Piston presentation and construction details । পিস্টন উপস্থাপনা এবং নির্মাণ বিবরণ.

 The tasks a piston is asked to perform in an I.C. engine are:

1. Transmitting blast energy to the crankshaft.

2. To form a seal so that the high pressure gases in the combustion chamber do not escape into the crankcase.(একটি সীল তৈরি করা যাতে দহন চেম্বারে উচ্চ চাপের গ্যাসগুলি ক্র্যাঙ্ককেসে পালাতে না পারে)

3. To serve as a guide and a bearing for the small end of the connecting rod.

In addition to the ability to perform the above functions efficiently, the piston must

have some more desirable features:

1. It should be silent in operation during both warming up and normal running. 

2. The design should be such that there are no twists. 

3. It should offer sufficient resistance to corrosion to corrosion due to some products of combustion e.g. sulfur dioxide.

4. It should be mechanically strong.

5. It should be light in weight so that the inertia energy caused by its speed of exchange is minimal.

6. For efficient heat transfer its component should have high thermal conductivity so that high compression ratio can be used without the occurrence of explosion.

7. It must have a long life. 

   

Constructional features and Materials.

A typical piston is shown in the picture. The top of the piston, called head or crown may be either flat as shown or with crown of dome or may be specially shaped to form desired shape of the combustion chamber, jointly with cylinder head. In case of a piston containing part of the combustion chamber in its crown, compression ratio can be controlled very accurately, but the disadvantage is that in this case a much larger amount of heat has to be dissipated to thrush the piston and the rings.

The supporting webs transmit the force of explosion directly from the crown to the piston pin bosses, thereby relieving the ring groove portion of large loads and thus preventing deformation of the ring grooves. The part of the piston below the rings is called skirt, which is kept quite close fitting in the cylinder, but even then it is separated from the cylinder walls by means of lubricating oil film, for smooth running.

The material used for pistons at one time was  cast iron which has good wearing qualities. As the technology developed, aluminum alloy containing silicon replaced cast iron as piston material because of two distinct advantages. First, it is three times lighter than cast iron, which makes it desirable from the point of view of inertia. Second, it has a high thermal conductivity, which makes it cool. But aluminum has its own disadvantages, too. It is not as strong as cast iron and hence thicker sections have to be used, as a result of which the weight of the piston is increased. It is seen that an aluminum piston, in actual practice, is only about 50 per cent in weight as compared to its cast iron counterpart. Fuhrer, aluminum is relatively soft, as a result of which fine particles in the lubricating oil become embedded in it.