Piston rings play a vital role in an engine to run smoothly and generate maximum power. Without piston rings, the engine can easily get seized and stop performing altogether.
So, what are piston rings? Piston rings are cast iron ring strips attached to the piston grooves along its outer walls. The main purpose of piston rings is to close the gap between the cylinder walls and the piston and ensure that the fuel mixture does not enter the crankcase.
There are lots of functions attached to each of the piston rings. Let’s dive into the piston ring types, the role of piston rings, and what an engine blowby means to understand piston rings further.
What Exactly Are Piston Rings?
Piston rings are cast iron ring strips attached to the piston grooves along its outer walls.
The piston has grooves for piston rings and usually, there are three grooves to accommodate the three piston rings along its outer diameter.
And what are piston rings made of? Piston rings are usually made of cast iron.
Cast iron is preferred in piston rings mainly because of its wear resistance. Along with that, cast iron also provides good machinability, high tensile strength and not to mention, and low cost of manufacturing.
The main purpose of piston rings is to close the gap between the cylinder walls and the piston and ensure that the fuel mixture does not enter the crankcase.
Now, why don’t we have the pistons directly closing the gap with cylinder walls? Pistons cannot be directly closing the gap with cylinder walls because – in case of thermal expansion, the piston will seize the cylinder – resulting in engine damage.
What Is The Role Of Piston Rings In An Engine?
There are three main roles that piston rings play in an engine.
First, piston rings seal the combustion chamber between the cylinder walls to avoid any leakage of the air-fuel mixture into the crankcase.
They keep whatever is in the combustion chamber out of the crankcase and whatever is in the crankcase – out of the combustion chamber.
Second, piston rings facilitate heat transfer between the piston and the cylinder. Piston is connected to piston rings and piston rings are connected to cylinder walls.
As a result, the heated piston due to the heat generated in the combustion chamber will transfer its heat to piston rings, which in turn, transfers it to the cylinder walls.
Third, piston rings help in oil flow across piston walls to keep the piston lubricated.
The piston has oil holes right below the piston rings. The oil in the crankcase moves up and the piston rings block them to go into the combustion chamber and force them to move through the oil holes.
As a result, the oil moves through the oil holes in the piston and lubricates both the outer and inner walls of the piston.
This helps in a smooth translating motion of the piston along the cylinder walls, thereby transferring the energy produced in the combustion chamber to the crankcase smoothly.
The Three Types Of Piston Rings
As you might have observed, there are three piston rings attached to the piston on its outer diameter, namely:
- Compression Ring
- Middle Ring
- Oil Scraper Ring
The compression ring is the ring that is helping to seal the combustion chamber so that the piston gets good compression.
It acts as the primary seal between the piston and the cylinder walls so that the air-fuel mixture does not go into the crankcase and conversely, the engine oil in the crankcase does not move up to the combustion chamber.
The oil scraper ring, the third ring, ensures lubrication along the piston by forcing the oil to move through the oil holes present in the piston, right below the oil scraper ring.
This ring does not allow the oil to move further and guides it down so that oil can enter the oil holes and lubricate the piston. That’s why the name, oil scraper ring.
The second ring, also called the middle ring, performs both the functions of the other two rings. It acts as the secondary seal between the piston and the cylinder walls and also as the secondary ring to scrape oil to the piston.
Engine Blowby
When the air-fuel mixture in the engine trickles down into the crankcase along the gap between the piston and cylinder walls, it is known as Engine Blowby.
Despite piston rings trying their best to seal the gap between the piston and the cylinder walls, the gaps come up over time.
This happens when the piston is moving up on the exhaust stroke, the piston rings are pressed down allowing the air-fuel mixture to travel down and accumulate in that piston ring gap.
Now, when the piston moves down on the intake stroke, the piston rings get pressed up, opening the gap towards the cranks case. This allows the air-fuel mixture to move down further, to the crankcase – causing blowby.
So, what are the effects of engine blowby? Engine blowby results in low compression on the piston, which in turn results in low power and low engine efficiency. In addition, some of the engine oil will also get burnt.
In short, engine blowby results in less engine power and more oil burning.
Signs of Bad Piston Rings
Bad piston rings essentially refer to piston rings not performing their jobs.
That is, not sealing the combustion chamber and not scraping oil for piston lubrication, and not facilitating heat transfer.
So, signs of bad or worn-out piston rings would include:
- White smoke from exhaust
- Less engine power
- Engine oil depletes faster
- Engine misfire and knocking
Since engine oil moves up the combustion chamber if piston rings are worn out, oil gets burnt resulting in white exhaust smoke and well as low oil levels in the motorcycle.
Next, if the piston rings are not sealing the gap, the fuel mixture will perforate down resulting in low compression. As discussed earlier, low compression leads to low engine power and misfire as well as low engine efficiency.
To Summarize
- Piston rings are cast iron ring strips attached to the piston grooves along its outer walls.
- Piston rings are usually made of cast iron.
- The main purpose of piston rings is to close the gap between the cylinder walls and the piston.
- Piston rings also facilitate heat transfer between the piston and the cylinder.
- Another function of piston rings is to help oil flow across piston walls to keep the piston lubricated.
- The three types of piston rings are – Compression Ring, Middle Ring, and Oil Scraper Ring.
- When the air-fuel mixture in the engine trickles down into the crankcase along the gap between the piston and cylinder walls, it is known as Engine Blowby.
- Engine blowby results in low compression on the piston, which in turn results in low power and low engine efficiency. In addition, some of the engine oil will also get burnt.
