Pistons, also known as “little powerhouses,” are small but mighty components that play a crucial role in many mechanical systems, from car engines to industrial pumps. But what exactly is a piston and how does it work? Let’s find out.
What is Piston?
A piston is a component of reciprocating engines, reciprocating pumps, gas compressors, hydraulic cylinders, and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-tight by piston rings. In an engine, its purpose is to transfer force from expanding gas in the cylinder to the crankshaft via a piston rod and/or connecting rod.
The piston is connected to a connecting rod, which is then connected to the crankshaft. Together, the piston, connecting rod, and crankshaft convert the up-and-down motion of the piston into the rotational motion that powers the wheels of the vehicle.
Internal combustion engines can have anywhere from one to twelve cylinders, with most automobiles having four, six, or eight. They also feature in external combustion engines, also known as steam engines, where the steam produced from boiling water propels the pistons in external cylinders.
Rotary engines do not have pistons, cylinders, or valves, instead, they have triangular-shaped rotors spinning around. However, currently, there are no Wankel rotary engines in production, the last one was the Mazda RX-8 which was discontinued in 2012.
In a four-stroke (gasoline or diesel) car engine, the intake, compression, combustion, and exhaust process take place above the crown of the piston. This forces the piston to move up and down within the cylinder, compressing the fuel and air mixture, which is then ignited by the spark plug.
The resulting explosion forces the piston downward, creating exhaust gases, which are then expelled out of the engine through the exhaust system.
What is a piston made of?
Engine components need to be hardwearing for longevity and lightweight to improve efficiency.
As a result, pistons are usually made from an aluminum alloy but the piston rings (usually comprising, from top to bottom, a compression ring, a wiper ring, and an oil ring) are made from cast iron or steel.
The oil ring wipes oil from the cylinder wall when the piston is moving but over time it and the other rings can wear, allowing oil from the crankcase to move into the combustion chamber.
Excessive oil consumption and white smoke from the exhaust tailpipes indicate piston ring wear.
Internal combustion engines can operate with a single cylinder – and therefore one piston (motorbikes and petrol lawnmowers) or as many as 12, but most automobiles have four or six.
Radial engines, commonly used in propeller-driven planes, have an odd number of cylinders and pistons for a smoother operation.
Pistons also feature in external combustion engines, otherwise known as steam engines, where water is heated in a boiler and the resulting steam is used to propel a pair of pistons (typically) in external cylinders, which then drive the wheels. Rotary engines do not have cylinders or pistons.
Piston parts diagram
Parts of a piston
The piston, as the moving part of the combustion chamber, has the task of converting this released energy into mechanical work. The basic structure of the piston is a hollow cylinder, closed on one side, with the segments piston crown with ring belt, pin boss, and skirt.
Major parts of a piston and their functions:
- Piston rings
- Piston skirt
- Piston pin
- Piston Head/Crown
- Connecting rod
- Piston bearings
1. Piston Ring
Piston rings maintain gas compression between the piston and the cylinder wall. Piston rings seal the cylinder so that combustion gas generated at the time of ignition does not leak into the opening between the piston and the cylinder.
There are usually 3 types of piston rings in a typical car engine:
- Compression ring: this is the top side ring and nearest the combustion chamber. It is also called the gas or pressure ring. The ring prevents combustion gases from leaking. Compression rings also help transfer heat from the piston to the walls of the cylinder.
- A wiper ring is the piston ring with a tapered face located in the ring groove between the compression ring and the oil ring. The wiper ring is used to further seal the combustion chamber and to wipe the cylinder wall clean of excess oil. Combustion gases that pass by the compression ring are stopped by the wiper ring.
- An oil ring is the piston ring located in the ring groove closest to the crankcase. The oil ring is used to wipe excess oil from the cylinder wall during piston movement. Excess oil is returned through ring openings to the oil reservoir in the engine block.
Related Posts: What is Piston Ring?
2. Piston Skirt
The skirt of a piston refers to the cylindrical material mounted on the round section of a piston. The part is usually made of cast iron due to its excellent wear resistance and self-lubricating properties. The skirt contains the grooves for mounting the piston oil ring and the compression rings. Piston skirts are available in different designs to meet specific applications.
There are two main types of piston skirts:
- Full skirt: It is also known as a solid skirt. The full skirt is tubular in shape. It is commonly used in the engines of large automobiles.
- Slipper skirt: The type of piston skirt is used for the pistons of motorcycles and some automobiles. Part of the skirt is cut away so that only the back and front surfaces remain on the cylinder wall. This helps to reduce weight and minimize the contact area between the cylinder wall and the piston.
