Four stroke petrol engine: Definition, Working, Cycle, Applications

What is four stroke petrol engine?

1) Four Stroke Petrol Engine is also called the Spark Ignition Engine.

2) Its working cycle is consists of four strokes, These are suction Stroke, compression Stroke, power Stroke, exhaust Stroke.

(Stroke:- The travel of piston from the TDC to BDC or vice versa is known as the stroke.)

3) Petrol is used as the fuel in the 4 Stroke Petrol Engine.  

4) Four stroke petrol engine works on the Otto Cycle.

Four stroke petrol engine working cycle:

Four stroke petrol engine working cycle
Four-Stroke-Petrol-Engine-working-cycle

1) Four stroke petrol engine works on the otto cycle.
2) Otto cycle is invented by the German engineer Nicolaus Otto.
3) It consists of the following process:-

a) Process ( 0-1 ):- At this process, the piston sucks (pulls) the air-fuel mixture inside the cylinder at constant pressure P1.

b) Process ( 1-2 ) :- It is adiabatic compression process. Piston compress the air-fuel mixture inside the cylinder from pressure P1 to P2

c) Process ( 2-3 ):- It is a constant volume heat addition process. In this process, spark plug produces a spark, and fuel starts burning.
At this process, the piston is at TDC.

d) Process ( 3-4 ):- It is an adiabatic expansion process. Due to the burning of fuel, High-pressure gases expand inside the cylinder from P3 to P4.

e) Process ( 4-1 ):- It is a constant volume process. During this process, heat rejects from the burnt gases. At this process, Piston is at BDC.

f) Process ( 1-0 ):- It is a constant pressure process in which burn gases are released to Exhaust Pipe.

Construction of four stroke petrol engine:

The Four Stroke Petrol Engine Consists of Cylinder, Piston, Connecting Rod, Crank, Inlet Port, Connecting Rod, Exhaust Port & Spark Plug.

1) Inlet Port:- Inlet Port is Connected to the Intake Manifold. Intake Port Consists of Inlet Valve. From Intake Manifold Air-fuel mixture enters into the Cylinder through Inlet Valve.

2) Exhaust Port:- Exhaust Port is Connected to the Exhaust pipe. Exhaust Port Consists of Exhaust valve. Burn gases from Cylinder are Released to Exhaust Pipe through Exhaust Valve.

3) Spark Plug:- Spark Plug is connected to Cylinder. Spark Plug Produces Spark in Cylinder. Spark helps in starts burning of fuel.

Spark Plug

4) Cylinder:- Piston Slides inside the cylinder. The cylinder head consists of an inlet port, Exhaust port, Piston & spark plug.

Different size cylinders are used in automobiles as per power requirements.

Cylinder

The size of the Cylinder means volume inside the cylinder when Piston is at the bottom dead center 

5) Piston:- Piston slides inside the cylinder. Piston suck (Pull) the air-fuel mixture from the intake manifold & compresses it inside the cylinder.

Piston

6) Connecting Rod:- Connecting Rod connects the piston to the crank.

One end of the connecting rod is connected to the piston.

The other end of the connecting rod is connected to the crank.

connecting rod

7) Crank:- It Connects the connecting rod to the crankshaft. One end of the crank is connected to the connecting rod. The other end of the crank is connected to the crankshaft.

Crank

Terms used in 4 stroke petrol engine:

Following are some of the different terms used in the case of the 4 stroke petrol engine, which will make it easy to understand the topic.

TDC (Top Dead Centre):- It is the nearest position of the piston from the cylinder head. When the piston is at TDC, the volume of the cylinder enclosed by the piston is minimum.

BDC (Bottom Dead Centre):- It is the farthest position of the piston from the cylinder head. When the piston is at BDC, the volume of the cylinder enclosed by the piston is maximum.

Stroke: The travel of the piston from the TDC to BDC or vice versa is known as stroke.

Stroke volume: The volume of the cylinder from the position TDC to the BDC is known as a swept volume. It is also known as the volume swept by the piston from TDC to BDC.

Clearance volume: When the piston is at TDC, the Volume enclosed by the cylinder and piston is known as clearance volume. It is the minimum volume inside the cylinder in the complete cycle.

A/F ratio (Air-fuel ratio): It is the ratio of the mass of air to the mass of fuel present into the air-fuel mixture.

