Pistons are an essential part of an engine, and the type of piston used can have a significant impact on the engine’s performance and efficiency. In this article, we’ll take a look at some of the different types of pistons and their uses.
A piston is a cylindrical component enclosed in a cylinder that is made gas-tight by piston rings. The piston converts the energy released during combustion into a mechanical action and transfers it to the crankshaft in the form of a torsional force via the piston pin and the connecting rod.
Types of Pistons
There are various types of pistons available on the market, each with its own unique features and characteristics.
A) This classification of pistons is based on the design of the skirt:
The skirt is the lower portion of the piston that surrounds the piston pin.
1. Solid Skirt Type Piston.
A solid skirt piston is a type of piston used in internal combustion engines, typically in reciprocating engines such as those found in cars and trucks.
A solid skirt piston is a piston where the skirt is made from one solid piece of metal, as opposed to a split skirt piston which has a split in the skirt allowing it to flex and expand more freely with temperature changes.
Solid skirt pistons are known for being more durable and have less chance of skirt fracture, but less flexible to thermal expansion.
2. Split Skirt Type Piston
A split skirt piston is a type of piston that has a split or divided skirt design. The skirt of the piston is the part that surrounds the bottom of the piston and it is typically used to guide and stabilize the piston as it moves up and down within the cylinder.
In a split skirt piston, the skirt is divided into two or more sections, which allows for greater flexibility in the piston design. This can provide benefits such as reduced noise and vibration, improved oil control, and increased durability. They are commonly used in high-performance engines, diesel engines, and heavy-duty applications.
3. Slotted Piston or Constant Clearance Piston
A slotted piston, also known as a constant clearance piston, is a type of piston used in reciprocating engines. It is designed with a slot or groove in the piston skirt that allows the piston to expand and contract with temperature changes in the engine.
This expansion and contraction can cause the piston to move slightly in the cylinder, which can lead to the increased clearance between the piston and cylinder wall.
The slotted piston helps to maintain a constant clearance between the piston and cylinder wall, which is important for preventing engine damage and reducing friction. This is particularly important in high-performance engines, where high temperatures and thermal expansion can cause significant changes in piston-to-cylinder clearance.
Slotted pistons are typically made from aluminum alloy or steel and are used in internal combustion engines, specifically high-speed diesel engines, gas turbine engines, and reciprocating compressors.
B) This classification of pistons is based on the shape and application of the piston:
4. Slipper Type Piston.
A slipper-type piston is a type of piston used in internal combustion engines, typically in large diesel engines. The slipper design is characterized by a reduced skirt area, which allows the piston to move more freely within the cylinder. This design helps to reduce friction and wear between the piston and cylinder, which can improve engine efficiency and longevity.
Additionally, this type of piston helps to reduce engine vibration by allowing the piston to move independently of the connecting rod, which can also help to improve engine efficiency.
Slipper pistons have the ability to reduce engine noise, improve fuel economy, and increase the engine’s power output.
5. Steel Inserted Type Piston.
A steel-inserted piston is a type of piston that is used in internal combustion engines. It is made by inserting a steel ring or sleeve into the top of the piston.
This steel ring or sleeve provides additional strength and support to the piston, which is useful in high-performance engines that are subject to high levels of stress and wear.
The steel insert can also help to improve heat dissipation and reduce the thermal expansion of the piston, which can improve the engine’s performance and efficiency.
6. Steel Belt Type Piston.
A steel belt piston is a type of piston used in diesel engines. The steel belt is a ring of steel that is wrapped around the piston skirt, providing added strength and durability to the piston.
The steel belt can help reduce piston expansion and increase the engine’s efficiency. It also helps to reduce the risk of piston failure, which can be costly and cause significant damage to the engine.
The steel belt piston can be used in high-performance and high-output engines, as well as in heavy-duty applications such as in trucks and construction equipment.
7. Cam Ground Type Piston.
A cam ground piston is a type of piston that has been ground (machined) to a precise shape to match the camshaft in the engine. This is done to ensure proper timing between the movement of the pistons and the opening and closing of the valves in the engine.
When the camshaft rotates, it pushes on the cam ground piston, which in turn pushes the connecting rod and crankshaft, causing the engine to turn. Cam ground pistons are typically used in high-performance and racing engines where precision and accuracy are critical.
8. Alfinz Type Piston.
This type of piston has a ring made of steel alloy which is a fixed in ring grow.
9. Heat Dam Type Piston
A heat dam piston is a type of piston used to reduce the amount of heat that is transferred from the top of the piston to the bottom. This is achieved by cutting an additional groove above the standard ring grooves on the piston. This additional groove is shallower and narrower than the standard ring grooves and acts as a barrier that prevents heat from flowing from the top of the piston to the bottom.
By adding the heat dam groove, the heat from the combustion process is blocked from flowing down to the bottom of the piston where it can cause damage or seizing of the piston in the liner.
Overall, the heat dam piston is used to increase the durability and longevity of the engine by reducing the amount of heat that is transferred to the bottom of the piston, which helps to prevent damage and seizing of the piston.
