What is a Piston in Engine and How Does it Work?

Pistons are an important part of internal combustion engines. Hence, the latter are often referred to as ‘piston engines.’ Besides converting the energy released during combustion into mechanical energy, this key component performs other functions as well.

In this piece, we’ll cover everything from how pistons are made, parts of a piston, functions, and types.

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What is a Piston?
How Does a Piston Work?
What are Pistons Made Up of
Major Parts of a Piston
Different Types of Piston
Functions of a Piston
Different Properties of a Piston
Frequently Asked Questions

What is a Piston?

A piston is a crucial component of various reciprocating machinery, such as gas compressors, hydraulic and pneumatic cylinders, pumps, and engines. The piston rings provide a gastight seal within the cylinder containing the moving component. Its function in an engine is to transmit the force of the expanding gas in the cylinder to the crankshaft through a connecting rod and/or piston rod.

The crankshaft is linked to the piston with a connecting rod. When the piston rises and falls, the connecting rod and crankshaft turn the crankshaft, which rotates the wheels. Most cars have four, six, or eight cylinders, but some have twelve. Steam engines, which use boiling water to move pistons in cylinders outside the engine, use them.

As the piston rises and falls in the cylinder, the fuel-air mixture compresses, starting the spark plug ignition process. The explosion pushes the piston down, releasing exhaust gases through the exhaust system.

How Does a Piston Work in an Engine?

Here is a step-by-step explanation of how a piston works:

Step 1: As the engine runs, the piston moves up and down in the cylinder. At the turning point, it experiences speed changes.

Step 2: The change in speed - first slowing down and then accelerating again leads to inertia forces acting on the piston. When combined with the gas pressure, these forces create the piston force.

Step 3: As a next step, piston force is transferred to the connecting rod and crankshaft, which converts it into rotational motion.

Step 4: The force from expanding combustion gases pushes the piston, leading to the crankshaft's rotation. Ultimately, the motion is transmitted through a system of gears, effectively transferring the power generated by the piston.

What Are Pistons Made Up Of?

In modern engines, pistons are primarily made of aluminium alloys owing to their lightweight nature and efficiency. While most pistons are cast, some might be forged to enhance strength and fatigue life.

Piston rings, on the other hand, are predominantly made of cast iron or steel alloys. The ring package comprises a wiper ring, a compression ring, and an oil ring.

What Are the Major Parts of a Piston?

Let’s dig deeper into the major parts of a piston and its functions.

1. Piston Rings

Typically made of cast iron and alloy cast iron, piston rings play an important role in engine operation. They serve the most important function of minimising friction between the piston and cylinder walls and preventing gas from leaking in the combustion process.

There are two main types of piston rings - Compressor rings, which seal the compression chamber, and oil control ring which helps regulate oil consumption.

2. Piston Head/Crown

Located just above the piston, it is the part that comes in contact with the combustion gases, making it experience high temperature and pressure.

3. Piston Skirt

The piston skirt refers to the cylindrical portion of the piston that travels within the cylinder. They are intentionally roughened during manufacturing to retain lubrication and withstand thrust.

4. Piston Pin

Also referred to as a gudgeon pin, this pin connects the piston to the connecting rod. They are made up of harder steel in order to offer good tensile strength.

5. Connecting Rod

The role of connecting rod is to connect the piston to the crankshaft. This allows piston movement within the cylinder and subsequently transmits motion to the engine. The connecting rod is made using cast aluminium alloy and is designed in a manner to withstand piston movement.

6. Connecting Rod Bolt

The primary role of the bolt is to secure the connecting rod to the crankshaft.

7. Piston Bearings

Piston bearings or wrist pin bearings provide support and reduce friction between the piston pin and connecting rod, thus enhancing engine performance and longevity.

What Are the Different Types of Pistons?

While there are many different types of pistons, when it comes to piston shapes, these are the notable ones:

Flat-top Pistons: As the name suggests, a flat-top piston boasts a flat-top surface. They have the smallest amount of surface space, enabling them to provide efficient combustion.
Dish Pistons: The shape of dish pistons resembles a plate with slightly curled-up outer edges. It is commonly used in boosted applications that do not require high compression ratios.
Dome Pistons: Dome pistons feature a raised centre for increased surface area and reduced compression, allowing for efficient energy performance.

What Are the Important Functions of a Piston?

The primary role of the piston is to transfer energy by receiving the force generated by the combustion gases within the cylinder. The energy is thereafter transmitted from the connecting rod to the crankshaft.

