A piston is a cylindrical engine main component  of reciprocating engines that slides back and forth in the cylinder bore by forces produced during the combustion process.  And also Piston are employed in reciprocating pumps, gas compressors and pneumatic cylinders, among other similar mechanisms.The piston acts as a movable end of the combustion chamber. The stationary end of the combustion chamber is the cylinder head. Pistons are commonly made of a cast aluminum alloy for excellent and lightweight thermal conductivity. Thermal conductivity is the ability of a material to conduct and transfer heat. . Depending on the application, engineers will put them to the test in a pressure chamber. Most pistons are made from die- or gravity-cast aluminum alloy. Cast aluminum alloy is lightweight and has good structural integrity and low manufacturing costs.

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The main Parts of Piston are as Follows:
  • Piston head
  • Piston Pin
  • Piston pin Bore
  • Skirt, ring grooves 
  • Piston rings.
  • Ring lands
Piston Head :-
Piston features include the piston head, piston pin bore, piston pin, skirt, ring grooves, ring lands, and piston rings. The piston head is the top surface (closest to the cylinder head) of the piston which is subjected to tremendous forces and heat during normal engine operation.

Piston Pin Bore :-
A piston pin bore is a through hole in the side of the piston perpendicular to piston travel that receives the piston pin. 

Piston Pin :- 
A piston pin is a hollow shaft that connects the small end of the connecting rod to the piston.

Skirt Ring Grooves :-
The skirt of a piston is the portion of the piston closest to the crankshaft that helps align the piston as it moves in the cylinder bore. Some skirts have profiles cut into them to reduce piston mass and to provide clearance for the rotating crankshaft counterweights.A ring groove is a recessed area located around the perimeter of the piston that is used to retain a piston ring. 

Ring Lands 
Ring lands are the two parallel surfaces of the ring groove which function as the sealing surface for the piston ring. 

Piston Rings :- 
A piston ring is an expandable split ring used to provide a seal between the piston an the cylinder wall. Piston rings are commonly made from cast iron. Cast iron retains the integrity of its original shape under heat, load, and other dynamic forces. Piston rings seal the combustion chamber, conduct heat from the piston to the cylinder wall, and return oil to the crankcase. Piston ring size and configuration vary depending on engine design and cylinder material.

Role Of Piston Rings :-
Piston rings commonly used on small engines include the compression ring, wiper ring, and oil ring. A compression ring is the piston ring located in the ring groove closest to the piston head. The compression ring seals the combustion chamber from any leakage during the combustion process. When the air-fuel mixture is ignited, pressure from combustion gases is applied to the piston head, forcing the piston toward the crankshaft. The pressurized gases travel through the gap between the cylinder wall and the piston and into the piston ring groove. Combustion gas pressure forces the piston ring against the cylinder wall to form a seal. Pressure applied to the piston ring is approximately proportional to the combustion gas pressure.

A piston ring must provide a predictable and positive radial fit between the cylinder wall and the running surface of the piston ring for an efficient seal. The radial fit is achieved by the inherent pressure of the piston ring. The piston ring must also maintain a seal on the piston ring lands.
In addition to inherent pressure, a piston ring seals the combustion chamber through applied pressure. Applied pressure is pressure applied from combustion gases to the piston ring, causing it to expand. Some piston rings have a chamfered edge opposite the running surface. This chamfered edge causes the piston ring to twist when not affected by combustion gas pressures.

Another piston ring design consideration is cylinder wall contact pressure. This pressure is usually dependent on the elasticity of the piston ring material, free piston ring gap, and exposure to combustion gases. All piston rings used by Briggs & Stratton engines are made of cast iron. Cast iron easily conforms to the cylinder wall. In addition, cast iron is easily coated with other materials to enhance its durability. Care must be exercised when handling piston rings, as cast iron is easily distorted. Piston rings commonly used on small engines include the compression ring, wiper ring, and oil ring.

Compression Ring :-
The compression ring is the top or closest ring to combustion gases and is exposed to the greatest amount of chemical corrosion and the highest operating temperature. The compression ring transfers 70% of the combustion chamber heat from the piston to the cylinder wall. Most Briggs & Stratton engines use either taper-faced or barrel-faced compression rings. A taper faced compression ring is a piston ring that has approximately a 1° taper angle on the running surface. This taper provides a mild wiping action to prevent any excess oil from reaching the combustion chamber.
A barrel faced compression ring is a piston ring that has a curved running surface to provide consistent lubrication of the piston ring and cylinder wall. This also provides a wedge effect to optimize oil distribution throughout the full stroke of the piston. In addition, the curved running surface reduced the possibility of an oil film breakdown due to excess pressure at the ring edge or excessive piston tilt during operation.

Wiper Ring :-
A wiper ring is the piston ring with a tapered face located in the ring groove between the compression ring and the oil ring. The wiper ring is used to further seal the combustion chamber and to wipe the cylinder wall clean of excess oil. Combustion gases that pass by the compression ring are stopped by the wiper ring.

Oil Ring :-
An oil ring is the piston ring located in the ring groove closest to the crankcase. The oil ring is used to wipe excess oil from the cylinder wall during piston movement. Excess oil is returned through ring openings to the oil reservoir in the engine block. Two-stroke cycle engines do not require oil rings because lubrication is supplied by mixing oil in the gasoline, and an oil reservoir is not required.

Advantages Of Piston Rings :-
  1. They transfer 60 percent of the heat from the piston to the cylinder wall.
  2. The main function of PISTON RING To give a gas-tight sealing of the clearance between the piston and the liner. 
  3. They prevent the fuel/air mixture and exhaust in the combustion chamber from leaking into the sump during compression and combustion.
  4. They keep oil in the sump from leaking into the combustion area, where it would be burned and lost.
  5. Piston rings are fitted in grooves which have been cut in the piston. They are split at one end so they can expand or slipped over the end of piston.
  6. Piston rings prevent the piston from knocking on the cylinder wall.

How Does Piston Works In Internal Combustion Engine ?
First, it sucks the air-fuel mixture by producing vacuum pressure into the cylinder through its downward motion then, the piston helps to compresses said mixture and, finally, it needs to withstand the sudden rise in temperature caused by the ignition with the help Spark Plug then a flame wave spreads through the combustion chamber at extreme speeds.While in these situation the intake and exhaust values are closed.

Of course, when all this is done, the piston has to expel the exhaust gases with help of outlet value. In an engine, its purpose is to transfer force from expanding gas in the cylinder to the crankshaft via a piston rod and/or connecting rod. In a pump, the function is reversed and force is transferred from the crankshaft to the piston for the purpose of compressing or ejecting the fluid in the cylinder. In some engines, the piston also acts as a valve by covering and uncovering ports in the cylinder wall.

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