# Define fuel efficiency

Fuel is a substance that will undergo combustion to produce heat when the substance comes in contact with oxygen at a temperature that is equal to or higher than the ignition temperature of the Fuel. Any combustible substance can act as a fuel.

Different types of fuels generate different amounts of heat. The heat produced by fuels is measured and expressed through its calorific value. For a fuel to be categorized as good fuel, it should have a high calorific value, in other words, greater the calorific value larger is the amount of heat produced on burning that fuel and vice versa. So fuel efficiency may be defined as the amount of energy released per unit mass of the fuel.

• A fuel that will be suitable for automobiles should have a high calorific value, should be available in abundance and should be easy to transport without emitting any pollutants.
• Petrol, Diesel and Compressed natural gas(CNG) are some fuels on which automobiles run.