What is Piston Ring?
Simply put, piston rings form a seal between the piston and cylinder wall, which prevents pressurized combustion gases from entering the oil sump. They also regulate oil consumption by preventing excessive oil from entering the combustion chamber and burning. Properly functioning rings are vital to maximum engine power and efficiency.
The main functions of piston rings in engines are:
- Sealing the combustion chamber so that there is minimal loss of gases to the crankcase.
- Improving heat transfer from the piston to the cylinder wall.
- Maintaining the proper quantity of the oil between the piston and the cylinder wall
- Regulating engine oil consumption by scraping oil from the cylinder walls back to the sump.
Importance of Piston Ring
Piston rings are designed to seal the gap between the piston and the cylinder wall. If this gap were too small, thermal expansion of the piston could mean the piston seizes in the cylinder, causing serious damage to the engine.
On the other hand, a large gap would cause insufficient sealing of the piston rings against the cylinder walls, resulting in excessive blow-by (combustion gases entering the crankcase) and less pressure on the cylinder, reducing the power output of the engine.
The sliding motion of the piston ring inside the cylinder wall causes friction losses for the engine. The friction caused by piston rings is approximately 24% of the total mechanical friction losses for the engine. The design of the piston rings is therefore a compromise between minimizing friction while achieving good sealing and an acceptable lifespan.
Lubrication of piston rings is difficult and has been a driving force to improvements in the quality of motor oil. The oil must survive high temperatures and harsh conditions with a high-speed sliding contact. Lubrication is particularly difficult as the rings have an oscillating motion rather than continuous rotation (such as in a bearing journal).
At the limits of piston movement, the ring stops and reverses direction. This disrupts the normal oil wedge effect of a hydrodynamic bearing, reducing the effectiveness of the lubrication.
Rings are also sprung to increase the contact force and to maintain a close seal. The spring force is provided by either the stiffness of the ring itself or by a separate spring behind the seal ring.
It is important that rings float freely in their grooves within the piston so that they can stay in contact with the cylinder. Rings binding in the piston, usually due to a build-up of either combustion products or a breakdown of the lubricating oil, can cause engine failure and is a common cause of failure for diesel engines.
Function of Piston Ring
Between the piston and cylinder, the piston ring is a part that is needed for an engine to work efficiently. Piston Rings have 4 main functions.
Function 1. Seal For Combustion Gas
The function is to fully maintain in-cylinder airtightness between the piston and cylinder wall. It works hard to keep a lid on combustion gas leaking into the gap between the piston and cylinder created at the time of an explosion.
If combustion gas leaked, full power could not be realized and fuel consumption would increase, which is detrimental both economically and environmentally.
Function 2. Control Lubricating Oil (Engine Oil)
Function to constantly create the minimum required lubricating oil layer to prevent burn. Inside an engine’s cylinder, the piston ring is subject to high-temperature combustion gas, and, with the piston, must travel and return many thousands of times a minute, or even more.
To ensure the piston ring and cylinder do not burn, and to prevent excess lubricating oil from getting inside the combustion chamber, the cylinder wall lubricating oil layer is controlled at the minimum required.
Function 3. Heat Conduction Function
A function that releases heat from the piston to the cylinder. When the gas explodes, the temperature inside the piston reaches a heat of approximately 300 degrees Centigrade.
If that heat remains inside the piston, the engine may be damaged, so that heat must be released from the interior. The piston ring helps to do that.
Function 4. Piston Attitude Support
A function that prevents the piston from making strong contact with the cylinder wall. If the piston is at an angle, as it moves it makes contact with various places, and this can cause engine damage. The piston ring supports the piston so the piston can travel smoothly up and down.
Types of Piston Ring
Piston rings commonly used on small engines include the three types of piston ring as mentioned below:
- Compression piston ring,
- Wiper piston ring
- Oil piston ring
1. Compression Ring
The compression ring is the topmost ring in the piston attached to its outside diameter. The main function of the compression ring is to seal the gap between the piston and the cylinder walls.
Sealing this gap with the outer diameter of the piston and the cylinder walls ensure that the air-fuel mixture in the combustion engine does not move down to the crankcase and causes low compression and power.
In addition, this sealing also makes sure that the engine oil in the crankcase, used for lubrication, does not move up into the combustion chamber and gets burnt.
Engine oil moving into the combustion chamber and getting burnt will effectively result in excessive consumption of oil in the engine and then to low engine oil levels.
So, essentially the compression piston ring ensures that the combustion chamber and the crankcase are segregated.
The air-fuel mixture in the combustion chamber is not allowed to trickle down into the crankcase and the engine oil is not allowed to move up into the combustion chamber.
The advantages of a compression ring sealing the gap between the piston and the cylinder walls include maintaining high compression in the combustion chamber, high power and acceleration in the vehicle, and no unnecessary burning of engine oil.
2. Wiper Ring
The wiper ring also called a Napier ring, or backup compression ring, are installed below the compression ring. Their main function is to clean the liner surface off the excess oil and to act as support backup ring on stopping any gas leakage further down which escaped the top compression ring.
