Two stroke Engine:
In this engine the cycle is completed in 2 strokes of the piston or in one revolution of crankshaft. In 2 strokes, Instead of valve ports are used for suction and exhaust purpose. As the moving parts are less the mechanical efficiency is high. But thermal efficiency of the engine is lower due to some charge escaping without burning.
Two stroke petrol engine: It has no valves but consists of inlet or induction port, exhaust port and third port called transfer port. let the piston be nearing the completion of its compression stroke. The ignition starts due to spark given by the spark plug and the piston is pushed down and performing the working strokes and in doing so the air fuel mixture already drawn from the inner port in the previous stroke is compressed to pressure of about 1.4 bar. The exhaust port is uncovered by the further downward movement of the piston, the transfer port which is only very slightly lower than exhaust port is also uncovered. The top of the piston is made of a particular shape the facilities thr deflection of fresh charge upwards a d avoids its escape along with the exhaust gases. This process is called scavenging.
After reaching the bottom dead center when the piston moves up, it first closes the inlet port, then the transfer port and exhaust port.The charge of the fuel which previously entered the cylinder us now compressed. After the ignition of the charge,the piston moves down from the power stroke and the cycle is repeated as before.
Two stroke diesel engine: In a two stroke cycle C.I engine all the operation are exactly same as S.I engine except in the case only the air is taken instead of air fuel mixture and the fuel is injected at the end of compression stroke, a fuel injector fitted instead of spark plug.
Four stroke Engine:
Here there are four strokes completing two revolution of the crankshaft. There are suction,compression, exhaust and power stroke. Only pure air is drawn into the cylinder during this stroke through inlet valve and exhaust valve is closed. These valves are operated by cam, push rod and rocker arm. The next stroke is the compression stroke in which the piston moves up with both the valves remaining closed. The air which has been drawn into the cylinder through suction stroke is compressed as piston ascends.The compression ratio varies from 14:1 to 22:1. The pressure at the end of the compression stroke ranges from 30 to 45. As the air is progressively compressed in the cylinder its temperature increases until when near the end of compression stroke. When the piston is near the top of the compression stroke diesel fuel is sprayed into the combustion chamber under high pressure. The fuel ignites being burnt with oxygen of the highly compressed air.
Difference between two stroke and four stroke engine.
Two stroke engine:
1. One working stroke in each cylinder per revolution in crankshaft.
2. Light in weight.
3. Moving parts are few.
4. Mechanical efficiency is more.
5. Thermal efficiency is less because a part of air fuel mixture gas as waste with the exhaust gas.
6. Noise is more.
7. Wear and tear is more due to the smaller size of the same power.
8. Construction is simple and easy to manufacture.
Four stroke engine:
1. One working stroke in each cylinder per two revolution of crankshaft.
2.Size and weight of the engine is heavy and larger.
3. Moving parts are more in number.
4. Mechanical efficiency is less.
5. Thermal efficiency is more.
6. Produces less noise.
7.Wear and tear is less.
8. Construction is more complicated due to valve mechanism.