What is Piston? | How does a Piston work?

An engine consists of a fuel pump, connecting rod, crankshaft, and fuel system. A piston is known as the heart of the piston engine. Without the piston, the reciprocating engine can’t compress the air-fuel mixture. Therefore, the maintenance and service of the piston are very important for the proper engine operation. Pistons are most commonly used in petrol engines and diesel engines. This article mainly explains the working, types, and some other aspects of the piston.

What is a Piston?

The piston is a reciprocating mechanical disc that reciprocates forward and backward inside the compression chamber of the engine. It transfers its motion to the crankshaft via a connecting rod.

The performance of an internal combustion engine depends on the piston operation.


This part of the IC engine has a movable part of metal with a piston ring. A piston pin uses to connect the connecting rod to the piston. This connecting rod further connects to the crankshaft via crankpins.

As the liquid or gas in the compression chamber contracts or expands, the piston disc starts moving in the chamber. During the air-fuel mixture combustion process, chemical energy produces.

As the combusted air-fuel mixture expands, the produced energy generates thrust. This thrust moves the piston forward and backward. It transfers its motion to the crankshaft, which further moves the vehicle.

Your piston must have high reliability and flexibility, but its weight must be as low as you can. The lightweight piston helps to decrease inertia generated due to its reciprocating mass.

It must have the capability to bear the high explosive force and temperatures produced in the compression chamber. Your engine piston must reciprocate with minimal friction in the compression chamber.

Working of Piston

A piston is a reciprocating component of the engine. It reciprocates inside the combustion chamber or compression cylinder. Its reciprocating motion helps to generate power from the air-fuel mixture and turns the vehicle wheel.

Working of piston

A piston works in the following way:

  1. For the suction stroke, the piston moves from TDC to BDC. During this motion, it generates a vacuum inside the combustion chamber. A vacuum generates as it reaches at BDC, which opens the suction valve. As the suction valve opens, the air-fuel mixture is introduced from the carburetor to the combustion chamber.
  2. After suction stroke, the piston performs compression stroke. For this stroke, it moves from BDC to TDC. During this motion, it reduces the volume of the combustion chamber.
  3. As the combustion chamber volume reduces, the air-fuel mixture compression occurs. As the piston reaches at TDC, the air-fuel mixture is fully compressed.
  4. A spark plug ignites the mixture when the mixture is fully compressed according to the requirements. Due to the ignition of the air-fuel mixture, heat energy is produced inside the combustion chamber.
  5. As the combusted air-fuel passes through an expansion valve, it expands and forces the piston to move from TDC to BDC.
  6. When the piston gets power by expanded air-fuel mixture, it reciprocates and further reciprocates the connecting rod. The connecting rod, along with the crankpin, converts the reciprocating motion into rotary motion and delivers it to the crankshaft. The crankshaft further delivers rotary motion to the flywheel, which turns the car wheels.
  7. At last, the piston performs the exhaust stroke. For this stroke, the piston again moves from BDC to TDC and expels the exhaust gases out of the combustion chamber. After this last stroke, the whole cycle repeats.

Types of Pistons

The piston has the following major types:

  1. Trunk Pistons
  2. Crosshead Pistons
  3. Sliding piston
  4. Deflector Pistons
  5. Racing pistons
  6. Invar strut piston
  7. Autothermic Pistons
  8. Specialliod Pistons

1) Trunk Pistons

These types of pistons have a long diameter. It is a dual functional function piston (i.e., it can work both as a cylindrical crosshead and a piston).

When the connecting rod tilts nearly all of its motion, a lateral force is still available there that responds to the cylinder wall on either side of the piston.

This is a most commonly used type of piston for reciprocating IC engines. They are employed in both diesel engine and gasoline engine, but high-speed engines now have lighter slipper pistons.

One of the most characteristic features of these pistons (especially for CI engines) is that in addition to the oil ring between the piston crown and gudgeon pin, they contain an oil ring groove under the gudgeon pin.

Read More: Different types of Engines

2) Crosshead Pistons

The high deceleration diesel engine may require extra assets for side forces on the pistons. Therefore, the high-speed diesel engine usually uses a crosshead piston. The main piston contains a large piston rod that extends downward from the piston to the secondary piston with a smaller diameter.

The main piston ensures the gas tightness and the piston ring motion. The smaller piston is mechanically driven. It works in a small compression chamber. It transmits the gudgeon pin and acts as a trunk guide.

