Piston - Internal Combustion Engine

The piston of an internal combustion engine transmits gas forces acting at the top, by the combustion of the fuel, to the crankshaft through the piston rod. Piston undergoes severe thermal stresses and fluctuating mechanical stress throughout its service life. It must be manufactured with materials that are resistant to acidic corrosion, erosion and high temperature creep. Material of construction depends on the size and power developed by the engine, type of fuel used, etc.

A Large 2 Stroke Cross Head Type Engine Piston
A Large 2 Stroke Cross Head Type Engine Piston

Pistons of large two stroke engines are made up of crown, skirt and cooling elements. Pistons can be oil cooled or water cooled. Periodic inspections must be carried out in both type of cooling. Freshwater cooling has the advantage that water has twice the thermal capacity when compared to oil. Also a higher temperature can be maintained at the outlet. Oil gets carbonized at higher temperature. But freshwater cooling has a disadvantage that the leakage of water to the lube oil system results in disastrous consequences.

Material of Construction

Part Name
Material
Piston Crown (2 Stroke Crosshead Engines)
Cast Steel
Piston (4 Stroke Trunk Piston Engines)
Aluminium
Piston Rod
Forged Steel
Piston Skirt
Cast Iron

Differences between Two Stroke and Four Stroke Pistons

A four stroke piston is usually of the trunk type, i.e. it is directly connected to the crankshaft, via connecting rod. it has a gudgeon pin, which helps in converting the reciprocating motion of the piston to the rotary motion of the crankshaft.

A two stroke piston is usually of the cross head type, i.e. it is connected via the piston rod to the cross-head bearing, which reciprocates along with the piston. The connecting rod connects the cross-head to the crankshaft.

A 4 Stroke Trunk Type Engine Piston with Connecting Rod
A 4 Stroke Trunk Type Engine Piston with Connecting Rod

The four stroke piston is usually shorter in length. It is cast in one piece, and carries compression as well as oil scraper rings. The piston is usually splash-lubricated, and hence the need for oil scraper rings. All of the above components are normally of different materials.

In the latest engines, oil cooling is enhanced by the so called 'Jet Shaker' principle, which increases the heat transfer rates to improve cooling efficiency.

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