The piston is a vital part of a reciprocating engine. It consists of a cylindrical piece of metal with piston rings that create an air-tight seal when fitted inside the engine cylinder. The piston is attached with a piston pin or gudgeon to a connecting rod, which is connected to the crankshaft.
In four-stroke (petrol and diesel) car engines, intake, compression, combustion and exhaust takes place above the piston, in the cylinder head which pushes the piston up and down (or in and out in a horizontally opposed – or flat – engine), causing the crankshaft to rotate.
What is a Piston?
The piston is found in reciprocating engines, reciprocating pumps, gas compressors, hydraulic cylinders and pneumatic cylinders. It is the moving element contained by a cylinder, and sealed by piston rings.
In an engine, the piston transfers force by expanding gas in the engine cylinder to the crankshaft, often through a piston rod and/or connecting rod. In a pump, the function is reversed and transfers force from the crankshaft to the piston to compress or expel the fluid within the cylinder.
In some engines, the piston also serves as a valve by covering and uncovering ports in the cylinder.
Why a piston is made of aluminium
Engine parts need to be tough and lightweight for efficiency and good running life.
For this reason, piston design typically uses an aluminium alloy to get the best overall performance in the engine of a normal car or motorcycle. The piston rings (a compression ring, a wiper ring and an oil ring from top to bottom) are built from cast iron or steel.
Function Of Piston
The piston acts as a forged moveable plug in the cylinder, forming the lower portion of the combustion chamber. There is a gas-tight seal between the piston and cylinder wall, so when the combustion gases are hot, the only way they can expand is by forcing the piston down.
This is the same motive force as the cannonball, except now the piston is pushed back up the cylinder by the rotating crankshaft and the cycle repeats itself.
More than 60% of the friction in the engine is due to the motion of the piston assembly, so this is an important area to focus on improving engine efficiency. The piston is also still under development and research, as we will see more detail in a moment.
Tremendous forces are generated during the change in direction of a piston during its up and down motion. The lighter the piston assembly, the less the mass means less momentum, meaning less force exerted, the possibility of higher RPM engines. This provides a constant effort to reduce weight in the connecting rod and piston.
The piston is connected to the crankshaft through a connecting rod. This is often just shortened to rod (or conrod). The two parts are often just referred to as a piston assembly. Both ends of the connecting rod are allowed to pivot: the part of the connecting rod that connects to the piston is the small end, and the part which attaches around the crankshaft is the big end.
The big end will have bearing inserts which provide a reduction in friction and put the connecting rod in exact oil clearance with the rod journal on the crankshaft. The connecting rod has a split, which is where the rod cap clamps around the big end bearing and crankshaft.
How piston rings work
The oil ring collects oil from the cylinder wall when the piston moves, but it (and the other rings) can wear and allow oil from the crankcase to enter the combustion chamber.
Worn piston rings are accompanied by excessive oil consumption and white smoke from the exhaust tailpipes.
Internal combustion engines operate on a single cylinder (one piston, like in most motorbikes and petrol lawnmowers or generators) up to a maximum of 12 (high-performance sports cars). Passenger vehicles typically have four or six cylinders.
Radial engines found in propellar-drivem planes typically have an odd number of cylinders and pistons to enable them to run smoother.
Piston are also found in an external combustion engine (steam-engine style) when water is heated in a boiler and steam forces a pair of pistons (typically) in external cylinders to turn wheels. A rotary engine does not have a cylinder or piston.
How Does a Piston Work?
In each four-stroke (gasoline or diesel) automotive engine, the intake, compression, combustion, and exhaust process not only take place above the crown of the piston, but this process is what causes the piston to move up and down in the cylinder.
The piston goes upward compressing the fuel and air into a smaller area in the cylinder head, where it will be fired by a spark plug, and the explosion will force the piston back down and make exhaust gases. A much more complete explanation can be read about here.
In any internal combustion engine, the piston performs four jobs on each and every cycle.
- First, as the piston goes down, it draws air and fuel into the cylinder/combustion chamber.
- Second, on the upward stroke, it compresses the air and fuel in the cylinder so it will explode when ignited.
- Third, the spark plug ignites the air-fuel mixture and the force flings the piston back down.
- Fourth, the piston comes to the top of the cylinder once more expelling the burnt gases (exhaust) out of the cylinder.
- Then the cycle repeats over and over again, hundreds or thousands of times a minute.
Piston Diagram with Name
Parts Of a Piston
Piston features include the piston head, piston pin bore, piston pin, skirt, ring grooves, ring lands, and piston rings.
