In an internal combustion engine, a piston is one of the greatest components that help the working of the combustion cycle. The engine part is enclosed in a cylinder block, which uses a piston ring to give no space for gas escape.
The pistons help in the transformation of heat energy into mechanical work and vice versa. It moves upward and downward inside the cylinder in order to expand and contract the air-fuel mixture. For this reason, a piston is inevitable 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 get started!
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 in order to convert heat energy into mechanical work. A piston follows a cyclical process for the continuation of the heat-converting process. The process is achieved in three ways:
- Providing heat to the gas inside the cylinder for useful work
- Removing heat from the cylinder in order to decrease the pressure so that the gas can easily be compressed.
- Applying work to the piston when it’s 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 spark ignition gasoline engine and compression ignition diesel engine. These two internal combustion engine processes are different, but they utilize pistons for their processes. 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 provides rotational momentum to a flywheel.
- It moves forward so that gases can be compressed and an explosion can occur for the backward movement.
- The piston contains a pin called 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 to carry the air-fuel mixture within the period of the combustion cycle.
- Pistons aid the control of oil flow in the cylinder walls using the oil control ring.
Related: What Is A Crankshaft? Its Function, Diagram And How It Works
Diagram Of a Piston
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. Furthermore, as the combustion cycle progresses, the magnitude and orientation of this force fluctuate in response to changes in piston force and the angle between the connecting rod axis and piston crown.
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.
Piston Material
Cast iron is the earliest material used for making pistons. However, modern engine 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. Billet pistons are also used in racing engines because they do not rely on the size and architecture of available forgings, making the design change at dying minutes. Although it’s not commonly visible to the naked eye.
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
Piston rings are the pieces of split rings that are mounted on a recess area of the piston. There are usually three piston rings in an engine. Sometimes the ring can be one, depending on the engine type.
Piston Bearings
The bearings are great piston parts that aid the effectiveness of the movement. It’s located at the points where the pivotal rotation occurs. These bearings are usually semicircular metal pieces that fit in the bores of these points.
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 function 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
Another piston part that cannot be left behind is the conrod bolt. It is used to mount the rod to the crankshaft. There are rod caps and bearings at the bottom end of the rod bolts. A nut is then used to lock the components down together with the bolt.
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 is also designed to bear mechanical strain, which is why it’s sturdy enough. The piston parts are made from forging and sometimes casting processes.
Related: What Is A Cylinder Block? Its Diagram and Types
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 in order to increase the surface area left on the top of the piston. Well, more surface area means less compression, while more compression indicates more force 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.
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 is supposed to work on.
In a diesel engine, the piston develops issues by a condition known as thermal fatigue. Thermal fatigue occurs when an engine is operated under a heavy load along with 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 some situations, the piston design is the cause. The manufacturer usually fixes the latter by sending a part that has been replaced. A piston skirt can still crack at the early stage of a repaired engine when the piston is not properly fitted to the rods. This causes stress fractures, which cause a serious crack in the skirt.