Difference between Petrol and Diesel Engine

The primary difference between Petrol and Diesel engines is that the Petrol engine works on the Otto cycle whereas the Diesel engine works on the Diesel cycle. Other differences can be attributed to the structure, types, and uses of these engines. The main parameter they are classified on is the type of fuel they use. Generally, Engines run on the principle of heat transfer.

Petrol Engine

  • Petrol engines are internal combustion engines which have spark-ignition. They run on relatively volatile fuels such as petrol.
  • In these engines, air and fuel are generally mixed post-compression.
  • Petrol engines work on the Otto cycle, which consists of two isochoric processes and two isentropic processes.
  • In petrol engines, air and petrol are usually mixed in a carburettor before being introduced to the cylinder.
  • Once the air and petrol are compressed, the fuel is ignited via an electric spark.

Difference Between Petrol and Diesel Engine

Diesel Engine

  • The Diesel engine is also an internal combustion engine which is also known as the compression-ignition engine. It is named after Rudolf Diesel.
  • In these engines, the fuel is injected into a combustion chamber and is then ignited by the high temperature of the air in the chamber.
  • The high temperature of the air in the cylinder is due to the adiabatic compression. These engines only compress the air and not the fuel.
  • When injected into the combustion chamber, the Diesel fuel undergoes spontaneous ignition.
  • These engines work on the Diesel cycle, which consists of a constant pressure process, a constant volume process, and two isentropic processes. Difference Between Petrol and Diesel Engine Diagram

Difference between Petrol and Diesel Engine

Apart from the type of fuel used, the engines are also divided on the basis of a lot of things, such as the presence of a spark plug in Petrol engines and a fuel injector in Diesel engines. We also know that lighter vehicles such as motorcycles, scooters, and cars typically use petrol in their engines whereas Diesel is used in much heavier machinery such as tractors, trucks, and buses. Thus, the types of fuel used also plays a major role in defining the major difference between Petrol and Diesel engines. More differences between these types of engines are listed in the tabular column below. 

Difference Between Petrol and Diesel Engine

Diesel Engine Petrol Engine
These engines work on the Diesel cycle Works on the Otto cycle
The fuel is mixed with air inside the cylinder Air and the fuel are mixed in a carburettor
Ignition is achieved with the help of the hot, compressed air. Fuel is ignited with an electric spark
High compression ratio Relatively low compression ratio
High power production Relatively low amounts of power are produced in a Petrol engine
These engines work with fuels that have low volatilities Highly volatile fuels are used in these internal combustion engines
Generally used in heavy vehicles such as trucks and buses Used in light vehicles such as motorcycles and cars.
Relatively low fuel consumption High fuel consumption.
High initial and maintenance costs Comparatively low initial cost and maintenance cost

Frequently Asked Questions – FAQs


List conditions under which combustion can take place.

The conditions required for combustion to take place are:
1. Fuel: Presence of a fuel
2. Air: Oxygen is must for combustion to occur
3. Ignition temperature: The minimum temperature at which a substance catches fire.


Explain how the use of CNG in automobiles has reduced pollution in our cities.

Combustion of fuels like petroleum causes the addition of many harmful pollutants like un-burnt carbon, particulate matter, and many fume gases enter the air and cause respiratory diseases.
Compressed Natural Gas (CNG) produces these harmful products in very little quantity.
It is a comparatively cleaner fuel. Therefore, the use of CNG has reduced pollution in our cities.


Give reasons : LPG is a better domestic fuel than wood.

LPG is a better domestic fuel than wood as

  • It does not produce smoke and unburnt carbon particles, which cause respiratory problems.
  • It has a low ignition temperature.
  • It is easy to store.

Name the unit in which the calorific value of a fuel is expressed.

The calorific value of a fuel is expressed in kilojoules per kilogram (kJ/kg).


Explain how CO2 is able to control fires.

CO2 is a non-combustible gas and it is also a non-supporter of combustion. It extinguishes fire in two ways:
Since it is heavier than oxygen, it covers the fire like a blanket around the burning material and cuts off the contact between oxygen and fuel.

In cylinders, CO2​ is kept in liquid form. When released, it expands enormously and cools down. This brings down the temperature of the fuel, which helps in controlling the fire.

Thus, the important differences between Petrol and Diesel engines are discussed briefly in this article. To learn more about these engines, and how they can be replaced with fuel cells, register with BYJU’S and download the mobile application on your smartphone.

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  1. it is very good diffrence between petrol cycle and diesel cycle.

  2. Thank you very much

  3. I appreciate every step taking in explaining the difference

  4. Thanks a lot for the info. It has really answered my questions…