Piston and Types of Piston, Definition, Function

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Piston and Types of Piston, Definition, Function

Editing Date：2025-12-20

Piston and Types of Piston, Definition, Function

Piston Definition
A piston is cylindrical in shape and forms the movable portion of the combustion chamber. A piston is a component of reciprocating engines, reciprocating pumps, gas compressors, and pneumatic cylinders among other similar mechanisms. In an engine, it’s purpose is to transfer force from expanding gas in the cylinder to the crankshaft via a piston rod and connecting rod.
The connecting rod helps to change the reciprocating motion of a piston into a revolving motion of the crankshaft. Pistons are commonly made of a cast aluminum alloy for excellent and lightweight thermal conductivity. (Thermal conductivity is the ability of a material to conduct and transfer heat).

How does pistons work?

Movement of the piston is inside the cylinder and it transfers the force of expanding gas via connecting rod to the crankshaft and from crank to piston in reverse direction. The compressing gas allows the injection of fluid in the cylinder. This also helps in controlling the valve. It serves as a heat transfer medium to the cylinder walls.

Pistons work by transferring the force output of an expanding gas in the cylinder to a crankshaft, which provides rotational momentum to a flywheel.

In every four-stroke (gasoline or diesel) car engine, the intake, compression, combustion, and exhaust process take place above the crown of the piston, which forces the piston to move up and down within the cylinder.

The piston pushes upward compressing the fuel and air into a much smaller space in the cylinder head, where it is then ignited by the spark plug. The resulting explosion forces the piston downward, creating exhaust gases. A much more complete explanation can be found here.

In any internal combustion engine, the piston performs four roles on each and every cycle.

•	First, as the piston goes down, it draws air and fuel into the cylinder or combustion chamber.
•	Second, on the upswing, it compresses the air and fuel in the cylinder so it will explode when it ignites.
•	Third, the spark plug ignites the air-fuel mixture, and the force flings the piston back down.
•	Fourth, the piston comes back to the top of the cylinder, expelling the burnt gases (exhaust) out of the cylinder.
•	Then, the cycle repeats over and over, hundreds or thousands of times per minute.

Piston Diagram with Name

Parts Of a Piston

Piston features include the piston head, piston pin bore, piston pin, skirt, ring grooves, ring lands, and piston rings.

1. Piston Ring

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

Piston rings commonly used on small engines include the compression ring, wiper ring, and oil 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.
•	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.
•	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. Two-stroke cycle engines do not require oil rings because lubrication is supplied by mixing oil in the gasoline, and an oil reservoir is not required.

2. Piston Skirt

The skirt of a piston refers to the cylindrical material mounted on the round section of a piston. The part is usually made from cast iron material due to its excellent wear resistance and self-lubricating properties.

The skirt contains the grooves to mount the piston oil ring as well as compression rings. Piston skirts come in different styles to suit specific applications.

3. Piston Pin

A piston pin is a hollow shaft that connects the small end of the connecting rod to the piston.

4. Piston Head/Crown

The piston head is the top surface (closest to the cylinder head) of the piston which is subjected to tremendous forces and heat during normal engine operation.

5. Connecting Rod

The connecting rod connects the piston to the crankshaft, and this is functions as a liver arm and transfers motion from the piston to the crankshaft.

The connecting rod is made of cast aluminum alloy. And it is designed in such a way that it can withstand the dynamic stresses of the combustion and piston movement.

6. Connecting Rod Bolt

Also, in the list of piston parts is the conrod bolt. These bolts clamp the rod to the crankshaft. The bottom end of rod bolts bolt are rod caps and bearings, held in place by a nut. A cotter pin on the nut prevents the assembly from coming undone.

7. Piston Bearings

The bearings are piston parts that are located at the points where pivotal rotation takes place. The are usually semicircular metal pieces that fit in the bores of these points. Piston bearings include the shells found at the big end where the rod connects to the crankshaft. There are also bearings at the small end where the rod links to the piston.

Piston bearings are usually manufactured using composite metals like lead copper, silicone aluminum, and others. The bearings are often coated to improve hardness and enable them to bear the load of piston and connecting rod movements.

Piston Characteristic

The piston should be strong enough to sustain the following:
•	Hammering effect of combustion gas pressure
•	Fluctuating load
•	High temperature of the gases

The piston should be:
•	Light in weight
•	Silent in operation
•	Mechanically strong

Due to the lightweight, there is Inertia losses.

