Internal combustion engine piston: structure, purpose, principle of operation

The piston is a key part of the cylindrical-shaped combustion engine, which is designed to transform fuel energy into mechanical work of a car engine. If the piston isn’t the only thing that broke because of the accident, turn the car in for recycling: https://scrapmycarnear.me.

The piston has a number of important functions:

provides the transmission of mechanical forces to the connecting rod;

is responsible for sealing the combustion chamber;

ensures timely removal of excess heat from the combustion chamber

The piston operates in a challenging and in many ways dangerous environment – at high temperatures and high loads, so it is particularly important for engine pistons to be efficient, reliable and wear-resistant. That is why they are manufactured using lightweight, but heavy-duty materials – heat-resistant aluminum or steel alloys. Pistons are made by two methods – casting or stamping.

Piston design

The engine piston has a fairly simple design, which consists of the following parts:

ICE piston head

Piston pin

Retaining ring



Steel insert

Compression ring first

Compression ring second

Scraper Ring

The design features of the piston depend in most cases on the type of engine, the shape of its combustion chamber and the type of fuel used.


The bottom can have different shapes depending on the function it performs – flat, concave and convex. The concave shape of the bottom makes the combustion chamber more efficient, but contributes to a greater buildup of deposits during combustion. A convex bottom shape improves piston performance, but also reduces the efficiency of the combustion process in the combustion chamber.

Piston Rings

Below the bottom there are special grooves (furrows) for installing piston rings. The distance from the bottom to the first compression ring is called the fireband.

Piston rings are responsible for the reliable connection of the cylinder and piston. They ensure reliable tightness by adhering tightly to the cylinder walls, which is accompanied by an intense process of friction.  Engine oil is used to reduce friction. A cast-iron alloy is used to manufacture the piston rings.

The number of piston rings that can be installed in a piston depends on the type of engine used and its purpose. Often systems with one wiper ring and two compression rings (first and second) are installed.

Wiper Ring and Compression Rings

The liner ring ensures that excess oil is removed from the inner walls of the cylinder in a timely manner, while the compression rings prevent gases from entering the crankcase.

The compression ring, located first, absorbs most of the inertial load as the piston operates.

To reduce the load, many engines install a steel insert in the ring groove to increase the strength and compression ratio of the ring. Compression-type rings can be made in the form of a trapezoid, barrel, cone, with a notch.

The liner ring is in most cases fitted with multiple oil drainage holes, sometimes with a spring expander.

It is a tubular piece that provides a reliable connection between the piston and the connecting rod. It is made of a steel alloy. When the piston pin is installed in the bosses, it is secured tightly with special retaining rings.

The piston, piston pin and rings form what is known as an engine piston group.

The guiding part of the piston unit, which can be in the form of a cone or barrel. The piston skirt is fitted with two bosses for connection to the piston pin.

To reduce frictional losses, a thin layer of sliding agent (often graphite or molybdenum disulfide) is applied to the skirt surface. The bottom of the skirt is fitted with an oil ring.

An indispensable process of the piston unit is its cooling, which can be accomplished by the following methods

by splashing oil through the holes in the connecting rod or by a nozzle;

movement of oil through a coil in the piston head;

Feeding oil into the ring area through an annular channel;

oil mist

Sealing part

The sealing part and the bottom join in the shape of the piston head. The piston rings – oil and compression rings – are located in this part of the unit. The channels for the rings have small holes through which the waste oil enters the piston and then drains into the engine crankcase.

In general, the piston of an internal combustion engine is one of the most heavily loaded parts, which is subjected to strong dynamic and, at the same time, thermal influences. This imposes high demands both on the materials used in the production of pistons and on the quality of their manufacture.

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