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.
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
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.
| S.no | Petrol Engines | Diesel Engines |
| 1 | A petrol engine draws a mixture of petrol and air during the suction stroke. | A diesel engine draws only air during the suction stroke. |
| 2 | The carburetor is installed in petrol engines to mix air and petrol in the required proportion and to supply it to the engine during the suction stroke. | The injector or atomizer is installed in diesel engines to inject the fuel at the end of the compression stroke. |
| 3 | The pressure at the end of the compression is about 10 bar. | The pressure at the end of the compression is about 35 bar. |
| 4 | The charge (i.e. petrol and air mixture) is ignited with the help of spark plug. | The fuel is injected in the form of a fine spray. The temperature of the compressed air is about 600° C at a pressure of about 35 bars. |
| 5 | A petrol engine has compression ratio approximately from 6 to 10. | A diesel engine has compression ratio approximately from 15 to 25. |
| 6 | The combustion of fuel takes place approximately at constant volume. In other words, it works on Otto cycle. | The combustion of fuel takes place approximately at constant pressure. in other words, it works on Diesel cycle. |
| 7 | The thermal efficiency is up to about 26%. | The thermal efficiency is up to about 40%. |
| 8 | Overheating trouble is more in petrol engine due to low thermal efficiency. | Overheating trouble is less in diesel engine due to high thermal efficiency. |
| 9 | The starting of petrol engine is easy due to low compression ratio. | The starting of the diesel engine is slightly difficult due to higher compression ratio compared to a petrol engine. |
| 10 | As the compression ratio is low, the petrol engines are cheaper and lighter in weight. | As the compression ratio is high, the diesel engines are costlier and heavier in weight. |
| 11 | The running cost of petrol engine is high because of the higher cost of petrol fuel. | The running cost of the diesel engine is low because of the lower cost of diesel fuel. |
| 12 | The maintenance cost is less. | The maintenance cost is more. |
| 13 | Petrol engines are high-speed engines. | Diesel engines are relatively low-speed engines. |
| 14 | The petrol engines are are installed in light duty vehicles such as scooters, motorcycles, cars. These are also used in airplanes. | The diesel engines are are installed in heavy-duty vehicles such as buses, trucks, tractors, earth moving machines, etc. |
