Working of Four Stroke Cycle Petrol Engine

Basic Engine nomenclature

Engine parts have been described in previous blog (Link is provided at the bottom of this blog). Some standard terminology commonly used for IC engine is described below:

a.       Bore

The inside diameter of the cylinder is known as bore.

b.      Stroke

The maximum distance travelled by the piston inside the cylinder in one direction is known as Stroke. It is equal to the twice the radius of crank.

c.       Top Dead Center (TDC)

The extreme position of the piston at the top of the cylinder (head end side) is known as top dead center. In case of horizontal engines, it is known as Inner Dead Center (IDC) position.

d.      Bottom Dead Center (BDC)

The extreme position of the piston at the bottom of the cylinder is called bottom dead center and in case of horizontal engines, it is known as Outer Dead Center (ODC) position.

e.       Stroke length

The distance between TDC and BDC is known as stroke length.

f.       Clearance volume

The volume contained in the cylinder above the top of the piston when the piston is at TDC, is called clearance volume.

g.      Piston displacement or Swept Volume

The total volume swept by the piston in moving between TDC and BDC is known as piston displacement or swept volume.

So cylinder volume is the sum of clearance volume and piston displacement volume.

h.      Compression ratio

The ratio of volume when the piston is at BDC to the volume when the piston is at TDC is known as compression ratio.

Numerically,

Compression ratio (Rc) =  cylinder volume / clearance volume

  

Working of Four stroke cycle petrol Engine:

The working cycle of the engine is said to be completed when it complete four strokes or two revolutions of crank and petrol is used as fuel.

Fig: Working of Four stroke petrol engine

Four strokes are described below:

a.       Suction stroke

The downward movement of piston from TDC to BDC is known as Suction stroke and the crank rotates by 180⁰ during this period. In this stroke, the piston is at the top most position (TDC) and is ready to move down drawing the air fuel (petrol) mixture. The inlet valve is open and the exhaust valve is closed during this stroke. As the piston moves downward, a fresh charge of air fuel mixture enters the cylinder through the inlet valve due to the suction created and is continued until the piston reaches BDC. At this position, inlet valve closes and steps towards next stroke.

b.      Compression stroke

During this stroke, both inlet and exhaust valve is closed and piston moves upward and compresses the charge enclosed in the cylinder. During this process, the pressure and temperature of the mixture increases rapidly. As the piston reaches the top dead center, the mixture is ignited with the help of spark generated by spark plug(s). The burning of the mixture is more or less instantaneous and the pressure and the temperature of gases increases at the constant volume (isochoric process).

c.       Power stroke or Expansion stroke

During this stroke, work is done. The increase pressure of gases exerts a large amount of force and pushes the piston down. During the expansion stroke, both valves remains closed. Piston moves from TDC to BDC reducing high temperature and high pressure gradually. The exhaust valve opens as the piston reaches BDC position and pressure falls suddenly to the atmospheric pressure at constant volume.

d.      Exhaust stroke

During the upward motion of piston, the exhaust valve is open and inlet valve remains closed. The upward movement of piston pushes the burnt gases outside the cylinder through exhaust valve. As the piston reaches TDC, again inlet valve opens and fresh charge is taken inside and the cycle repeats.

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