Know the Different Components of Piston in a Car

The piston is placed at the heart of a reciprocating engine and it consists of a moving cylinder of metal with piston rings to achieve an airtight seal, when it is introduced into the engine cylinder. The piston is attached with the help of a piston pin to a connecting rod, which in turn is connected to the crankshaft.

The piston slides back and forth in the cylinder bore due to the forces produced during the combustion process. It acts as a movable end of the combustion chamber and 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 which means they can easily conduct and transfer heat. Aluminum expands when heated and proper clearance must be provided to maintain free piston movement in the cylinder bore. Insufficient clearance can cause the piston to seize in the cylinder. Excessive clearance can cause a loss of compression and an increase in piston noise.

Different components of a piston

The different components of a piston can be classified into the following:

• Piston head
• Piston pin bore
• Skirt
• Ring grooves
• Piston rings

Piston head

The piston head lies near the cylinder head and is the top surface 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. There is a piston pin, which is a hollow shaft that connects the small end of the connecting rod to the piston.

Skirt

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.

Ring grooves

A ring groove is a recessed area located around the perimeter of the piston that is used to retain a piston ring. Also, there are ring lands which are the two parallel surfaces of the ring groove and function as the sealing surface of the piston ring.

Piston rings

A piston ring is an expandable split ring used to provide a seal between the piston and 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.

The piston rings that are commonly used on small engines include:

• Compression ring
• Wiper ring
• Oil ring.

Compression 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 towards the crankshaft.

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.

To summarize

The piston acts as the movable end of the combustion chamber and must withstand pressure fluctuations, thermal stress, and mechanical load. Piston material and design contribute to the overall durability and performance of an engine.

The lightweight of aluminum reduces the overall mass and force necessary to initiate and maintain the acceleration of the piston. This allows the piston to utilize more of the force produced by combustion to power the application. Piston designs are based on benefits and compromises for optimum overall engine performance.

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