What is a Piston? – Function, Parts, and Types

A piston is a cylindrical component that is used to transfer force from expanding gases in an engine to a crankshaft or other mechanical component.
Engine Piston Diagram

If you’re a car enthusiast, you’ve probably heard the term “piston” thrown around before. But what exactly is a piston, and why is it so important for the functioning of your car’s engine? In this article, we’ll take a closer look at what a piston is, how it works, and why it’s such a crucial part of your car engine.

What is a piston?

A piston is a cylindrical component that is used to transfer force from expanding gases in an engine to a crankshaft or other mechanical component. In an engine, the pistons are moving component that is contained by a cylinder and is made gas-tight by piston rings.

A piston is connected to a crankshaft, which converts the linear motion of the piston into rotational motion. This rotational motion is then used to drive a flywheel, which is used to store energy and provide a consistent output of power.

The process of converting heat energy into mechanical work is known as the thermodynamic cycle, and the piston plays a key role in this process. The cycle can be broken down into three steps:

  • Inputting heat into the gas inside the cylinder causes it to expand, increasing the volume in the cylinder and providing useful work.
  • Removing heat from the cylinder causes the gas’s pressure to decrease, making it easier to compress.
  • Inputting work into the piston compresses it back to its initial state, ready to start the cycle again.

Pistons are used in a wide range of applications, from internal combustion engines in cars to reciprocating engines, reciprocating pumps, gas compressors, hydraulic cylinders, and pneumatic cylinders.

Pistons are typically made from cast or forged from aluminum alloys and are designed to withstand the high temperatures and pressures found in engines.

what does a piston do?

The main function of the piston is a transformation of heat energy into mechanical work and vice versa. The piston converts the energy released during combustion into a mechanical action and transfers it to the crankshaft via a piston rod and/or connecting rod.

The basic function of a piston is to move up and down inside the cylinder, compressing and expanding the combustion chamber. The combustion chamber is where the fuel and air mixture is ignited, creating the power that drives the engine.

As the fuel and air mixture is ignited, the pressure in the chamber increases, forcing the piston down. This downward motion of the piston is transferred to the crankshaft via the connecting rod, which in turn drives the wheels of the car.

How Does a Piston Work?

The piston work by converting the energy released during combustion into a mechanical action and transfers it to the crankshaft in the form of a torsional force via the piston pin and the connecting rod.

In an internal combustion engine, the piston is located inside the cylinder and is sealed off at the top by a cylinder head. The cylinder is filled with a mixture of fuel and air, which is ignited by a spark plug, causing the mixture to burn and expand.

This expansion pushes the piston down, which in turn rotates the crankshaft, converting the linear motion of the piston into rotary motion. This rotary motion is then used to drive the wheels of a car or other vehicle.

According to ms-motor service, Pistons are equipped with piston rings. They seal the combustion and working chambers from the crankcase. They also remove the oil from the cylinder walls, thereby controlling oil consumption.

Piston rings also dissipate the heat absorbed by the piston during combustion to the cooled running surface of the cylinder liner.

Engine Piston Diagram

Engine Piston Diagram

Parts of a piston

There is a total of eight parts of the piston assembly which are: piston head, piston pin bore, piston pin, skirt, ring grooves, ring lands, and piston rings.

1. Piston head.

The piston head is the top of the piston, which is typically dome-shaped and made of aluminum or other lightweight metal. It is designed to withstand high temperatures and pressures that are generated in the combustion chamber and to transfer heat from the combustion chamber to the cylinder walls.

The top of the piston head is usually flat and has a small depression called the valve relief, which is designed to provide clearance for the valve train components.

2. Piston pin bore.

A piston pin bore is a hole in the side of the piston perpendicular to piston travel that receives the piston pin.

3. Piston pin.

The piston pin is a small rod that connects the piston to the connecting rod, which is responsible for transmitting the power generated by the piston to the crankshaft. The piston pin is usually made of steel and is a press fit into the piston, which means that it is held in place by friction.

