Elements of Mechanical engineering (18ME15/25) a VTU syllabus topic.
Module 3: Internal Combustion engines introductions. working of Two stroke petrol engine.

Two stroke petrol engines

•In a 2-stroke engine, ports are present in the cylinder in place of valves. The ports are the openings in the cylinder opened and closed by the movement of piston with in the cylinder. There are three ports, namely

•Inlet port: Through which admitting of charge in to the crankcase takes place.

•Transfer port: Through which the charge is transferred from the crankcase to the cylinder.

•Exhaust port: Through which the burnt gases are discharged out of the cylinder.

•In a 2-stroke engine, piston performs two different strokes or crankshaft completes one revolution to complete all the operations of the working cycle. In these engines there are no suction and exhaust strokes, instead they are performed while the compression and power strokes are in progress.

First stroke(Downward stroke)

vAt the beginning of this stroke, the piston is in the TDC as shown in the figure(a). At this position, inlet port is opened and hence fresh air petrol mixture enters into the crankcase. At this position, compressed air-petrol mixture present in the cylinder in the previous cycle is ignited by the spark generated by the spark plug. The combustion of fuel releases hot gases which increases the pressure in the cylinder. The high pressure gases exerts a pressure on the piston and hence the piston moves from TDC to BDC. Thus piston performs power stroke. The power impulse is transmitted from the piston to the crankshaft through the connecting rod. This causes the crankshaft to rotate at high speeds. Thus work is obtained in this stroke.

vAs the piston moves downwards, it uncovers the exhaust port and hence burnt gases escape out of the cylinder as shown in the figure(b). As piston moves downwards further, opens the transfer port and the charge in the crank case is compressed by the underside of the piston as shown in figure(b). the compressed charge from the crankcase rushes into the cylinder through the transfer port as shown in fig(b). the charge entering the cylinder drives away the remaining exhaust gases through the exhaust port.

vThe process of removing the exhaust gases with the help of fresh charge is known as scavenging. The piston is provided with a projection at its top known as ‘deflector’. The purpose of providing a deflector is to deflect the fresh charge coming through the transfer port to move towards the top end of the cylinder. By doing this, the fresh charge will be able to derive the entire burnt gases out of the cylinder.

Second stroke(Upward stroke):

vAt the beginning of the stroke, piston is in BDC and it covers the inlet port as shown in figure(c) and stops the flow of fresh charge into the crankcase. During the stroke, piston ascends and move towards TDC. As the piston moves upwards, it closes the transfer port, there by stopping the flow of fresh charge into the cylinder kas shown in figure(d).

v  Further upward movement of the piston closes the exhaust port and actual compression of the charge begins. In the meantime, the inlet port is opened, and the upward movement of piston creates a suction in the crankcase. Fresh charge enters into the crankcase through the inlet port as shown in the figure(a). the compression of the charge in the cylinder continues till the piston reaches the TDC. This completes the cycle.