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.
The piston ring is an expandable ring that provides a seal between the piston and the cylinder wall. Piston rings are commonly made from cast iron which retains the integrity of its original shape once heat, load, and other dynamic forces are applied.
Piston rings are used to seal the combustion chamber, transfer heat away from the piston to the cylinder wall, and to return oil back to the crankcase. The size and configuration of piston rings will differ depending upon the engine design and the material of the cylinder.
The piston rings usually found on small engines will include the compression ring, the wiper ring, and the oil ring.
- The compression ring is the piston ring that is located in the ring groove closest to the piston head. The compression ring will seal the combustion chamber from any possibility of leakage during the combustion process.
- The wiper ring is the piston ring with a tapered face that is 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. It will stop combustion gases that may pass by the compression ring.
- The oil ring is the piston ring that is 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. It will return the excess oil through ring openings back into the oil reservoir in the engine block. Two stroke cycle engines do not require oil rings since lubrication is provided by mixing oil into the gasoline, an oil reservoir is not required.
2. Piston Skirt.
The skirt is the cylindrical part attached to the bottom of a piston. The skirt is made from cast iron, as it has great wear resistance and self-lubricating properties. The skirt will have grooves to mount the piston oil ring and the compression rings. Skirts of pistons have styles that differ for applications.
3. Piston Pin.
The 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 of the piston, when the piston is in the down position, or closest to the cylinder head. This top surface of the piston must withstand significant force and heat when the engine is working normally.
5. Connecting Rod.
The connecting rod brings the piston up to the crankshaft. The function of a connecting rod is to be the lever arm which transfers the motion of the piston to the crankshaft.
The connecting rod is made of cast aluminum alloy for maximum strength, while limiting weight. The connecting rod must be constructed to endure the dynamic stresses of the combustion process and movement of the piston.
6. Connecting Rod Bolt.
Further down the list of piston parts is the conrod bolt, which holds the rod to the crankshaft. The bottom of the rod bolts are rod caps and bearings, and these rod caps and bearings are clamped in place with a nut. A cotter pin on the nut mechanically secures the assembly.
7. Piston Bearings
The bearings are the piston parts at each pivotal rotation point. Bearings are typically made to be semicircular metal pieces that fit in bores of the points of rotation. Piston bearings consist of shells located at the big end where the rod connects with the crankshaft. There are also bearings located at the small end where the rod connects with the piston.
Piston bearings are usually made of composite metals like lead copper, silicone aluminum, or others. Piston bearings are coated to increase hardness and help sustain the load incurred by the movement of the piston and connecting rod.
Types of Pistons
There are three types of pistons, each named for its shape: flat top, dome, and dish.
1. Flat-top Pistons.
Flat-top pistons are just what they sound like; they have flat tops. Flat-top pistons have the smallest surface area, which allows them to generate the most force. Flat-top pistons are great for forming good combustion.
Flat-top pistons produce the most uniform flame distribution. The problem with this is that if you have a small combustion chamber it could create too much compression.
2. Dish Pistons.
Also known as bowl, dish pistons have a dish-shaped piston with raised edges that form a plate-like shape. Because dish pistons have a larger volume in the combustion chamber, a lower compression ratio is attained. While this is a detriment for some applications, it is beneficial for some applications where high reciprocating forces are not needed.
Dish piston applications include turbocharged or supercharged combustion engines. They limit the likelihood of knock or detonation due to boosted compression.
In some engines, pistons assist in containing fuel spray, thereby improving combustion. This is dependent on many other factors in older engines like type of piston, carburetor design, cylinder bore volume, etc.
3. Dome Pistons.
Dome pistons create a smaller combustion chamber and contribute to a higher compression rate theoretically.
Dome pistons create other problems including low burn or incomplete combustion, valve interference, and inefficient efficiency, etc. This type of piston should only be used for specific purposes.
5 Things That Go Wrong with Pistons And How To Prevent Them
Pistons in modern engines are built to last but occasionally things can go wrong. Below are the most common symptoms of piston failure:
1. Burned Piston.
You can usually see a burned piston as soon as you pull the top of the engine off. You should see burned holes and possibly signs of melting on the piston. Burned diesel pistons come from using dirty fuel injectors.
2. Cracked Piston.
After continued use of poor-quality gasoline, a failure will develop into a cracked piston. Exhaust gas recirculation system failure can fracture a piston. Chip tuning, no piston cooling, or a mono-directional nozzle are other failures that can lead to cracked pistons.
3. Timing belt snapped.
The timing belt is important because it ties the pistons and valves to their alternating motion. Once the belt snaps, it can create collision between the two, leading to damage.
To prevent the belt from snapping, you must change it according to the car manufacturer’s instructions.
4. Worn piston rings.
The piston rings, after years of wear, no longer create a seal between the piston and the cylinder. As a result, oil travels from the crankcase past the piston and into the firing chamber. White smoke from the tailpipe and low engine oil are signs.
5. Piston slap.
If the noise does not stop once the engine warms up, the piston or the cylinder is worn. A noisy piston is simply too much gap between the piston and the cylinder wall.