Petrol and Diesel Engine

There has been constant debate between petrol and diesel engines. Some think that diesel engines are better since they are more efficient. While others find petrol engines better since the price of petrol is more economical. But, very few people know the key differences between them.

Let us know about the key differences between the petrol engine and diesel engine.

What Exactly is a Petrol Engine?

Many people don’t know that but petrol engines  are internal combustion engines that contain spark ignition. Petrol (a volatile fuel) is usually used to run petrol engines. They are also commonly referred to as gasoline engines. 

Petrol Engine: Characteristics

• It works on the Otto cycle
• It has 2 isochoric processes and 2 isentropic processes

In the older version of the petrol engine, air and petrol are two components that are generally mixed post-compression. These two components are mixed in a carburettor.

Nowadays, petrol engines use fuel injector. Now, you may ask what does a fuel injector do? A fuel injector has the ability to inject the fuel directly into the combustion chamber. Once the fuel is injected the mixing process starts. Once the compression is done the fuel is ignited via an electrical spark. That’s why they are also known as SI (spark ignition) engines. The spark plug has a high voltage current which helps in producing a spark. 

The spark plug is usually found on the combustion chamber’s top that helps in quickly igniting the air-petrol mixture. This ignition produces heat which is then converted into mechanical work in the power stroke of the piston.

How does a Petrol Engine Work?

Intake or Suction Stroke

• In the Intake or Suction Stroke, the piston moves downward
• This creates a vacuum inside the combustion chamber
• The air-fuel (petrol) mixture which is available on the inside comes outside into the combustion chamber
• This stroke causes the suction valve to open
• At the same time the exhaust valve closes

2. Compression Stroke

• When the suction process is completed the piston moves upwards to accomplish the air-fuel mixture compression
• The upward movement of the piston creates pressure on the mixture into the combustion chamber
• During this stroke, both the valve and exhaust valve are closed
• The temperature of the air-fuel mixture becomes very high due to the compression process
• This compression process fires up the spark plug and the ignition process of the air-fuel mixture starts up
• This spark starts the combustion process of the air-fuel mixture inside the combustion chamber
• The combustion moves the piston further that later increases the temperature of the mixture which produces heat

3. Expansion Stroke

• The heat which was generated in the previous stage makes the piston move downward. This further turns the crankshaft
• The downward movement of the piston causes the air-fuel mixture to expand inside the chamber
• This leads to a decrease in the pressure of the mixture

4. Exhaust Stroke

• In this stage, there is an upward movement of the piston that opens the exhaust valve
• Due to this the useless gases are released
• Once the exhaust stroke is done the piston again moves down
• Again, all four strokes are again repeated

What Exactly is a Diesel Engine?

Diesel engines are also internal combustion engines in which the ignition of the fuel (diesel) is generated by the high temperature of the air in the cylinder due to adiabatic compression. 

Due to this working mechanism, this engine is also called a compression-ignition engine (CI engine).

Diesel engines work by only compressing air and not fuel. The diesel engine was first developed by Rudolf Diesel in 1893. These engines run at higher rotation speeds.

How Does a Diesel Engine Work?

1. Suction Process

• To create a suction of air the piston moves downward
• The fresh air starts entering the compression cylinder
• The exhaust valve is closed and the suction valve is opened

2. Isentropic Compression

• Once the suction process is completed the suction valve is closed and the piston moves upward
• The upward movement compresses the air inside the cylinder
• During this whole process, there is no change in enthalpy
• The compression of the air is done at a very high temperature and pressure
• This leads the air-fuel mixture to ignite itself, and it doesn’t need any extra external heat source

3. Heat Addition at Constant Pressure

•  A fuel injector injects diesel fuel into the cylinder which is later mixed with the compressed air
• As soon as the diesel fuel touches the compressed air, the air-fuel mixture ignites
• This ignition adds heat to the compressed air-fuel mixture

4. Isentropic Expansion

• In this stage, there is an expansion of the mixture into the cylinder
• This expansion moves the piston down and rotates the crankshaft
• This moves the car

5. Constant Volume Heat Rejection

• After the expansion process, in order to remove the waste heat from the cylinder the piston moves further down
• Once the waste heat is removed the piston again sucks air and the whole process is again repeated

Petrol and Diesel Engine: Difference 

Conclusion

As you can see there is a lot of difference between petrol and diesel engines. Both of them have their own pros and cons. After reading this article you can now make a more informed decision on which vehicle to buy. The key thing to remember here is that a petrol engine works on the ignition which is generated by an electric spark. While in the diesel engine the ignition is generated from the compression of hot air.