C) This classification of pistons is based on the design of the head or crown of the piston:
10. Flat-top Pistons.
A flat-top piston is a type of piston that has a flat top surface, rather than a dome-shaped top. The flat top surface area is the smallest among all other types of pistons, which allows for the most force to be generated during the combustion process.
This makes flat-top pistons ideal for efficient combustion, as they create an even flame distribution throughout the combustion chamber.
However, using a flat-top piston in a smaller combustion chamber can create too much compression, which can lead to engine damage if not properly managed.
11. Dish Pistons.
Dish pistons are a type of piston used in internal combustion engines. They are shaped like a plate with the outer edges slightly curling up, and are typically used in applications where the engine is being boosted (such as with a turbocharger or supercharger) and does not require a high-lift camshaft or high compression ratio.
Because of their design, dish pistons present the least problems for engineers when compared to other types of pistons, primarily because they are used in specific types of applications where they do not need to withstand high levels of stress. Additionally, they are less prone to wear and tear, resulting in less maintenance and repair costs.
12. Dome Pistons.
The dome piston is designed with a raised area, or “bubble,” in the middle of the piston. This raised area increases the surface area available on the top of the piston, which in turn reduces the compression rate of the combustion chamber.
The reason for this is that while more compression generally generates more force, there is a limit to how much force the engine can safely handle. By reducing the compression rate, the dome piston helps prevent the engine from experiencing excessive force that could cause damage.
Different Types of Pistons
1. Trunk pistons
A Trunk piston is a type of piston used in internal combustion engines. It is characterized by its long length relative to its diameter. This design allows it to act both as a piston and a cylindrical crosshead. The connecting rod of the engine is angled for much of its rotation, which creates a side force that acts against the side of the piston and against the cylinder wall. A longer piston helps to support this side force.
Trunk pistons have been a common design in engines since the early days and were used for both petrol and diesel engines. They were mostly replaced by the lighter-weight slipper piston in high-speed engines.
Trunk pistons, particularly for diesel engines, have a groove for an oil ring below the gudgeon pin, in addition to the rings between the gudgeon pin and crown.
The name ‘trunk piston’ comes from the ‘trunk engine’, an early design of the marine steam engine. These engines were designed to be more compact by avoiding the use of a separate crosshead and instead placing the gudgeon pin directly within the piston. These trunk engine pistons bore little resemblance to the trunk piston of internal combustion engines, they were extremely large in diameter and double-acting. Their ‘trunk’ was a narrow cylinder mounted in the center of the piston.
2. Crosshead pistons
Crosshead pistons are a type of piston used in large slow-speed diesel engines to support the side forces on the piston. The main piston has a large piston rod extending downwards from the piston to a second smaller-diameter piston.
The main piston is responsible for gas sealing and carries the piston rings. The smaller piston, called the crosshead, is purely a mechanical guide. It runs within a small cylinder as a trunk guide and also carries the gudgeon pin.
The lubrication of the crosshead has advantages over the trunk piston as its lubricating oil is not subject to the heat of combustion. The oil is not contaminated by combustion soot particles, it does not break down owing to the heat, and a thinner, less viscous oil may be used. This results in less friction between the piston and crosshead, which may be only half of that for a trunk piston.
3. Slipper pistons
A slipper piston is a type of piston that is designed for use in a petrol engine. It is characterized by its reduced size and weight, which is achieved by removing as much material as possible while still maintaining the structural integrity of the piston.
This typically includes the removal of material from the sides of the piston skirt, leaving only two lands to prevent the piston from rocking in the bore. The purpose of this design is to reduce the reciprocating mass of the engine, which makes it easier to balance and allows for higher speeds.
In racing applications, slipper piston skirts can be configured to be extremely lightweight while still maintaining the necessary rigidity and strength. This reduction in inertia also improves the mechanical efficiency of the engine, as the forces required to accelerate and decelerate the reciprocating parts cause less friction with the cylinder wall.
Additionally, there may be some reduction in friction with the cylinder wall, as the area of the skirt that slides up and down in the cylinder is reduced. However, most friction is caused by the piston rings and the bearing surfaces of the wrist pin, so the benefit of this reduction is limited.
4. Deflector Pistons
Deflector pistons are used in two-stroke engines with crankcase compression, which is a type of engine where the fuel and air mixture is compressed in the crankcase before being pushed into the cylinder. The gas flow within the cylinder needs to be directed in a specific way to ensure efficient scavenging, which is the process of removing the exhaust gases from the cylinder to make room for a new mixture of fuel and air.
In cross scavenging, the transfer (inlet) and exhaust ports are on directly facing sides of the cylinder wall. To prevent the incoming mixture from passing straight across from one port to the other, the piston has a raised rib on its crown called a deflector which is intended to deflect the incoming mixture upwards, around the combustion chamber.
However, cross scavenging was never as effective as hoped, and much effort and many different designs of the piston crowns went into developing improved scavenging. The crowns developed from a simple rib to a large asymmetric bulge, usually with a steep face on the inlet side and a gentle curve on the exhaust.
5. Racing Pistons
In racing engines, piston strength and stiffness is typically much higher than that of a passenger car engine, while the weight is much less, to achieve the high engine RPM necessary in racing.