Most of the wiper rings have a taper angle face which is positioned toward the bottom to provide a wiping action as the piston moves toward the crankshaft.
If the wiper ring is incorrectly installed with the tapered angle closest to the compression ring, it results in excessive oil consumption. This is caused by the wiper ring wiping excess oil toward the combustion chamber.
3. Oil Ring
The main function of the oil scraper ring is to guide the engine oil scraping along the piston’s outer surface back into the crankcase. Engine oil lubricates the piston as well as all the components within the crankcase.
Engine oil is essential for an internal combustion engine since it performs two functions – i) lubrication and ii) cooling of the components.
Without engine oil or the oil at low levels, the engine will not be able to perform and there can be harmful effects on the motorcycle components. That’s why the oil scraper ring which helps in guiding the oil along the piston as well as into the crankcase to lubricate and cool down the components is such an essential component in itself.
Now, how does this oil scraper ring help exactly?
Firstly, what the engine oil does is it moves up along the piston’s outer diameter as a result of piston’s up and down movements. The oil scraper rings barricade further upward movement of the oil and forces it to move downward.
Now, this is where things get interesting. The oil scraper ring although forces the engine oil to move downward, it does not necessarily mean along the piston surface. There are oil holes on the piston right below the oil scraper ring. These holes allow the engine oil which are forced to move downward to enter into the crank case through these holes.
Thus, the engine oil which was moving along the piston outer surface is forced back down into the crankcase by the oil scraper piston ring.
That’s why this ring is so vital for engine oil movement within the crankcase. Oil scraper ring helps the crankcase components from heating too much or getting worn out by facilitating smooth oil movements.
Ring Installation: How To Install Piston Rings
All piston ring sets contain information relative to the proper installation of the rings packaged in the set.
Three Piece Flex Vent
First, place the spacer in the grove. Then spiral the first rail into the groove below the spacer followed by the remaining rail into the groove above the spacer.
One- & Two-Piece Oil Rings
Using a proper ring installation tool, open the oil ring just enough to allow the ring to go over the top diameter of the piston. Work the opened ring down from the top of the piston to the oil groove. Place the ring squarely into the piston groove, taking care to keep it flat. Release the tension of the ring installer tool.
When using a two-piece oil, install the hump-style spring in the groove then follow the above instructions.
Although in many cases most compression rings appear similar to the mechanic installing them, there are, however, many subtle design changes that dictate how the ring is correctly installed.
Not only must the ring be installed with the proper side toward the top of the piston, but it is also imperative that the ring be installed in the proper groove.
Rings installed in the wrong groove or wrong side up can lead to excessive oil pumping, excessive blow-by, and in some cases completely dry up the bore, causing ring and cylinder scuffing as well as accelerated wear. Any of these problems, of course, constitute a failure as far as an engine overhaul or rebuild is concerned.
Lay the rings out in groups of tops ring and 2nd rings on the bench. Check for TOP marks (DOT or Lasor marks).
With the oil, control rings loaded on the piston, using a proper ring installation tool, open the second groove rings with the top mark up, just enough to allow the ring to go over the top diameter of the piston.
Work the opened ring down from the top of the piston to the second groove, place the ring squarely into the piston groove taking care to keep it flat, release the tension of the ring installer tool (the ring is now installed).
Repeat this same process with the Top groove rings. (Compression rings must never be spiraled on the piston.)
Once the piston rings are on the piston, “stagger” the end gaps. It is important that the ring end gaps are not lined up. This prevents oil to flow past the rings.
Rings installed in the wrong grooves or the wrong side up can lead to excessive oil consumption and blow-by and possible scuffing of the cylinders. These problems can cause engine failure.
How Do Piston Rings work?
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.
The topmost groove of the piston consists of a compression ring whose main function is to seal any kind of leakage inside the combustion chamber during the combustion process. When the air-fuel mixture is ignited. Pressure from the combustion gases is applied to the piston head, forcing the piston towards the crankshaft.
Pressurized gases pass through the gap between the cylinder wall and the piston and into the groove of the piston ring. During the combustion process, the force of high-pressure gases presses the piston ring against the cylinder liner wall which helps it to form an effective sealing. This pressure pushing the piston ring is proportional to the combustion gas pressure.
The next set of rings in the piston which are placed below the compression ring and above the oil rings are called wiper rings.
They have a tapered face construction and it is used to further seal the combustion chamber. As the name suggests, they assist in wiping the liner wall clean of any excess oil and impurities. If any of the combustion gases were able to pass by the compression ring. These gases will be blocked by the wiper ring in good condition.
The last set of rings is oil rings which are located at the bottom grooves of the piston closest to the crankcase. The main function of the oil ring is to scrape any excess oil from the walls of the cylinder liner when the piston is in motion.
The majority of the wiped oil is directed into the crankcase back to the oil sump. These oil rings come with a spring fitted at the back in a 4-stroke engine to provide an additional push for wiping the liner.