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 Fuel Types: Definition, Characteristics, Classification and Properties of Fuel, Practice Problems and FAQs

What if I say cow dung can be used as fuel?

Yes, it is possible to make fuel from a cow dunk. You must be familiar with the Gobar gas plant, where the major constituent of cow dung, that is methane, is used as fuel for many utility purposes such as cooking, heating and drying. 

Whenever we hear fuel, only petrol, diesel and very rarely kerosine comes into our mind due to their numerous applications in our day-to-day life. 


But fuel is a broader term and has many categories. The above-stated example of Gobar gas falls under the category of biofuels. 

Let’s unfold a few more mysterious and interesting categories of fuels!

Table of Content:

  • Definition of Fuel
  • Characteristics of a Fuel
  • Properties of an Ideal Fuel:
  • Classification of Fuels
  • Practice Problems
  • Frequently Asked Questions

Definition of Fuel:

Fuels are materials that, when burned, release thermal or chemical energy. This energy is necessary for carrying out a variety of tasks and is either used in its natural state or transformed into a usable form of energy with the aid of machinery. 

Petrol, the fuel used to power cars, is an example of conversion; when it burns, it produces heat energy, which is then transformed into mechanical energy to power the car. Our bodies can be compared to machines; to run our bodies' metabolic processes, we need fuel, which comes from food.

"Energy is neither created nor destroyed; it is only transformed from one form to another," states the first law of thermodynamics. In this situation, fuels are a source of energy that we use in a variety of ways depending on our needs. Fuels include things like gasoline, diesel, and coal.

Characteristics of a Fuel:

Fuels are substances that, when burned, release a significant amount of heat and energy. A fuel can be distinguished by two factors: its calorific value and its fuel efficiency.

  • Calorific Value: The calorific value of a fuel is the amount of energy released during combustion expressed in joules per unit mass of the fuel.
  • Fuel Efficiency: As far as we are aware, energy can only be transferred. Therefore, fuel efficiency refers to how effectively a fuel transforms its innate energy into a form that can be used. It is significant to remember that no fuel is 100% efficient.

Properties of an Ideal Fuel:

Before selecting the proper fuel, it is important to consider the following characteristics:

  • The fuel should burn efficiently and have a high calorific value.
  • low ash and moisture content.
  • Fuel should have a moderate ignition temperature.
  • The cost of handling the fuel should be kept to a minimum, and it should be simple to store and transport.
  • Finally, no harmful byproducts from the fuel's combustion should be produced, further harming the environment.

Classification of Fuels:

Fuels can be classified into the following categories based on their state and occurrence.

Classification of fuels on the basis of occurrence:

  • Natural Fuels
  • Fossil Fuels

Classification of fuels on the basis of state:

  • Solid Fuels
  • Liquid Fuels
  • Gaseous Fuels


Solid fuels:

Solid fuels are generally defined as fuels that are found at room temperature in their solid state. They were the first known form of fuel used by humans to start a fire, essentially made of wood. As it paved the way for the industrial revolution by powering steam engines and igniting furnaces, coal is one of the most significant fuels known to man. Other examples include wood, charcoal etc. 


  • Solid fuels can be easily stored and transported from one place to another
  • The production cost or manufacturing cost of solid fuel is less.
  • Solid fuels have a moderate ignition temperature.


  • Even if the manufacturing cost is low but handling cost is very high
  • Energy is wasted in large quantities.
  • The ash content of solid fuels is very high and they burn by forming clinkers.

Liquid Fuels:

By being subjected to heat and pressure in the Earth's crust, the majority of liquid fuels are created from the fossilized remains of extinct plants and animals. Instead of the liquid fuel itself, the fumes are flammable.

Crude oil, which is produced from organic material that has been fossilized under high pressure and temperatures, is an example of a liquid fuel.

Gaseous fuels:

Along with being produced from solid and liquid fuels, gaseous fuels also exist in nature. Hydrocarbons, carbon monoxide, hydrogen, or a combination of all three make up the majority of gaseous fuels. 

Gaseous fuels include compressed natural gas, which is delivered through pipes to the kitchens of most homes.


  • The heat content of liquid fuels are quite high
  • Liquid fuels don't need any special burner technology.


  • Storing liquid fuels requires a large tank.
  • Due to their extreme flammability, there is a very high likelihood of fire hazards, so strict safety precautions must be taken.


All the categories mentioned above are included in the special category of fuels known as biofuels. In layman's terms, all gaseous, solid, and liquid fuel is referred to as biofuel when it is produced from natural biomass. Biofuels come from organic materials that are a rich source of carbon, like plants. A new fuel innovation, bioethanol is currently being investigated extensively for use in the automotive sector.

Fossil Fuels:

Let's define each term related to fossil fuels individually: fossilized remains of a living thing (either plants or animals) Energy-producing substances are referred to as fuel. Thus, a compound mixture of buried organic matter that can be transformed into crude oil, coal, or natural gas—all of which are useful energy sources—can be referred to as fossil fuel.

Practice Problems:

Q.1 Which of the following energy is produced during the combustion of petrol in a vehicle?

a. Light energy
b. Heat energy
c. Electrical energy
d. None of the above

Answer: (B)
Heat energy is produced during the combustion of petrol in a vehicle. This energy is further converted to mechanical energy to drive the vehicle.

Q.2 Energy is neither created nor destroyed, it is only transferred from one form into another. Which law of thermodynamics states the above statement?

a. 1st law of thermodynamics
b.  Zeroth law of thermodynamicsc
c. 3rd law of thermodynamics
d. 2nd law of thermodynamics

Answer: (A)
According to the first law of thermodynamics: ‘energy is neither created nor destroyed; it is only transformed from one form to another. 

Q.3 Which of the following is not a characteristic of fuel?

a. The fuel should have a high calorific value and efficiency rate.
b. High moisture and ash content.
c. The ignition temperature of the fuel should be moderate.
d. The fuel should be easy to store and transport and must incur minimum expenses to handle it.

Answer: (B)
Fuel should always contain low moisture and ash content. Hence, option B is the correct choice.

Q.4 Which of the following is not an example of solid fuel?

a. Coal
b. Coke
c. Wood
d. Crude oil

Answer: (D)
Coal, coke and wood are examples of solid fuel whereas crude oil is an example of liquid oil.

Frequently Asked Questions-FAQs: 

Q.1 Which fuel is more environmentally friendly?
Answer: Compared to gasoline and gasoline engines, diesel and diesel engines emit fewer and less harmful emissions. They are more environmentally friendly.

Q2. Fuel: Is it bad for the environment?
Answer: Carbon dioxide, a greenhouse gas, is released into the atmosphere in large quantities when fossil fuels are burned. Global warming is caused by greenhouse gasses, which trap heat in our atmosphere.

Q.3 What makes it a fossil fuel?
Answer: because they are made of the remains of dead plants and animals. Hence, these are called fossil fuels.

Q.4 Can humans become fossil fuels?
Answer: Yes is the obvious response here. The hard parts of the body, such as the teeth, bones, and shells, are what, according to research, are responsible for the formation of fossil fuels.

Related Topics:

Acid and base difference Difference between alkali and base
Physical and Chemical changes Chemical composition
Mixtures Difference between mixtures and solutions

FOUNDATION 2022Important Concepts of Physics




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