1. Piston Ring
A piston ring is an expandable split ring used to provide a seal between the piston the cylinder wall. Piston rings are commonly made from cast iron. Cast iron retains the integrity of its original shape under heat, load, and other dynamic forces.
Piston rings seal the combustion chamber, conduct heat from the piston to the cylinder wall, and return oil to the crankcase. Piston ring size and configuration vary depending on engine design and cylinder material.
Piston rings commonly used on small engines include the compression ring, wiper ring, and oil ring.
- A compression ring is the piston ring located in the ring groove closest to the piston head. The compression ring seals the combustion chamber from any leakage during the combustion process.
- 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. Two-stroke cycle engines do not require oil rings because lubrication is supplied by mixing oil in the gasoline, and an oil reservoir is not required.
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 from cast iron material due to its excellent wear resistance and self-lubricating properties.
The skirt contains the grooves to mount the piston oil ring as well as compression rings. Piston skirts come in different styles to suit specific applications.
3. Piston Pin
A piston pin is a hollow shaft that connects the small end of the connecting rod to the piston.
4. Piston Head/Crown
The piston head is the top surface (closest to the cylinder head) of the piston which is subjected to tremendous forces and heat during normal engine operation.
5. Connecting Rod
The connecting rod connects the piston to the crankshaft, and this is functions as a liver arm and transfers motion from the piston to the crankshaft.
The connecting rod is made of cast aluminum alloy. And it is designed in such a way that it can withstand the dynamic stresses of the combustion and piston movement.
6. Connecting Rod Bolt
Also, in the list of piston parts is the conrod bolt. These bolts clamp the rod to the crankshaft. The bottom end of rod bolts bolt are rod caps and bearings, held in place by a nut. A cotter pin on the nut prevents the assembly from coming undone.
7. Piston Bearings
The bearings are piston parts that are located at the points where pivotal rotation takes place. The are usually semicircular metal pieces that fit in the bores of these points. Piston bearings include the shells found at the big end where the rod connects to the crankshaft. There are also bearings at the small end where the rod links to the piston.
Piston bearings are usually manufactured using composite metals like lead copper, silicone aluminum, and others. The bearings are often coated to improve hardness and enable them to bear the load of 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.
This piston type is also known as bowl piston. It features a plate-like shape with raised outer edges. Because of the increased combustion chamber volume, dish pistons achieve a lower compression ratio. Although a drawback, this characteristic is an advantage in some situations where high reciprocating forces are not necessary.
Dish pistons are often used in engines with turbocharged or supercharged combustion. They help to prevent knock or detonation that would be caused by the boosted compression.
In some engines, pistons help to contain the fuel spray, helping to enhance the combustion process. In older engines, the process will be influenced by several factors: piston type, carburetor design, and the volume of the cylinder bore.
3. Dome Pistons.
Domed pistons create a more compact combustion chamber, creating higher compression rates, in theory.
However, domed pistons often create other issues, including slow-burning or incomplete combustion, valve interference, and reduced efficiency. This type of piston should only be used in very specific applications.
5 Things That Go Wrong with Pistons And How To Prevent Them
Pistons in modern engines are built to last but sometimes things can go wrong. These are the most common piston failures:
1. Burned Piston.
A burned piston can be seen instantly once the top of the engine has been removed. You should be able to recognize visible signs of melting and sometimes a burnt hole in the piston. A burned diesel piston is generally caused by dirty fuel injectors being used.
2. Cracked Piston.
Continued usage of low-quality gasoline can lead to cracked pistons. A fractured piston might also be the result of the exhaust gas recirculation system failing. Chip tuning, lack of piston cooling, and a faulty injection nozzle are other possible causes of a cracked piston.
3. Timing belt snapped.
The timing belt plays a key role as it keeps the movement of the pistons and the valves in perfect alternating time. Once the belt has snapped, it can cause a collision between the two which can then lead to further damage.
To prevent the timing belt from snapping, it’s important to change the timing belt according to the car manufacturer’s instructions.
4. Worn piston rings.
Piston rings eventually start to wear and the seal between the piston and cylinder is no longer airtight. As a result, oil makes its way from the crankcase past the piston and into the firing chamber. Symptoms of this are white smoke coming from the tailpipe and a drop in engine oil level.
5. Piston slap.
If the noise doesn’t go away once the engine is up to temperature, the piston or the cylinder could be worn. A noisy piston is caused by too large a gap between the piston and the cylinder wall.