What are Four Strokes in Four Stroke Petrol Engine:

Four Stroke Petrol Engine diagram

The working of the 4 stroke petrol engine comprises of following 4 important strokes.

1) Suction Stroke:- While suction stroke, the piston moves from TDC to BDC with the intake valve is in open condition and the exhaust valve in closed condition.
During this stroke, as the piston moves from TDC to BDC, a partial vacuum is created inside a cylinder that helps to suck (pull) the air-fuel mixture from the intake manifold into the cylinder.
At the end of the suction stroke when the piston reaches the BDC, the inlet valve becomes closed.

2) Compression Stroke:– During the compression stroke, the piston compresses the air-fuel mixture to high pressure inside the cylinder.   For this purpose, the piston moves from the BDC towards TDC. During a compression stroke, both valves (Inlet and Exhaust) remain in a closed condition.  At the end of the compression stroke, the spark plug produces the spark to ignite the air-fuel mixture.

3) Power Stroke / Expansion Stroke:- It is a stroke, in which power is obtained from the engine by the expansion of combustion products. It is also called an Expansion Stroke because due to the burning of fuel, High-pressure gases expand inside the cylinder and force the piston downwards. During power stroke piston moves from the TDC towards BDC. At the end of the power stroke, the exhaust port opens.

4) Exhaust Stroke:- In Exhaust stroke, the piston moves from BDC to TDC. During this stroke, burn gases are released to Exhaust Pipe through an exhaust port. The exhaust valve closes when the exhaust stroke is completed.

Working of four stroke petrol engine:

1) When the piston moves from TDC to BDC, the inlet valve is open, Hence when the piston sucks (Pull) the air-fuel mixture from the Intake Manifold. At the end of this stroke inlet valve closes.

2) After Suction Stroke, Piston compresses the air-fuel mixture to high pressure inside the cylinder.  During the compression stroke, the piston moves from the BDC towards TDC.
During the Compression stroke, Inlet and Exhaust valve are closed.
At the end of the compression stroke, the spark plug produces the spark to burn the fuel.

4) After compression stroke, Due to the burning of fuel, High-pressure gases expand inside the cylinder and force the piston downwards. In Expansion stroke piston moves from TDC to BDC.
It is a stroke, in which power is obtained from the engine by burning the fuel hence this stroke is known as Power Stroke.
At the end of the power stroke, the exhaust port opens.

6) After Power Stroke, Piston moves from BDC to TDC. During this stroke, burn gases are released to Exhaust Pipe through the exhaust port.
The exhaust valve closes when the exhaust stroke is completed.

After Exhaust valve closes, the fresh Air-fuel mixture again enters the cylinder Therefore cycle continues.

Advantages of four stroke petrol engine:

The four stroke petrol engine has the following advantages:-

  1. The 4-stroke petrol engines work at high speed and low torque.
  2. The 4-stroke petrol engines work at a comparatively lower compression ratio.
  3. It does not require a high-pressure fuel injection system and fuel injectors.
  4. The engines are light in weight.
  5. The working of 4 stroke petrol engine is less noisy.
  6. Cold starting is easier in 4 stroke petrol engines.
  7. The initial cost of 4 stroke petrol engine is less.
  8. It has a lower maintenance cost.
  9. The engine creates fewer vibrations.

Disadvantages of four stroke petrol engine:

The four stroke petrol engine has the following disadvantages:-

  1. It requires an external device like a carburetor for the mixing of air and fuel.
  2. The fuel used by the four stroke petrol engine is highly volatile.
  3. The spark ignition system is required to ignite the compressed mixture.
  4. The cost of fuel used in four stroke petrol engine is comparatively high.
  5. It creates less torque than diesel engines.
  6. The 4 stroke petrol engine has lower thermal efficiency.
  7. It is not suitable for heavy load application due to the less torque generation.
  8. This engine consumes more fuel for a unit amount of power generation, Thus it has lower specific fuel consumption.

Applications of four stroke petrol engine:

The four stroke petrol engine has applications in the following vehicles:-

  1. Auto-rickshaw
  2. Scooter
  3. Petrol Cars

Read also:

Thanks to:

Four stroke petrol engine gif By Zephyris – Own work, CC BY-SA 3.0

2 thoughts on “Four stroke petrol engine: Definition, Working, Cycle, Applications”

Leave a Comment