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.
Today we’ll be looking at the definition, functions, working, types, parts, materials, and diagram of an automobile piston.
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 transform 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 in internal combustion engines
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 process is different but they utilize piston for their processes. Below are the functions of the piston on an automobile engine:
- The major function of a piston is 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.
How does a piston work?
Asking how a piston works, you will be exposed to the entire working principle of the internal combustion engine. This is because the piston does the major work during the four-stroke cycle.
Just as earlier mentioned, an internal combustion engine is of two types and they work in two different manners. One of them works with a spark plug, hence it’s named a “spark ignition engine” and the other a “compression ignition engine”. Their work is quite different. Well, the working of this engine has been covered in another article.
Cast iron is the earliest material used for making pistons. However modern engine benefits 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.
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.
Below is the diagram of a piston:
Major parts of pistons and their functions
Below are the explanations of the major parts of the piston:
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 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.
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.
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.
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 is a 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.
A connecting rod is one of the major parts of the piston most time 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 designed to bear mechanical strain which is why it’s sturdy enough. The piston parts are made from forging and sometimes casting process.
Types of pistons
Below are the three 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. It is usually caused by excessive compression or over-advanced ignition timing as a result of combustion pressure in petrol engines. 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. Most time manufacturer corrects the latter by supplying a superseded part. 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.
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That’s for this article that highlights the definition, working, parts, types, material, and the problem of a piston. I hope the knowledge is attained if so, let me know your thoughts, and don’t forget to share. Thanks!