Engine piston

Posted On December 18th, 2019

In the crank mechanism, the piston performs several functions, including the perception of gas pressure and the transfer of forces to the connecting rod, sealing the combustion chamber and removing heat from it. The piston is the most characteristic part of an internal combustion engine , as it is with its help that the thermodynamic process of the engine is realized.

The conditions in which the piston operates are extreme and are characterized by high pressure, temperature and inertial loads. Therefore, pistons on modern engines are made of lightweight, durable and heat-resistant material – aluminum alloy, less often steel. Pistons are made in two ways – by injection molding or stamping, the so-called forged pistons.


Engine piston diagram

The piston is an integral structural element, which is conditionally divided into a head (in some sources it is called the bottom) and a skirt. The shape and design of the piston is largely determined by the type of engine, the shape of the combustion chamber and the combustion process taking place in it. The piston of a gasoline engine has a flat or near-flat surface of the head. Grooves can be made in it to fully open the valves. Pistons for engines with direct fuel injection have a more complex shape. In the piston head of a diesel engine , a combustion chamber of a certain shape is made, which provides a good turbulence and improves mixture formation.


Below the piston head, grooves are made for installing the piston rings. The piston skirt has a conical or curved ( barrel-shaped ) shape. This shape of the skirt compensates for the thermal expansion of the piston during heating. When the engine reaches operating temperature, the piston assumes a cylindrical shape. To reduce friction losses, a layer of antifriction material ( molybdenum disulfide, graphite ) is applied to the side surface of the piston . The piston skirt has holes with tides ( bosses ) for attaching the piston pin.

The piston is cooled from the side of the inner surface in various ways:

  1. oil mist in the cylinder;
  2. splashing oil through the hole in the connecting rod;
  3. spraying oil with a special nozzle;
  4. oil injection in a special annular channel in the zone of rings;
  5. oil circulation through a tubular coil in the piston head.

Piston rings form a tight connection between the piston and the cylinder walls. They are made of modified cast iron. Piston rings are the main source of friction in an internal combustion engine. Loss of friction in the rings reaches up to 25% of all mechanical losses in the engine.

The number and arrangement of rings depends on the type and purpose of the engine. The most common scheme is two compression and one oil scraper ring. Compression rings prevent the breakthrough of gases from the combustion chamber into the crankcase. The first compression ring operates under the most severe conditions. Therefore, on the pistons of diesel and a number of forced gasoline engines, a steel insert is installed in the groove of the ring, which increases strength and allows to realize the maximum compression ratio. Compression rings can have a trapezoidal, barrel-shaped, conical shape, some are made with a cut (cut).

The oil scraper ring removes excess oil from the surface of the cylinder and prevents oil from entering the combustion chamber. The ring has many drainage holes. Some ring designs have a spring expander.

The piston is connected to the connecting rod using a piston pin, which has a tubular shape and is made of steel. There are several ways to install the piston pin. The most popular so-called a floating finger that can be rotated in the bosses and the piston rod of the connecting rod during operation. To prevent finger movement, it is fixed with retaining rings. Much less often, rigid fastening of the ends of the finger in the piston or rigid fastening of the finger in the piston head of the connecting rod is used.

The piston, piston rings and piston pin are known as the piston group.


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