted

Sample Problem
in an experiment, 4.5kg of a fuel was completely burnt. The heat produced was
measured to be 180000KJ. Calculate the calorific value of the fuel.

Answer. The calorific value of a fuel is the heat produced by burning 1Kg of fuel. Now,
the heat produced by burning 4.5kg fuel = 180000 kJ

180000 x 1
So, the heat produced by burning 1 kg fuel = = 40000kJ/kg.
4. 5
Thus, the calorific value of the given fuel is 40000kJ/kg.

Characteristics of an ideal fuel

1. It has a high calorific value, or it produces a large amount of heat.
2. It burns easily in the air at a moderate rate.
3. It has a proper ignition temperature.
4. It does not produce any harmful gases or leaves any residue after burning.
5. It is cheap, readily available, and easy to transport.

Flame
A flame is a region where the combustion of gaseous
substances takes place. Only those solid and liquid fuels which
vaporise on being heated, burn with a flame.

Some of the substances which burn by producing flames are

LPG, biogas, wax, etc. Some fuels burn without flame, for

example, charcoal, coal, etc.

When fuels burn, the type of fame produced depends on the
proportion of oxygen which is available for burning the fuel.

1. When the oxygen supply is insufficient, then the fuels burn
incompletely producing mainly a yellow flame. The yellow flame is caused by the glow of
hot unburnt carbon particles produced due to incomplete combustion fuel. This yellow
flame produces light, so it is said to be a luminous flame.
2. When the oxygen supply is sufficient, then the fuels burn completely producing mainly a
blue flame. This blue flame does not produce much light, so it is said to be a non -
luminous flame.