Engine piston

Pistons are made of aluminum in small engines or cast iron in larger slower- speed engine .The piston both seals the cylinder and transmits the combustion-generated gas pressure to the crank pin via the Connecting rod .

Aluminium alloy pistons almost used in all modern engines have . Because the aluminium alloy is of
lower strength than cast iron , thicker sections have to be used so not all the advantage of the light
weight of this material is realised.Moreover,
because of its higher Coefficient of thermal
expansion , larger running clearances have
to be allowed.and, The thermal conductivity
of aluminium is about three times that of iron.
this , enables aluminium pistons to run at
temperatures about 200°C lower than cast-
iron ones.

The thermal flow in a piston is from the crown,
to the ring belt ,whence the heat is transferred
through the rings to The cylinder walls and
thence to the coolant.

A small proportion is transferred down to The
skirt and then across the bearing surfaces
between the skirt and cylinder walls but this
is not of such great significance,partly because of the relative remoteness of the skirt from the source of heat and partly its fairly light contact with the cylinder walls . Some heat is also taken away with the Lubricant but, again, this is not a very significant proportion unless the underside of the crown is positively cooled by a jet of oil or some other system.

The main parts of a piston are :

1 - The top ,which may also called the Head or Crown .

2 - The Ring belt .

3 - The Pin bosses .

4 - The Skirt .

The top is part of the Combustion chamber .The top may be flat , or a combustion chamber may be cut into the top of the piston,the top may be raised or have a bowl cut into it.soot contamination of the lubricating
oil in Diesel engines is reduced when the combustion chamber is located in the piston,as opposed to the Cylinder head .

The piston skirt ,which wraps around the lower part of the piston,distributes the side loads and prevents the piston from rocking in the cylinder . long pistons rock less than short ones and are used in diesel engine
to reduce the number of required compression rings .

The pin boss supports the piston pin and transmits the force of combustion to the pin.it is one of the most highly loaded ares of the piston .The piston pin is usually hollow to reduce it's weight.

The piston iS fitted with rings which ride in grooves cut in the piston head to seal against gas leakage and control oil flow.

So we can say that The piston performs the following functions :

(1) Forms a movable gas-tight plug to confine the charge in the cylinder .

(2) Transmits to the connecting rod the forces generated by combustion of the charge .

(3) Forms a guide and a bearing for the small end of the connecting rod,and takes the lateral thrust
due to the obliquity of that rod.