FOUR STROKE SI ENGINE (OR) PETROL ENGINE

 FOUR STROKE SI ENGINE (OR) PETROL ENGINE : 

   The four-stroke cycle petrol engine operates on Otto (constant volume) cycle. Since ignition in these engines is due to a spark they are also called spark ignition engines 

The four different strokes are:

1.Suction stroke (or)inyake stroke 

2.Compression stroke 

3.Working (or) Power (or)      Expansion stroke 

4. Exhaust stroke 

                                PV -diagram.                        

1. Suction stroke :

    

     During suction stroke, the piston is moved from the top dead center to the bottom dead centre by the crank shaft. The crank shaft is revolved either by the momentum of the flywheel or.by the electric starting motor. The inlet valve remains open and the exhaust vakve is closed durimg this stroke. The proportinate air-fuel mixture is sucked inti the cylinder due to downwards movement of the  piston. 

2. COMPRESSION STROKE:,

      During compression stroke, the piston from bottom dead center to the top dead centre, thus compressing air petrol mixture. Due to compression, the pressure and temperature are increased. Just before the end of this stroke the spark -plug initates a spark, which ignites the mixture and combustion takes place at constant volume. Both the inlet and exhaust valves remains closed during this stroke. 

3.WORKIN STROKE :

   The expansion of hot gases exerts a pressure on the piston. Due to this pressure, the piston moves from top dead centre to bottom dead centre and thus the work is obtained in this stroke. 

4.EXHAUST STROKE :

    During this stroke, the inlet valve remains closed and the exhaUST valve opens. The greater part of the burnt gases escapes because of their own expansion. The drop in pressure at constant at constant volume is represented by the line EB. The piston moves from bottom dead centre to top dead centre and pushes the remaining gas to the atmosphere. When the piston reached the top dead centre the exhaust valve closes and cycle is completed. This operations are repeated over  and over again in running the engine . Thus the four stroke engine completes one working cycle, during this the crank rotated by two revolutions.