Piston rings undergo lots of wear due to constant friction between the piston and the inner cylinder walls. As a result, piston ring material should have high wear resistance.
So what are piston rings made of? Piston rings are made of cast iron. This cast iron is usually alloyed with elements like silicon, manganese and molybdenum to improve wear resistance and elasticity.
Let us deep dive into cast iron properties and why they are used in piston rings.
What Material Are Piston Rings Made Of?
Piston rings are made of cast iron alloy.
Cast iron is a subset of iron-carbon alloys with the carbon content making up more than 2% in the alloy.
So why cast iron is used in piston rings? Why not other materials?
Cast iron is used in piston rings mainly because they provide strong wear resistance and offer excellent machinability – making them one of the most used engineering materials out there.
That’s why cast iron is the preferred material in most of the engine and automotive components. Any strong automotive component you take, chances are its most likely made of cast iron.
Desired Material Properties Of Piston Rings
When piston rings are manufactured, here are the main required properties of the material that piston rings will made of.
First, the piston ring material should have high wear resistance. This is because the piston rings will be placed to fill the gap between the piston outer diameter and the inner cylinder walls.
As piston moves up and down every combustion cycle (2 times to be precise), the piston rings will continuously brush against the cylinder inner walls throughout these movements.
Hence, its is absolutely necessary for the piston rings to have high wear resistance. If not, they will be damaged too soon for all the wear that piston and cylinder walls puts them through.
Second, the piston ring material should have good elastic properties. Now, this elastic characteristics is because you don’t want the piston ring to be too brittle and should be able to expand if subjected to high temperatures.
Third, the piston ring material should have strong heat resistance characteristics. This is again because the combustion chamber burns the fuel and generates immense amount of energy and heat.
As a result, the piston ring material should withstand high temepratures.
To summarize, the piston ring material should have strong wear resistance, good elastic properties and high resistance to heat.
Cast iron fits the bill mostly. Add to it they have good tensile properties and have excellent machinability.
We say ‘mostly’ because cast iron does not have great elasticity. They are too brittle. So, that’s why alloying elements like silicon, manganese and molybdenum are added.
Why Alloying Elements Are Used In Cast Iron
As discussed in the previous section, a good piston ring material should have strong wear resistance, good elastic properties and high resistance to heat.
While cast iron fits most of the bill in addition with high tensile strength and great machinability for ease of manufacture, they fall short on elasticity.
Cast iron is too brittle to satisfy the required elasticity requirements for piston rings.
Now, what do you do? Find another material that fits all the bill, or improve the characteristics of cast iron by adding alloying elements.
Cast iron alloy is a much easier option and economical as well.
That’s the reason why elements like silicon, manganese and molybdenum are alloyed with cast iron.
Adding these elements with the cast iron makes the alloy much more elastic as well as improves the wear resistance.
So, adding alloying elements to cast iron will:
- Adds elasticity since cast iron is too brittle to function as piston ring material
- Improves the wear resistance of the cast iron
Now, what are the elements added to the cast iron alloy?
Usually silicon, manganese and molybdenum are the elements alloyed to the cast iron. In addition, copper and tin are sometimes added as well depending on the manufacturer.
How Are Piston Rings Manufactured?
Piston rings are manufactured as a combination of casting and machining.
Typically, a cast iron cylinder block is either sand casted or cement casted. Die casting is usually not preferred since cast iron itself has such high tensile strength.
Once you get the casted cylindrical cast iron block, you bore the cylinder to get a tube with defined inner and outer diameters.
Then, you come along with a parting tool and machine the tube at required thickness to get the piston rings.
Lastly, each piston ring will have a clearance gap which can be created through various ways, with using a tapered cylinder and expand from within to put pressure and break the ring as one of the used method.
For more details on how piston rings are made, here is a video.
Purpose Of Piston Rings
The main purpose of the piston rings is to seal the gap between the piston outer surface and the cylinder inner walls. This ensures that the combustion chamber and the crank case remain segregated.
This segregation between the combustion chamber and the crank case helps in two ways.
First, the air fuel mixture will not flow down to the crank case. As a result the engine compression ratio remains intact, translating in higher power and acceleration.
Second, the lubricating oil in the crank case does not move up into the combustion chamber through the gaps between the piston and the cylinder walls. As a result, there is no loss of oil which will get burnt if moved to combustion chamber and no excessive consumption of engine oil.
For a more detailed overview check out our post on piston rings’ purpose.
Apart from the above mentioned roles, piston rings also help in heat transfer and engine oil lubrication.
What are the three types of piston rings?
The three piston rings placed on the piston outer diameter are:
- Compression ring
- Middle ring
- Oil scraper ring
Compression ring seals the combustion chamber from the crankcase whereas the oil ring helps in engine oil lubrication. The middle ring performs both functions and acts as a backup for the other two rings.
What are the signs of bad piston rings?
Here are the symptoms of a bad piston ring in an internal combustion engine.
- Excessive engine oil consumption
- White smoke from exhaust
- Low compression in the engine
- Decreased power and acceleration
- Piston slapping
Piston rings are made of cast iron alloy with elements like silicon, manganese and molybdenum included. Cast iron is used in piston rings mainly because they provide strong wear resistance and offer excellent machinability.
The piston ring material should have strong wear resistance, good elastic properties and high resistance to heat. While cast iron fits most of the bill in addition with high tensile strength and great machinability for ease of manufacture, they fall short on elasticity.
So, adding alloying elements to cast iron will add elasticity since cast iron is too brittle to function as piston ring material and improves the wear resistance of the cast iron.