The inertia loads reduce on the bearing, due to changes in motion.

Piston Application or uses

The main application of the Piston are as follows:
•	Reduced inertia improves the mechanical efficiency of the engine.
•	It compresses the fluid inside the cylinder hence increases the pressure and temperature of the fluid inside the cylinder.
•	It also provides the direction.

Piston Advantages

The main advantages of the Piston are:
•	Mechanical simplicity
•	Flexibility and reliability
•	Power to weight ratio
•	Multi-fuel capability
•	Low turbine operating temperature
•	Less vibration and noise
•	Less maintenance
•	Easy to start the piston
•	Highly suitable for waste heat recovery
•	Give a high degree of maneuverability
•	Less manufacturing cost
•	Low NOx emissions
•	It offers the HCCI combustion process
•	Internally balanced
•	Modularity

Piston Disadvantages

The main disadvantages of the Piston are:
•	Poor fuel economy
•	Stability of fuel supply
•	Poor part load efficiency
•	High combustion rate
•	Reduction gearing is require

Types of Piston

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


Flat-top Pistons	Dish Pistons	Dome Pistons

1. Flat-top Pistons.
As simple as it sounds, a flat-top piston has a flat top.
Flat-top pistons have the smallest amount of surface space; this enables them to create the most force.
This type of piston is ideal for creating efficient combustion.
Flat-top pistons create the most even flame distribution.
The difficulty that comes with this is that it can create too much compression for smaller combustion chambers.

2. Dish Pistons.
This piston type is also known as bowl piston. It features a plate-like shape with raised outer edges. Because of the increased combustion chamber volume, dish pistons achieve a lower compression ratio. Although a drawback, this characteristic is an advantage in some situations where high reciprocating forces are not necessary.
Dish pistons are often used in engines with turbocharged or supercharged combustion. They help to prevent knock or detonation that would be caused by the boosted compression.
In some engines, pistons help to contain the fuel spray, helping to enhance the combustion process. In older engines, the process will be influenced by several factors: piston type, carburetor design, and the volume of the cylinder bore.

3. Dome Pistons.
Domed pistons create a more compact combustion chamber, creating higher compression rates, in theory.
However, domed pistons often create other issues, including slow-burning or incomplete combustion, valve interference, and reduced efficiency. This type of piston should only be used in very specific applications.

FAQ's

What is a piston in a car?

A piston is a crucial component of an internal combustion engine used in cars. It is a cylindrical metal object that moves up and down in the engine cylinder, creating the energy required to propel the car forward. The piston is connected to the connecting rod, which in turn is connected to the crankshaft. As fuel and air are burned in the combustion chamber, the piston is driven down, transferring the energy to the crankshaft, and in turn, the wheels of the car. The piston is designed to withstand high temperatures and pressure, making it a durable component of the engine. It is an integral part of the engine, and any issues with it can lead to significant engine problems. Regular maintenance and inspections can help ensure the longevity and efficiency of the piston in a car.

What are the 3 main types of piston?

The following are the main types of Piston:
1.	Flat top Piston
2.	Dome Piston
3.	Dish Piston

Which piston is the strongest?

Forged Pistons are the strongest type because during manufacturing the aluminum is not melted instead the ingot is forced into the shaping mold.

How to replace piston rings without removing engine?

Replacing piston rings is a crucial part of engine maintenance, and many car owners dread the thought of going through the hassle of removing the engine to get the job done. However, it is possible to replace piston rings without removing the engine, albeit it is a bit more challenging. One way to do so is by using a piston ring compressor tool that allows you to compress the rings and slide them into the cylinder. You can also use a leak-down tester to pinpoint which cylinder has a bad ring and focus your efforts on that specific cylinder. It's important to note that replacing piston rings without removing the engine requires a good level of technical knowledge and skills. If you're not confident in your abilities, it's best to leave the job to a professional mechanic to avoid further damage to your engine.

Conclusion

In conclusion, the piston of a car is a vital component that plays a significant role in the engine's functioning. The piston's primary function is to convert the fuel's energy into mechanical energy, which propels the car forward. There are several types of pistons available in the market, including taper piston, cam ground piston, special alloys, wire wound, autothermic, bimetal, cast steel, tinned, oil-cooled, and two-piece pistons.

Each type has specific features that cater to different needs and requirements. Therefore, it's essential to choose the right piston for your car to ensure optimal performance and longevity. Proper maintenance and regular check-ups can also help increase the piston's lifespan and prevent engine damage.