Calorific value is one of the most important characteristics of a fuel. It judges the efficiency of fuel.
Calorific value is defined as "the total quantity of heat liberated, when a unit mass or volume of the fuel is burnt completely."
Units of heat
1. Calories: amount of heat required to raise the temperature of 1 gm of water by 1^{0}C is known as calorie.
2. Kilo-calorie (kcal): amount of heat required to raise the temperature of 1 kg of water by 10C is known as kilo-calorie.
1 kcal = 1000 cal.
3. Centigrade heat unit (C.H.U): it is defined as the amount of heat required to raise the temperature of 1 pound (lb) of water by 1 ^{0}C.
4. British thermal unit (B.Th.U) : it is defined as the amount of heat required to raise the temperature of 1 pound (lb) of water by 1 ^{0}F (degree Fahrenheit).
Calorific value can be expressed in two ways.
1. Higher calorific value,
2. Lower calorific value.