Two stroke Spark ignition (SI) engine

In two stroke engine, the cycle is completed in two strokes of a piston or in one complete rotation (360°) of crank.
– Let us start with the piston at Top dead centre (TDC). As the piston reaches the TDC, the spark is produced by spark plug. Due to increase in pressure and temperature, the piston is pushed downward by gases to produce power stroke.
– During its downward movement, the piston uncovers the exhaust port and the expanded burnt gases leave the cylinder through exhaust port.
– During downward stroke the charge (fuel + air) is compressed by the underside of the piston. When the piston uncovers the transfer port, fresh charge is transferred through transfer port to the upper part of the cylinder. The burnt gases are also swept out by fresh charge. This process is called ‘scavenging’. This process continued until the piston reaches Bottom dead centre (BDC).
– As the piston moves upward it first uncovers the transfer port, and exhaust port a little later. After that the actual compression of charge begins. Due to upward motion of the piston, the pressure in the crankcase falls below atmosphere. It helps in introducing the fresh charge into the crank case through the inlet port.
– The compression of charge continues till the piston reaches TDC. In this way a cycle is completed.
– The fresh charge induced in the crank case is sucked through the carburetor.