Farm Power And Machinery (1+1)

Second stroke (Power and Exhaust)

 
• The rise in pressure in the cylinder caused by the burning gases forces the piston to move down in the cylinder. When the piston goes down, it covers and closes the suction port, trapping the mixture drawn into the crankcase during the previous stroke then compressing it. Further downward movement of the piston uncovers first the exhaust port and then transfer port. This allows the burnt gases to flow out through exhaust port. Also the fresh mixture under pressure in the crankcase is transferred into the cylinder through transfer port during this stroke. Special shaped piston crown deflect the incoming mixture up around the cylinder so that it can help in driving out the exhaust gases. The Second stroke is depicted in Plate 3.3.
• When the piston is at the top of its stroke, it is said to be at the top dead centre (TDC), when the piston is at the bottom of its stroke, it is said to be at its bottom dead centre (BDC). In two stroke cycle engine, both the sides of the piston are effective which is not in the case of four stroke cycle engine.
  
  Plate : 3.3 Second stroke in two stroke engine

Scavenging
The process of removal of burnt or exhaust gases from the engine cylinder is known as scavenging. Entire burnt gases do not go out in normal stroke, hence some type of blower or compressor is used to remove the exhaust gases in two stroke cycle engine.