3. Piston pin/Gudgeon pin
The piston pin is also known as a wrist pin or Gudgeon pin, that used to connect the piston to the connecting rod and provides a bearing for the connecting rod to pivot upon as the piston moves.
In very early engine designs, including those driven by steam, and many very large stationary or marine engines, the gudgeon pin is located in a sliding crosshead that connects to the piston via a rod.
The gudgeon pin is typically a forged short hollow rod made of a steel alloy of high strength and hardness that may be physically separated from both the connecting rod and piston or crosshead.
Piston pin design, especially in small, high revving automotive engines, is challenging. The piston pin must operate under some of the highest temperatures encountered in the engine and its location makes it difficult to lubricate while remaining small and light in order to fit within the piston diameter and not unduly increase the piston mass.
The requirements for lightness and compactness call for a small diameter rod that is subject to high shear and bending loads and has some of the highest compressive loads of any bearing in the entire engine.
To overcome these problems, the materials from which the piston pin is made and the way in which it is made are among the most sophisticated of all mechanical components found in internal combustion engines.
These give rise to the following types of pins.
- Stationary/fixed pin: the pin attaches to the bosses of the piston via a screw. The piston rod then pivots on the pin.
- Semi floating: the pin attaches to the connecting rod in the middle, and the pin ends move freely within the piston bearing and at the bosses.
- Full-floating: in this pin type, the pin is not attached to the pin or piston connecting rod. Instead, it is secured by plugs, clips, or snap rings attached to the piston bosses. The pin can then oscillate at the bosses as well as the rod.
4. Piston Head/Crown
It is also known as a piston crown or dome, the head of a piston is its top. It is the part that comes into contact with combustion gases. This heats it to extremely high temperatures. To prevent melting, piston head parts are made from special alloys, including steel alloys.
A piston head is usually built with channels and cavities. This helps create a swirl that improves combustion. Different types of piston heads are used in different engines. Reasons for the differences vary. The preferred piston head design depends on many factors, such as the expected performance and the type of engine.
5. Connecting rod
A connecting rod also called a con rod, is the part of a piston engine that connects the piston to the crankshaft. Together with the crank, the connecting rod converts the reciprocating motion of the piston into the rotation of the crankshaft.
MORE: What is Connecting Rod?
6. Piston bearings
The bearings are piston parts that are located at the points where pivotal rotation occurs. These are usually semi-circular pieces of metal that will fit into the holes on these points. The piston bearings include the cups at the large end where the rod is connected to the crankshaft. There are also bearings on the small end where the rod connects to the piston.
Piston bearings are usually made from composite metals such as lead copper, silicon aluminum, and others. The bearings are often coated to improve hardness and to support the load from the piston and connecting rod movements.
Types of Pistons
There are three types of pistons, each named for its shape: flat top, dome, and dish.
1. Flat-top Pistons
As simple as it sounds, a flat-top piston has a flat top. Flat-top pistons have the smallest amount of surface space; this enables them to create the most force. This type of piston is ideal for creating efficient combustion.
Flat-top pistons create the most even flame distribution. The difficulty that comes with this is that it can create too much compression for smaller combustion chambers.
2. Dish Pistons
Dish pistons present the least problems for engineers. That is more because of where they are used than any property, they themselves hold.
They are shaped just like a plate with the outer edges slightly curling up. Typically, dish pistons are used in boosted applications that do not require a high-lift camshaft or high compression ratio.
3. Dome Pistons
Opposite in concept to the dish pistons, these bubble in the middle like the top of a stadium. This is done to increase the surface area available on the top of the piston. More surface area means less compression.
While more compression does mean more force is generated, there is an upper limit of what each combustion chamber can handle. Reducing the compression rate in this way essentially prevents the engine from ripping itself apart.
It’s just one tool in limiting the amount of force generated to what the engine is capable of handling safely.
If you are just beginning, this is only the start. You can’t understand the whole puzzle without putting the pieces in context with one another.
So while this explains what pistons do and how differences in shape matter, it needs to be understood in the context of the whole engine to get the full picture. Keep studying and you’ll be on your way.
Different Types of pistons
Following are the Types of Pistons:
- Trunk pistons
- Crosshead pistons
- Slipper pistons
- Deflector pistons
- Racing Pistons
1. Trunk pistons
Trunk pistons are long relative to their diameter. They act both as a piston and cylindrical crosshead. As the connecting rod is angled for much of its rotation, there is also a side force that reacts along the side of the piston against the cylinder wall. A longer piston helps to support this.