Crosshead Piston

Crosshead has better lubricating oil than trunk piston lubricating oil. The heat of combustion does not affect crosshead lubrication. The lubricating oil of the crosshead pistons doesn’t contaminate with flammable soot particles, is not damaged by heat, and can use thinner.

3) Sliding Piston

These types of pistons are best suitable for gasoline engines. These pistons have the smallest size and weight. In risky cases, they have just piston skirt, piston ring support, and piston crown to leave two platforms that prevent the piston from vibrating in the bore.

Sliding Pistons

The edge of the piston skirt extends away from the cylinder wall around the gudgeon pin. The goal of this process is to decrease the mass of the reciprocating movement, which makes it able to easily balance the engine and create high speeds.

4) Deflector Pistons

These types of pistons are most commonly used in a 2-stroke engine with crankshaft compression, which carefully directs the airflow into the cylinder to ensure an efficient exhaust. For the side vacuum cleaner, the inlet port and exhaust port are on the side facing directly to the cylinder wall.

These pistons have a raised rib at the top to stop the direct passage of the incoming air-fuel mixture from one port to the second port. This serves to divert the incoming mixture around the combustion chamber.

5) Racing Pistons

The racing engines have stiffer and stronger pistons than passenger car engines. They are lighter to attain the desired racing speed of the engine.

Racing piston

6) Invar Strut Piston

The invar strut pistons have Invar which is an alloy composed of 64% steel and 36% nickel. Its expansion coefficient is negligible (i.e., 000000063/°C). In a piston, the Invar strut secures the skirt and piston pin bosses that allow the piston to expand to approximately the size of the cylinder. 

Invar Strut pistons

7) Autothermic Pistons

This type of piston has a low-expansion steel insert in the bosses of the piston pin. The shape of these inserts is such that their ends are fixed to the piston skirt.

8) Specialliod Pistons

Specialloid pistons manufacture a wide range of zero piston CI engines and SI engines for main ship propulsion, railway traction, industrial stationery, commercial vehicles, and auxiliary applications.

The latest specialloid diesel pistons have vertical gears on the inner surface of the skirt and sturdy piers that transfer loads directly from the top to the bearing area of ​​the gudgeon pin. 

Parts of Piston

The piston has the following major parts:

  1. Cap
  2. Connecting rod bearing
  3. Piston Rings
  4. Bolt
  5. Connecting rod 
  6. Piston Head or Crown
  7. Piston pin
  8. Piston skirt
  9. Piston ring grooves

Piston parts

1) Piston Rings

The piston ring is a most vital part of the engine piston. As the piston reciprocates inside the combustion chamber, the combustion of the air-fuel mixture occurs. The ring uses to stop the leakage of combustion gases through the piston and to reduce the friction. This ring provides a seal between the cylinder valve and piston.

The alloy cast iron or cast iron is used to construct these rings.  

The piston rings have the following types:

  1. Oil Controller Ring
  2. Compressor Ring and
2)  Piston crown or Piston head

The piston head installs on top of the piston. It has the capability to bear very high temperature and pressure due to its position. The crown uses to limit the confinement process timing, the pigment coming out of the exhaust assists to carry it out of the engine.

3) Piston ring grooves

It consists of a groove on the top of the piston that uses a ring.

4) Piston skirt

It is a cylindrical material attached to the circular portion of the piston. The cast iron part is most commonly used for construction skirts because it has excellent self-lubricating and wear resistance characteristics.

The piston skirt has grooves to install the compression and piston oil rings. These skirts have multiple designs according to the nature of the applications:  

These skirts have the following major types:  

  1. Full Skirt: This skirt also refers to as a solid skirt. It has a tubular design. The full skirts are most commonly used for large vehicle engines.
  2. Slipper skirt: These skirts are most commonly used for motorcycle pistons and some other vehicles. Only the back and front are left on the wall of the cylinder because part of the skirt has been cut off. This saves weight and minimizes the contact area between the piston and the cylinder wall.
5) Piston pin

It is also known as a gudgeon pin. It uses to link the connecting rod to the piston. These pins are made of hard steel.

6) Bolt

It uses to link the connecting rod and clamp.  

7) Connecting rod bearing

The connection rod bearing installs in two parts. These two parts connect in such a way that they make a complete circle. This bearing installs between the crank pin and the connecting rod.

