What is Four Stroke Engine?

Before starting to know about four stroke engine, we must have the knowledge of what does stroke means. Here we will go to learn about working, types, main components, application, and advantages of 4 stroke engines. I hope that you will really enjoy this.

What is a Stroke?

When the piston moves from TDC to BDC, it is called 1 stroke. Stroke means the movement of something in a direction. Here we mean the movement of the piston. Before defining 1 stroke we should know about some basic terminology used in a four stroke engine.

Main Components of Four Stroke Engine

Main parts of four stroke engine

1. Inlet Manifold: It is a passage for the entry of the fuel inside the cylinder.

2. Exhaust manifold: Passage for the escape of the burnt gases.

3. Intake valve: It is a cam operated valve through which the fuel or air enters within the cylinder.

4. Exhaust valve: It is also a cam operated valve through which the burnt gases escape out of the cylinder.

5. Spark plug: It is used to generate the spark for the burning of the fuel.

6. Cylinder: It is a part of the engine where piston does reciprocating motion in order to produce all four strokes necessary for the working of an engine.

7. Piston: It is a part of engine which does reciprocation motion within the cylinder. It performs suction, compression, and exhaust process during the working of the four stroke engine.

8. Connecting rod: It connects the piston and crankshaft of the engine. It transfers the power generated by the engine from piston to the crankshaft. And the reciprocating motion of the piston changes to the circular motion.

9. Crankshaft: It is used to convert the reciprocating motion of the piston into circular motion.

10. Crank case: It holds the crankshaft assembly in it and protects them from damage.

Basic Terminology Used:

1. Bore:

The inner diameter of the cylinder is called bore. It is generally expressed or measured in millimeter (mm).

2. Stroke:

The distance travelled by the piston from one of its dead center positon to other dead centre position is called stroke. The distance between the two dead centre is called stroke length.

3. TDC:

TDC (Top Dead Center)

The full form of TDC is Top Dead Centre. It is the maximum upper limit up to which the piston can move.

4. BDC:

BDC (Bottom Dead Center)

The full form of the BDC is Bottom Dead Centre. It is the lower maximum limit up to which the piston can move.

5. Combustion chamber:

It is a chamber where the combustion of the fuel takes place

6. Clearance volume:

It is the Volume included between the Piston and the Cylinder Head when the Piston is at its Top Dead Center in Vertical Engines and inner Dead Center in Horizontal Engines. The Clearance Volume is generally expressed as percentages of Swept Volume.

7. Swept volume:

It is the volume through which the Piston sweeps for its one Stroke. It is equal to the area of cross section of the Piston multiplied by its Stroke Length. It is also known as swept volume.

8. Total volume:

The summation of the clearance and swept volume is called total volume.

9. Compression ratio:

The ratio of the total volume to the clearance volume is called compression ratio. For Petrol Engines the value of Compression Ratio varies from 10:1 to 14:1 and for Diesel Engines varies from 18:1 to 23:1

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What is Four Stroke?

There are four stroke in a 4 stroke engine. It means piston moves 4 times to complete its one cycle of power stroke. One cycle of power stroke includes suction, compression, power and exhaust stroke.

1 stroke: suction stroke
In this stroke, piston movement is from TDC to BDC, and suction of air or fuel takes place inside the cylinder.

2 stroke: Compression stroke
In this stroke, the piston movement is from BDC to TDC and the Compression of the air or fuel takes place.

3 stroke: Power stoke
In this stroke, the piston movement is from TDC to BDC and the combustion of the fuel takes place.

4 stoke: Exhaust stroke
In this stroke, the piston moves from BDC to TDC, and the burnt exhaust gases are thrown out of the cylinder.

What is four Stroke Engine?

Any mechanical device which is capable of converting chemical energy of the fuel into mechanical energy is called an engine. Also in a four stroke engine, the chemical energy is converted into mechanical energy in which the piston does four times movement to produce a power stroke ( 2 times from TDC to BDC and 2 times from BDC to TDC).


The four stroke engine are of two types and these are

1. Petrol Engine or Gasoline Engine:

When petrol is used as fuel in four stroke engine then it is called a four stroke petrol engine. The construction of the petrol engine is slightly different from the diesel engine. In petrol engine there is a spark plug for the combustion of the fuel. And air-fuel mixture is sucked in the cylinder. the air-petrol mixture is prepared by the carburetor.

