Two stroke cycle engines

TWO STROKE CYCLE ENGINES

Description

As the name implies, a two-stroke cycle engine performs two strokes to complete one cycle in one revolution of the crankshaft. Necessarily, two strokes out of the four strokes of a cycle are to be eliminated. The two strokes that are eliminated are suction and exhaust strokes.

Suction and exhaust strokes are combined with compression and power strokes. The power is developed in every revolution of the crankshaft.

See Fig. 6.

Two-stroke engine has only ports at the cylinder walls and has no valves. In the case of single cylinder two stroke engines (used in scooters and motorcycles), three ports are provided, namely, inlet, transfer and exhaust.

These ports are opened and closed by the movement of the piston itself. Note that the exhaust port is located slightly above the transfer port.

Through the inlet port, fresh charge from the carburetor is taken into the cylinder crank case. Through the transfer port, the fresh charge from the bottom of the piston is supplied to the cylinder. Through the exhaust port, the hot gases are pushed out.

1. TWO STROKE PETROL ENGINE

Scavenging

 In the two-stroke petrol engine, the exhaust gases are removed from the cylinder with the help of fresh compressed charge. This process of removing exhaust gases is called Scavenging. A specific shape is given to the piston, called deflector. The deflector helps to prevent the loss of incoming charge and helps for exhausting the hot gases effectively.

It will be easier to describe the cycle beginning at the point when the piston reaches TDC at the end of the compression stroke. In the two-stroke petrol engine, the draw of petrol-air mixture into the cylinder will not take place in a separate stroke. Therefore, the method of draw of petrol-air mixture should be understood properly.

First Stroke

Fig. 7(a) shows the position of the piston at the end of compression. The spark is produced by the spark plug as the piston reaches TDC. The pressure and temperature of the gases are increased and hence the gases push the piston downwards producing the power stroke.

Refer Fig. 7(b). When the piston opens the exhaust port during the downward stroke, burnt gases.leave the cylinder through the exhaust port.

Refer Fig. 7(c). A little later, the piston uncovers the transfer port and the crank case is directly connected to the cylinder through the transfer port. The downward stroke of the piston compresses the charge in the crank case by the underside of the piston.

Scavenging: In the above position of the piston, the compressed mixture of petrol and air is transferred through the transfer port to the upper part of the cylinder. The exhaust gases are pushed out, with the help of compressed charge. This is known as Scavenging. Scavenging is continued until the piston reaches BDC.

Second Stroke

As the piston moves upwards, it covers the transfer port. Hence, flow of charge into the cylinder is stopped. The upward motion of the piston lowers the pressure in the crank case below atmosphere and fresh air is induced in the crank case through the inlet port as it is uncovered. A little later, the piston covers the exhaust port and actual compression of the charge starts as shown in Fig. 7(d).

The compression is continued until the piston reaches TDC. The ratio of compression ranges from 1:7 to 1:10. The cycle is thus completed within two strokes.

Note: The two-stroke engine requires only two strokes of the piston to complete one cycle of operation. The crankshaft makes only one revolution to complete one cycle. Power is developed in every revolution of the crankshaft.

Uses: Two-stroke petrol engines are used in mopeds, scooters, motorcycles, because they run at high speeds with moderate power outputs.

2. TWO STROKE DIESEL ENGINE

Refer Fig. 8. In two-stroke diesel engine, the draw of pure air into the cylinder will not take place in a separate stroke.

First Stroke

 Fig. (a) shows the position of the piston at the end of compression. Diesel is injected using fuel injector just before completing the compression. It starts burning. The high pressure, high temperature gases push the piston downwards, producing the power stroke. As the piston moves little down, the supply of diesel stops.

Refer Fig. (b). In this, the piston uncovers the exhaust port during the downward stroke. Hence the burnt gases leave the cylinder through the exhaust port.

Refer Fig. (c). A little later, the piston uncovers the transfer port as shown. Now, the crank case is directly connected to the cylinder through the transfer port. Air in the crank case is compressed by underside of the piston and the compressed air is transferred to the cylinder through transfer port. The exhaust gases are pushed out with the help of fresh air until the piston reaches BDC (similar to two stroke petrol engine).

Second Stroke

 The piston moves upwards. It first covers the transfer port and stops the flow of air into the cylinder. A little later, the piston covers the exhaust port as shown in Fig. (d), and actual compression of air starts. The upward motion of the piston lowers the pressure in the crank case below atmosphere and fresh air is induced in the crank case through the inlet port as it is uncovered. The compression of air is continued until the piston reaches TDC.

The fuel supply starts just before the piston reaches TDC and the cycle is completed.

3. COMPARISON OF FOUR STROKE AND TWO STROKE ENGINES

4. COMPARISON OF PETROL ENGINE AND DIESEL ENGINE