Trunk pistons have been a common design of pistons since the early days of the reciprocating internal combustion engine. They were used for both petrol and diesel engines, although high-speed engines have now adopted the lighter-weight slipper piston.
A characteristic of most trunk pistons, particularly for diesel engines, is that they 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’ derives from the ‘trunk engine’, an early design of the marine steam engine.
To make these more compact, they avoided the steam engine’s usual piston rod with a separate crosshead and were instead the first engine design to place the gudgeon pin directly within the piston.
Otherwise, these trunk engine pistons bore little resemblance to the trunk piston; 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
Large slow-speed Diesel engines may require additional support for the side forces on the piston. These engines typically use crosshead pistons.
The main piston has a large piston rod extending downwards from the piston to what is effectively a second smaller-diameter piston. The main piston is responsible for gas sealing and carries the piston rings.
The smaller piston is purely a mechanical guide. It runs within a small cylinder as a trunk guide and also carries the gudgeon pin.
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.
The friction of both piston and crosshead maybe only half of that for a trunk piston. Because of the additional weight of these pistons, they are not used for high-speed engines.
3. Slipper pistons
A slipper piston is a piston for a petrol engine that has been reduced in size and weight as much as possible.
In the extreme case, they are reduced to the piston crown, support for the piston rings, and just enough of the piston skirt remaining to leave two lands so as to stop the piston rocking in the bore.
The sides of the piston skirt around the gudgeon pin are reduced away from the cylinder wall.
The purpose is mostly to reduce the reciprocating mass, thus making it easier to balance the engine and so permit high speeds. In racing applications, slipper piston skirts can be configured to yield extremely lightweight while maintaining the rigidity and strength of a full skirt.
Reduced inertia also improves the mechanical efficiency of the engine: the forces required to accelerate and decelerate the reciprocating parts cause more piston friction with the cylinder wall than the fluid pressure on the piston head.
A secondary benefit may be some reduction in friction with the cylinder wall, since the area of the skirt, which slides up and down in the cylinder is reduced by half. However, most friction is due to the piston ring, which are the parts that actually fit the tightest in the bore and the bearing surfaces of the wrist pin, and thus the benefit is reduced.
4. Deflector pistons
Deflector pistons are used in two-stroke engines with crankcase compression, where the gas flow within the cylinder must be carefully directed in order to provide efficient scavenging.
With cross scavenging, the transfer (inlet to the cylinder) 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. This is intended to deflect the incoming mixture upwards, around the combustion chamber.
Much effort, and many different designs of the piston crown, 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.
Despite this, cross scavenging was never as effective as hoped. Most engines today use Schnoodle porting instead. This places a pair of transfer ports in the sides of the cylinder and encourages gas flow to rotate around a vertical axis, rather than a horizontal axis.
5. Racing Pistons
In racing engines, piston strength and stiffness are 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.
The most important tasks that the pistons must fulfill are:
- Transmission of force from and to the working gas
- Variable bounding of the working chamber (cylinder)
- Sealing off the working chamber
- Linear guiding of the conrod (trunk piston engines)
- Heat dissipation
- Support of charge exchange by drawing and discharging (four-stroke engines)
- Support of mixture formation (by means of the suitable shape of the piston surface on the
- combustion chamber side)
- Controlling charge exchange (in two-stroke engines)
- Guiding the sealing elements (piston rings)
- Guiding the conrod (for top-guided conrods)
As the specific engine output increases, so do the requirements on the piston at the same time.
- The pistons should be strong enough to sustain:
- Hammering effect of combustion gas pressure,
- Fluctuating load and
- A high temperature of the gases.
- The piston should be:
- Light in weight
- Silent in operation and
- Mechanically strong.
- Due to the lightweight:
- Inertia losses and
- The inertia loads reduce on the bearing, due to change in motion
Piston Application or uses:
The main application of the Pistons is:
- Reduced inertia also improves the mechanical efficiency of the engine.
- It compresses the fluid inside the cylinder hence increases the pressure and temperature of the fluid inside the cylinder.
- It also provides direction.
The main advantages of the Pistons are:
- Mechanical simplicity
- Flexibility and reliability
- Power to weight ratio
- Multi-fuel capability
- Low turbine operating temperature
- Less vibration and noise
- Less maintenance
- Easy to start the piston
- Highly suitable for waste heat recovery
- Give a high degree of maneuverability
- Less manufacturing cost
- Low NOx emissions
- It offers the HCCI combustion process
- Internally balanced
The main disadvantages of the Pistons are:
- Poor fuel economy
- Stability of fuel supply
- Poor part load efficiency
- High combustion rate
- Reduction gearing is required