2. Diesel Engine:

When the fuel used in the four stroke engine is diesel than it is called a diesel engine. In diesel engines, there is a fuel injector for the injection of the fuel within the cylinder. During suction, only air is sucked within the cylinder. Hot compressed air is used for the burning of the fuel in this type of four-stroke engine.

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Working of Four Stroke Engine

The various strokes in four stroke engine are

  1. Suction stroke
  2. Compression stroke
  3. Power stroke
  4. Exhaust stroke

Let’s understand what actually happens in these strokes one by one in detail.

1. Suction Stroke:

suction stroke

  • The piston moves from TDC to BDC
  • Opening of intake valve
  • The suction of the air or air-fuel mixture ( air in diesel engine and air-petrol  mixture in petrol engine)

In suction stroke what happens, first the piston moves from TDC to BDC. As the piston moves the inlet valve opens and the air-fuel mixture in case of a petrol engine and only air in diesel engines enters into the cylinder. The exhaust valve remains closed during this stroke.

2. Compression Stroke

compression Stroke

  • The piston moves from BDC to TDC.
  • Compression of air or air-fuel mixture (air in diesel engine and air-fuel mixture in a petrol engine)
  • Intake and Exhaust valve remains closed.

In compression stroke, the piston moves from BDC to TDC. The inlet and exhaust valve remains closed during this stroke. As the piston moves upward (from BDC to TDC) the compression of air- fuel mixture in case of petrol engine and only air in case of diesel engine takes place. The compression processes complete when piston reaches the TDC. The compression is done to increase the temperature of the air or air-fuel mixture. The temperature is increased so that it can easily catch fire during sparking in case of a petrol engine and spraying of diesel in case of the diesel engines.

3. Power/ Expansion/Working Stroke

Power stroke

  • Sparking and burning of the air-fuel mixture in the Petrol engine.
  • Burning of diesel by the hot gases.
  • The piston moves from TDC to BDC.
  • Intake and Exhaust valve remains closed.
Petrol engine:

The air-fuel mixture is ignited by the spark plug. Due to the ignition the burning process starts. The burning of the air-fuel mixture creates a very high pressure burnt gases. This high pressure burnt gases exerts a thrust on the top face of the piston and it starts to move downward from TDC to BDC. This is the power stroke of the engine. In this stroke, we get power that is utilized to run the vehicle. The intake and exhaust valve remains closed during this stroke.

Diesel engine:

As the Piston approaches TDC the injection of the diesel in the form of spray by fuel injector takes place. As the diesel sprayed by the fuel injector come in contact with the hot compressed gases it catches fire and burning processes starts. Due to burning high pressure hot burnt gases originates and it puts a very high thrust on the top face of the piston. Due to the thrust impact on the piston, it starts to move in a downward direction i.e. from TDC to BDC.

4. Exhaust Stroke

Exhaust stroke

  • The piston moves from DBC to TDC.
  • Opening of the Exhaust Valve.
  • Escaping of the hot burnt gases through exhaust valve.

In this stroke the piston moves upward i.e. from TDC to BDC. As the piston moves upward the exhaust valve opens and all the burnt gases left after power stroke starts escaping out of the cylinder. The burnt gases escape out in the environment through exhaust Valve. When the piston reaches at TDC the exhaust process completes. And after this again all the four stroke repeat themselves.

Working of Four Stroke Engine (petrol engine)

 Working of Four Stroke Diesel Engine


  • The four-stroke petrol engines are used in light vehicles such as motorcycles, cars, scooters, etc.
  • Four-stroke diesel engines are used in heavy vehicles like buses, trucks, Trains, etc where a very large power output is required.

Advantages of Four Stroke Engine Over Two Stroke Engine

  • It has high fuel efficiency.
  • It runs quieter than 2 stroke engines.
  • It remains for last longer than 2 stroke engine and does not cease as quickly.
  • We don’t have to mix oil in the fuel  as done in the 2 stroke engine.

In this article, we have studied what is a four-stroke engine. If you found this piece of information useful then don’t forget to like and share it.

Pankaj Mishra

Pankaj Mishra is a blogger by passion and Mechanical Engineer by profession. He has completed his B.Tech degree in mechanical engineering in the year 2015. He loves to share his knowledge and help others.

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