In every engine piston plays an important role in working and producing results.Piston forms a guide and bearing for the small end of connecting rod and also transmits the force of explosion in the cylinder,to the crank shaft through connecting rod.
Construction Of Piston:-
Its top known by many names such as crown,head or ceiling and thicker than bottom portion.Bottom portion is known as skirt.There are grooves made to accommodate the compression rings and oil rings.the groove,made for oil ring,is wider and deeper than the grooves made for compression ring.The oil ring scraps the excess oil which flows into the piston interior through the oil return holes and thus avoiding to reach the combustion chamber but helps to lubricate the gudgeon pin to some extent.In some designs the oil ring is provided below the gudgeon pin boss .The space between the grooves are called as lands.
The diameter of piston always kept smaller than that of cylinder because the piston reaches a temperature higher than cylinder wall and expands during engine operation.The space between the cylinder wall and piston is known as piston clearance.The diameter of the piston at crown is slightly less than at the skirt due to variation in the operating temperatures.Again the skirt itself is also slightly tapered to allow for unequal expansion due to temperature difference as we move vertically along the skirt the working temperature is not uniform but slightly decrease.
Which Material Is Most Suitable For Piston?
Cast Iron,Aluminium Alloy and Cast Steel etc. are the common materials used for piston of an Internal Combustion Engine.Cast Iron piston are not suitable for high speed engines due its more weight.these piston has greater strength and resistance to wear.
The Aluminium Alloy Piston is lighter in weight and enables much lower running temperatures due to its higher thermal conductivity.The coefficient of expansion of this type of piston is about 20% less than that of pure aluminum piston but higher than that of cast iron piston and cylinder wall.To avoid seizure because of higher expansion than cylinder wall,more piston clearance required to be provided.It results in piston slap after the engine is started but still warming up and tends to separate the crown from the skirt of the piston.
Cutting a vertical slot will avoid this disadvantage.this slot helps in taking up thermal expansion and so the overall diameter of the piston is not required to be so reduced as to obstruct the safe operation between the cylinder walls and the pistons.
To increase the life of grooves and to reduce the wear,a ferrous metal rings are inserted in the grooves of high speed engines.
I hope,this information will help you to select proper piston.The details about other parts of Internal Combustion Engine will be cover in coming posts.
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