4. Skirt.

The skirt is the cylindrical section of the piston that surrounds the piston pin bore. It is designed to provide support for the piston and to maintain a tight seal within the cylinder.

The skirt also helps to guide the piston as it moves up and down within the cylinder. The bottom of the skirt is usually tapered to reduce friction and ease installation into the cylinder.

5. Ring groove.

The ring groove is a groove cut into the outer surface of the piston, in which the piston rings are seated. The ring lands are the areas of the piston between the ring grooves, which are designed to support the piston rings and keep them in place.

The ring grooves are usually cut at an angle to match the angle of the cylinder walls, this is known as a taper cut.

6. Ring lands.

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

7. Piston ring.

The piston ring is a thin metal ring that sits in the ring groove and is designed to seal the combustion chamber and prevent the fuel-air mixture from escaping. There are typically three piston rings, each with a specific function.

  • The top ring also called the compression ring, is responsible for sealing the combustion chamber to maintain compression.
  • The second ring also called the oil control ring, is responsible for preventing oil from entering the combustion chamber.
  • The oil control ring typically consists of two or three smaller rings, which scrape oil off the cylinder walls and return it to the oil pan.

8. Connecting Rod.

The connecting rod is a component that connects the piston to the crankshaft, which converts the up-and-down motion of the piston into a rotary motion that is used to power the vehicle. It’s made of strong metal alloys like steel or aluminum to withstand the high loads and stresses generated by the engine.

The connecting rod has two ends, one that attaches to the piston and the other to the crankshaft. The big end of the connecting rod is usually designed with a bushing or bearing that allows it to rotate smoothly around the crankshaft journal.

Types Of Pistons

There are three types of pistons, each named for its shape: flat top, dome, and dish, according to Mondello Performance.

1. Flat-top Pistons.

These are the most basic and simple types of piston design. They have a flat top and the smallest amount of surface space. This design allows them to create the most force and is ideal for creating efficient combustion.

Flat-top pistons are known for creating the most even flame distribution, which results in more complete combustion of the fuel-air mixture. The downside to this design is that it can create too much compression for smaller combustion chambers, which can cause pre-ignition and engine damage.

Flat-top pistons are commonly found in low-performance engines, where the compression ratio is low and the camshaft is designed for low lift and low duration.

2. Dish Pistons.

These pistons are shaped like a plate with the outer edges slightly curling up. The dish on the top of the piston reduces the total volume of the combustion chamber, which reduces the compression ratio. This design is typically used in boosted applications, such as turbocharged or supercharged engines, where a high compression ratio is not necessary.

Dish pistons are also used in engines that require a low-lift and low-duration camshaft. The design of the dish piston makes it ideal for engines where a high amount of power is required, but the engine can’t handle high compression.

3. Dome Piston.

These pistons are shaped like the top of a stadium, with a bubble in the middle to increase the surface area available on the top of the piston. This design increases the total volume of the combustion chamber, which reduces the compression ratio.

Dome pistons are typically used in high-performance engines that require a high compression ratio. The high compression ratio allows for more power to be generated, but it also increases the risk of engine damage from pre-ignition.

Dome pistons are used in engines that have a high-lift and high-duration camshaft, which allows for more air and fuel to be drawn into the combustion chamber.

What Are Pistons Made Of?

Pistons are made from either low-carbon steels or aluminum alloys, but they can also be made from other materials such as titanium. Aluminum is the most common material because it is lightweight, durable, and has good heat dissipation properties.

To improve the performance and longevity of the pistons, manufacturers often use coatings on the piston’s surface. These coatings can include:

  • Chrome plating. This is a thin layer of chrome that is applied to the piston to improve wear resistance and reduce friction.
  • Molybdenum disulfide (Moly) coating. This is a dry lubricant that is applied to the piston to reduce friction and improve wear resistance.
  • Thermal barrier coatings. These coatings are applied to the piston to insulate it from heat and reduce thermal expansion.

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