How does four stroke engine works ?

After understanding about the classification of I.C Engines lets try understand the "How does four stroke engine works" ?
These are the basic engines used in Automotive Industry, in this post we would be learning about Four Stroke Gasoline or Petrol Engine

The name itself says that piston is going to complete four stroke while rotating the crankshaft twice, so one operating cycle requires 720° rotation of crankshaft, so what is stroke ?
Basically stroke is movement of piston in either direction from TDC(Top Dead Centre) to BDC(Bottom Dead Centre) or vice versa, the below pictorial represents TDC, BDC and Stroke.
On the engine head we can see Spark Plug for Gasoline Engine or Fuel Injector for Diesel Engine

Diagram denoting Engine Cylinder Parts

One operating cycle gets completed when all the four strokes gets completed, this also known as Otto cycle since it was demonstrated by Nikolaus Otto in 1876.

Piston  – In an engine, piston is used to transfer the expanding force of gases to mechanical rotation of crankshaft via a connecting rod.

Crankshaft  – A crankshaft is a part which is able to convert the reciprocating motion of piston  to rotational motion.

Connecting rod  – A connecting rod transfers motion from a piston to crankshaft

Flywheel – Flywheel is a rotating mechanical device which is used to store energy.
The functions of a flywheel are the following :

1. It stores up energy to help the engine over idle strokes of the piston i.e. suction,compression and exhaust.
2. It dampens out speed fluctuations of the crankshaft due to the varying effect of the firing impulses during the engine cycle.
3. It provides a convenient mounting point for the clutch and starter ring gear.

Spark Plug  – A spark plug delivers electric current to the combustion chamber which ignites the air-fuel mixture coming from Carburetor

Carburetor -  Its main function is to mix the fuel with air in a perfect ratio. It depends upon the engine’s speed and load conditions. Theoretically, the ideal mixture contains 15 parts of air and 1 part of gasoline fuel

Intake Stroke :

The intake is when the air-fuel mixture from carburetor  is introduced to fill the combustion chamber. The intake event occurs when the piston moves from TDC to BDC and the intake valve is open. The movement of the piston toward BDC creates a low pressure in the cylinder. Basically due to pressure difference the air-fuel mixture through the open intake valve moves into the cylinder to fill the low pressure area created by the piston movement. The intake valve then closes and the air-fuel mixture is sucked inside the cylinder is also called Suction Stroke

The below diagram depicting Intake Stroke, with the direction of piston movement along with valve position

Intake Stroke

Main Points :

1.Intake valve - Open, Exhaust valve - Closed
2.Crankshaft rotation - 180°
3.Piston Movement - from TDC to BDC

Compression Stroke :

The compression stroke is when the trapped air-fuel mixture is compressed inside the cylinder. Compressing the air-fuel mixture allows more energy to be released when the charge is ignited. Intake and exhaust valves must be closed to ensure that the cylinder is sealed to provide compression. Compression is the process of reducing or squeezing a charge from a large volume to a smaller volume in the combustion chamber. The flywheel helps to maintain the momentum necessary to compress the charge. Due to reduce in the volume of charge (Air fuel mixture), temperature in the chamber increases which would help the Ignition.

The below diagram depicting Compression Stroke, with the direction of piston movement along with valve position

Compression Stroke

 Main Points :

1.Intake valve - Closed, Exhaust valve - Closed
2.Crankshaft rotation - 180° ( Total 360°)
3.Piston Movement - from BDC to TDC


Power Stroke:

The ignition (combustion) event occurs when the charge is ignited with the help of Spark Plug, It (Charge) gets rapidly oxidized through a chemical reaction to release heat energy. The spark at the spark plug initiates combustion at approximately 20° of crankshaft rotation before TDC.

The power stroke is an engine operation Stroke in which hot expanding gases forces the piston head away from the TDC or top cylinder head. Piston force reciprocated motion is transferred through the connecting rod to apply torque to the crankshaft. This torque makes crankshaft rotate. The amount of torque produced or rotation of piston  depends upon by the pressure on the piston head, the size of the piston head, and other factors. During the power Stroke, both  Intake and Exhaust valves are closed.


The below diagram depicting Power Stroke, with the direction of piston movement along with valve position

Power Stroke

Main Points :
1.Intake valve - Closed, Exhaust valve - Closed
2.Crankshaft rotation - 180°( Total 540°)
3.Piston Movement - from TDC to BDC


Exhaust Stroke

The exhaust stroke occurs after the Power Stroke when exhaust gases (NOx,CO,SOx etc) are generated after the combustion needs to be expelled from the combustion chamber. The exhaust stroke is the final stroke and occurs when the exhaust valve is open and the intake valve is closed. Piston movement evacuates exhaust gases to the exhaust management system where lot of treatment strategies applied on the gases is done before expelling it to atmosphere.
The exhaust valve opens, and inertia of the flywheel and other moving parts push the piston back to TDC, forcing the exhaust gases out through the open exhaust valve. At the end of the exhaust stroke, the piston is at TDC and one operating cycle has been completed.


The below diagram depicting Exhaust Stroke, with the direction of piston movement along with valve position.

Exhaust Stroke

Main Points :
1.Intake valve - Closed, Exhaust valve - Opened
2.Crankshaft rotation - 180°( Total 720°)
3.Piston Movement - from BDC to TDC

Now after looking in to all the stroke individually, the animation shows Four stroke in proper sequence

Four Stroke