8) Cap

It is the lower part of the piston assembly. It is the lower half of the connecting rod and generates housing for supporting the connecting rod.

9) Connecting rod bolt

The connecting rod bolt is the most important part of the piston. This bolt uses to connect the connecting rod and crankshaft. This bolt has a bearing and rod cap at the bottom end. The assembly is then bolted with nuts.

The role of the bolt is to attach the connecting rod to the crankshaft so that the connecting rod can survive the stress produced by the rotation of the crankshaft.

Steel is used for the construction of bolts, but aluminum is employed to make lightweight bolts.

Nickel is best suitable for making strong bolts. The nickel bolts also have a long service life and are used for heavy-duty vehicles.

Function of Piston

  • The main function of the piston is to compress only air or air-fuel mixture inside the cylinder and receive power by the combusted mixture.
  • It receives thrust generated by the combusted air-fuel mixture in the cylinder and delivers it to the connecting rod.
  • It has reciprocating motion inside the combustion chamber. It performs suction, compression, expansion, and exhaust strokes. After the completion of these strokes, it rotates the crankshaft, which further rotates the vehicle wheel.

Piston Characteristic

  • The engine piston must have high reliability and flexibility.
  • It has the capability to bear the explosive force, high pressure, and temperatures of the combusted air-fuel mixture in the compression chamber.
  • It must have the capability to endure the effects of fluctuating loads.
  • It must have lightweight. The lightweight piston helps to decrease inertia generated due to its reciprocating mass.
  • Your engine piston must-have silent operation and is lightweight.
  • It should be mechanically strong.

Piston Advantages and Disadvantages

Advantages of Piston

  • Simple design
  • Low weight
  • High reliability and flexibility
  • High power to weight ratio
  • Easy manufacturing
  • Generate very low vibration because there is no working part in contact
  • Multi-fuel capability
  • Modularity
  • Low turbine working temperature
  • Less noise
  • Require low maintenance
  • Low exhaust gases emissions
  • Easy to start the piston engine
  • Low manufacturing cost
  • Deliver a high degree of maneuverability
  • Best appropriate for waste heat recovery
  • Internally balanced
  • It offers the HCCI combustion process

Disadvantages of Piston

  1. Low efficiency of the fuel
  2. low stability
  3. Requires reduction gearing
  4. Stability of fuel supply
  5. High combustion rate
  6. Not suitable for part load efficiency
  7. Not ideal for hauling heavy loads over long distance

 Piston Application

  • The pistons are most commonly used in engines to compress the air-fuel mixture. It is a reciprocating part of the engine.
  • They also use in reciprocating pumps. The reciprocate inside the pump cylinder. Their main purpose is to increase the pressure of the fluid and pump it to the desired area.
  • They use in compressors to compress the gases or air.

FAQ Section

What is an Engine Piston?

A piston is known as the heart of the piston engine. It compresses the air or air-fuel mixture inside the combustion chamber. This compression of the air-fuel mixture generates an explosion which generates thrust. This thrust reciprocates the piston, and it further transfers its motion to the crankshaft via a connecting rod.

What are the types of pistons?

The pistons have the following types:

  1. Specialliod Piston
  2. Trunk Piston
  3. Autothermic Piston
  4. Invar strut Piston
  5. Crosshead Piston
  6. Racing Piston
  7. Sliding Piston
  8. Deflector Piston

What are the pistons used for?

The pistons are used to compress the air-fuel mixture. As the air-fuel mixture is combusted, it generates thermal energy, which exerts a force on the piston. As the piston gets force, it moves the vehicle. The pistons are most commonly used in piston engines, diesel engines, 2-stroke engines, and 4-stroke engines.

What is the function of the piston?

In a reciprocating engine, the main function of the piston is to compress the air-fuel mixture and transfer thrust generated by the combusted air-fuel to the crankshaft, which moves the vehicle wheels.

In a pump and compressor, the piston receives rotary motion by the crankshaft and compresses the working fluid inside the compression cylinder.

What are the components of the piston?

The piston has the following major components:

  1. Piston Rings
  2. Piston crown or Piston head
  3. Piston ring grooves
  4. Piston skirt
  5. Piston pin
  6. Bolt
  7. Connecting rod bearing
  8. Cap
  9. Connecting rod bolt
Read More
  1. Different types of Piston Engines
  2. Working of Camshaft
  3. Working of Crankshaft
  4. Types and working of Connecting Rod
  5. Function of Engine Cooling System

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