Additionally, the piston plays an important part in dissipating the heat generated during combustion by transferring it to the cylinder walls.

Overall, the functions of a piston include:

Energy Transfer: Applying pressure to molecules of gas or liquid facilitates the transformation of that pressure into usable energy in motion. Different air pressures caused by the piston's motions turn the crankshaft, generating the force that drives the vehicle forward.

Heat Dissipation: Heat transmission from burned gas is significantly higher than in any other cycle phase. Heat transfer degrades engine performance. Convective heat transfer is influenced by gas temperature, heat transfer coefficient, charge motion, and transfer area, which are influenced by flame/combustion chamber geometry.

Supporting Piston Rings: The potential damage caused by the piston slamming into the cylinder is averted by using piston rings. These rings, with their tension properties, ensure a tight seal, preventing the catastrophic failure that could result from the piston's uncontrolled movement.

Guiding the Connecting Rod: The piston, piston pin, and connecting rod form a precision-engineered assembly, translating the piston's linear motion into the crankshaft's rotational motion. When functioning in perfect coordination, this intricate system ensures the engine's smooth operation, with the piston lifting the connecting rod and turning the crankshaft as it rises in the engine.

What Are the Properties of a Good Piston?

A good piston must possess the following properties:

Strength and Durability: To avoid breaking under the strain of the power stroke, it must be able to endure high pressure and temperatures.
Lightweight: A lightweight piston helps improve engine efficiency and reduces inertia forces.
Low Heat Expansion: During operation, the piston's design should minimise the impact of thermal expansion.
Balanced Design: The ideal piston design considers the crown shape, the design of the piston skirt, and other aspects to achieve a balance between the piston's weight, strength, and performance.

Thus, the piston is a key component of internal combustion engines. From withstanding high temperatures and pressures, it plays a crucial role in ensuring optimal engine performance and efficiency!

FAQs About Pistons in Automobiles

All

What is the working principle of a piston?

At its core, the fundamental working principle of a piston revolves around transferring the force produced by the expanding gases within the cylinder to the crankshaft. The force is subsequently converted into rotational motion, which is then utilised by the flywheel.

Why is the speed of the piston important?

Piston speed holds great importance as it determines the potential power loss and maximum achievable RPM (Revolutions Per Minute) of an engine. In brief, good speed is directly proportional to the optimal functionality of the car’s engine.

Why is it important for pistons to be lightweight?

The lightweight nature of the piston results in lower inertia forces, allowing the engine to achieve higher RPMs more easily.

What is the function of pistons?

The operation of a pump is reversed, with force transferred from the crankshaft to the piston to compress or discharge the fluid in the cylinder. In some engines, the piston serves as a valve by covering and uncovering apertures in the cylinder—a piece of an internal combustion engine piston revealing the gudgeon pin.

What are the three types of pistons?

Flat-top Pistons
Dish Pistons
Dome Pistons

What is a piston made of?

Pistons are typically manufactured using aluminium alloys and low-carbon steels. In addition to being very hot, the piston is very inert, vibrates, and experiences friction. The differential thermal expansion of the piston and cylinder wall is reduced to a minimum using carbon steel.

Why do we need a piston?

Many mechanical systems rely on pistons, modest but powerful parts used in everything from automobile engines to industrial pumps.

What are the seven parts of a piston?

Piston Rings
Piston Head/Crown
Piston Skirt
Piston Pin
Connecting Rod
Connecting Rod Block
Piston Bearings

How can someone prevent piston damage?

A forged piston compresses aluminium alloy under high pressure, making it stronger and more durable than a cast piston.

Can a damaged piston be repaired?

It may be possible to repair a piston with little damage, but replacing the damaged piston is usually the more economical choice.

How often should pistons be replaced?

Pistons typically last for the engine's life but must be replaced if damaged or worn.

What is piston clearance?

Piston clearance is a small gap between the piston and the cylinder wall, allowing the thermal expansion of the piston when the engine is running.

What if a piston fails?

If a piston fails, it can lead to severe damage to the engine.

What is a piston skirt?

The piston skirt is the lower part of the piston that helps stabilise its movement within the cylinder and reduce side-to-side rocking.

What are the signs of a lousy piston?

Signs of a bad piston include excessive oil consumption, exhaust smoke, engine power loss, knocking or slapping noises, and poor compression.

What are piston rings, and what are they used for?

Metallic split rings called piston rings are fastened to the outer diameter of pistons in internal combustion engines and steam engines.

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