The unit of calorific value of a fuel is:

The calorific value of a fuel is the amount of heat energy produced when a unit mass (1 kilogram) of the fuel is burnt completely in air or oxygen. It is expressed in kilojoules per kilogram (kJ/kg). For gaseous fuels, it is sometimes expressed per unit volume, such as kJ/m³.

The calorific value is a measure of how efficient a fuel is: the higher the calorific value, the more heat is produced from the same amount of fuel. It is measured using an instrument called a bomb calorimeter.

Approximate calorific values of some common fuels:

• Hydrogen: about 1,50,000 kJ/kg, the highest of all common fuels.
• Methane (natural gas) and CNG: about 50,000 kJ/kg.
• LPG: about 55,000 kJ/kg.
• Petrol: about 45,000 kJ/kg.
• Kerosene: about 45,000 kJ/kg.
• Diesel: about 45,000 kJ/kg.
• Biogas: about 35,000-40,000 kJ/kg.
• Coal: about 25,000-33,000 kJ/kg.
• Charcoal: about 33,000 kJ/kg.
• Wood: about 17,000-22,000 kJ/kg.
• Cow-dung cake: about 6,000-8,000 kJ/kg.

Characteristics of an ideal fuel:

• A high calorific value.
• A moderate ignition temperature (the lowest temperature at which a substance catches fire).
• Low cost and easy availability.
• Easy to store and transport.
• Burns steadily at a moderate rate.
• Produces little smoke, ash or harmful gases when burnt.

Hydrogen has the highest calorific value and burns cleanly to produce only water, so it is considered a fuel of the future. However, it is difficult and expensive to produce, store and transport safely. India launched the National Green Hydrogen Mission in 2023 to promote the production of hydrogen using renewable energy. ISRO uses liquid hydrogen as a fuel in the cryogenic engines of its GSLV and LVM3 rockets.

Burning fossil fuels releases carbon dioxide (a greenhouse gas), and incomplete combustion releases carbon monoxide, a poisonous gas.

Why not the other options?

• ✗kJ/m is not a unit of calorific value.
• ✗Joule-second is the unit of Planck's constant and angular momentum.
• ✗kg/m³ is the unit of density.