A piston in an internal combustion engine plays a crucial role in facilitating the operation of the combustion cycle. A cylinder block encloses the engine part, using a piston ring to prevent gas from escaping.
The pistons help in the transformation of heat energy into mechanical work and vice versa. It moves upward and downward inside the cylinder to expand and contract the air-fuel mixture. For this reason, a piston is essential in an internal combustion engine.
In this reading, we’ll explore what a piston is, its function, diagram, and how it works. We’ll also learn its built material, parts, and types.
Let’s begin!
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What Is a Piston?
A piston is a mechanical device that moves upward to compress gas and downward due to an explosion in a cylinder to convert heat energy into mechanical work. A piston follows a cyclical process to continuously convert heat into mechanical work. The process is achieved in three ways:
- Providing heat to the gas inside the cylinder for useful work
- Removing heat from the cylinder to decrease the pressure so that the gas can easily be compressed.
- The process involves applying work to the piston when it is in its initial state, ready to perform the cycle again.
Functions Of a Piston
Pistons play a vital role in an automobile engine, including the gasoline engine with spark ignition and the diesel engine with compression ignition. These two internal combustion engine processes are different, but both utilize pistons. Below are the functions of the piston on an automobile engine:
- The piston helps to transfer the force output of a small gas explosion in the cylinder to a crankshaft. This process provides rotational momentum to a flywheel.
- It moves forward so that gases can be compressed and an explosion can occur during the backward movement.
- The piston contains a piston pin, which allows the gas in the chamber to escape.
- A connecting rod attached to the piston’s bottom allows the mechanical work to be transferred.
- Pistons help carry the air-fuel mixture during the combustion cycle.
- Pistons aid the control of oil flow in the cylinder walls using the oil control ring.
Parts Of a Pistons
A piston is a critical component of an internal combustion engine, and it consists of various parts, each playing a specific role in its operation. Here’s a breakdown of the main parts of a piston:
Piston Skirt
A piston skirt is a cylindrical material attached to the round portion of a piston. It is usually made from cast iron in order to resist wear and its self-lubricating properties. There are grooves on the skirt that allow the piston rings to perfectly sit. The function of the piston skirt is to travel up and down the cylinder.
Piston Rings
The pieces of split rings that mount on the piston’s recess area are known as piston rings. There are usually three piston rings in an engine. Occasionally the ring can be one, depending on the engine type.
Piston Bearings
The bearings are excellent piston parts that aid the effectiveness of the movement. The bearings are located at the points where the pivotal rotation occurs. These bearings are usually semicircular metal pieces that fit in the bores of these points.
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Piston Pin
The piston pin is a piston part that is also known as a wrist pin or gudgeon pin. This pin is the hollow or solid shaft in the skirt section. There is a piston rod pivoted on this pin, held in the piston ring bushing. The purpose of the piston pin is to offer bearing support so the piston can function properly.
Piston Head
This part of a piston is also known as a crown or dome, which is the top surface. It’s the part that contacts the combustion gases, making it experience an extremely high temperature. The function of the piston is to receive pressures, temperatures, and other stresses of the expanding gas.
Connecting Rod Bolt
The conrod bolt is another essential component of the piston. It is used to mount the rod to the crankshaft. There are rod caps and bearings at the bottom end of the rod bolts. Next, we use a nut to secure the components and bolt together.
Connecting Rod
A connecting rod is one of the major parts of the piston, most often shortened as a conrod or rod. It connects the piston to the engine’s crankshaft and allows the piston’s movement in the chamber. The component’s robustness stems from its design to withstand mechanical strain. We make the piston parts through forging and occasionally casting processes.
Diagram Of a Piston
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How Does A Piston Work?
The piston moves up and down in the cylinder when the engine is running. As the piston approaches the turning point, it decelerates before rapidly accelerating again. The movement of the piston generates inertia forces.
The piston force, which is a combination of the gas pressure forces, is transmitted to the connecting rod and crankshaft.
There are only two points where connecting rods are precisely vertical: at the upper and lower turning points. A connecting rod angle forces the piston against the cylinder wall.
As the combustion cycle continues, the size and direction of this force change based on the piston force and the angle between the connecting rod and the piston top.
Piston rings are part of a piston. They ensure that the combustion and working chambers are sealed in relation to the crankcase. In addition to regulating oil use, they remove oil from the inside of the cylinder. The cooling running surface of the cylinder liner receives the heat that the piston rings release during combustion.
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Piston Material
Cast iron is the earliest material used for making pistons. However, modern engines benefit from lighter materials for engine balancing. Good pistons must be able to resist the engine’s combustion temperatures. Alloys such as Y alloys and aluminum are specifically used to obtain such properties. Pistons are made from aluminum alloys with a casting process.
Furthermore, some pistons used in racing vehicles require better strength and fatigue life, which is why they are forged. Racing engines also utilize billet pistons, as they do not depend on the size and architecture of available forgings, allowing for last-minute design changes. Although it’s not commonly visible to the naked eye.
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Types of Pistons
Pistons come in various types, each designed for specific engine applications and performance needs. Here are the most common types of pistons:
Dish Pistons
A dish piston is shaped like a plate with slightly curling up outer edges. It is easy and simple, as well as giving fewer issues to engineers. It’s often used in boosted applications that do not require a high-lift camshaft or high compression rate.
Flat-Top Pistons
The flat-top type of piston has a flat top. It has the smallest amount of surface space, giving the chance of creating the most force. It’s perfect for creating efficient combustion. The flat-top pistons create a high explosion in the chamber, but compression might be too much for smaller combustion chambers.
Dome Pistons
The concept of dish pistons is quite opposite to the dish type. The middle bubble to increase the surface area left on the top of the piston. More surface area means less compression, which results in less force being created.
The combustion chamber has an upper limit it can handle, so reducing the compression rate is the best option for preventing the engine from breaking down.
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Common Piston Problem
The problem the piston develops is nothing less than a crack. This crack occurs on the top of a piston head, which is known as a crown.
Due to the combustion pressure in petrol engines, it is typically the result of excessive compression or over-advanced ignition timing. The piston head cracks because it operates outside the pressure it is supposed to work on.
In a diesel engine, the piston develops issues due to a condition known as thermal fatigue. An engine experiences thermal fatigue when it operates under both a heavy and a light load.
These constant, drastic changes in combustion temperature suddenly result in thermal cracks in the piston head. Cracks also happen in the piston skirt due to the constant excessive loading of the engine and high mileage fatigue.
In certain instances, the design of the piston is the root cause. Usually, the manufacturer resolves this issue by sending a replacement part. A piston skirt can still crack at the early stages of a repaired engine when the piston is not properly fitted to the rods. This condition causes stress fractures, which cause a serious crack in the skirt.