Piston Rings: Design, Functions & Construction

Piston ring

What is the function of Piston Rings?

The piston rings are a vital part of internal combustion engines. They move up and down alongwith pistons. Their purpose is to provide a seal between the piston and cylinder wall. Therefore, an engine’s efficiency and economy primarily depend upon the smooth working of the piston and rings.

Both of them should operate in the cylinder with minimum friction. Besides, the rings should be able to withstand high temperatures and pressures in the cylinder. They should also be strong enough. Again, they should withstand the pressure difference in the combustion chamber and crankcase.

The Functions of the Piston are as follows.

They:

  1. Form a seal and prevent the combustion gases from leaking into the crankcase.
  2. Provide an easy path for the heat to flow from the piston crown to the cylinder walls.
  3. Maintain sufficient lubricating oil on cylinder walls throughout the entire length of the piston travel.
  4. Minimize the wear & tear of the cylinder walls by maintaining a constant oil film between the surfaces.
  5. Prevent the lubricating oil from entering the combustion chamber. Here, it will burn eventually, leaving the carbon deposits.

Piston Rings Construction:

Generally, manufacturers cast the piston ring individually. It is machined carefully to exert uniform pressure against cylinder walls when in its position. First, a gap needs to be cut at the ends to expand and slip over the piston head. Then, release it while inserting it into the ring groove.

Piston ring construction
Piston ring construction

Furthermore, the gap almost closes when the piston is inside the cylinder. Thus, the ring could exert pressure on cylinder walls. Therefore, it is a must for better sealing. Besides, the end gap also ensures any circumference-wise expansion of the ring at higher operating temperatures. As a result, the piston rings always operate at temperatures higher than the cylinder walls. Thus, the heat flows from the rings to the walls.

Ring Gap:

The manufacturers carefully determine the amount of end gap. If the gap is excessive, it results in blow-by (leakage of gases into the crankcase). It may also cause scuffing of the rings. In contrast, a lesser gap causes the piston rings to ‘butt’ at higher temperatures. It results in excessive and non-uniform pressure on the cylinder walls. Besides, it causes excessive wear. Ideally, the piston ring gap is around 0.30mm to 0.35mm when installed.

Types of ring end gaps
Types of ring end gaps

There are three types of piston ring end gaps. They are Straight, Tapered, and Seal type. Out of these, the straight type is the most common type due to its simplicity and cost-effectiveness. However, the Tapered and Seal types are more effective in preventing leakage. However, they are costly.

Piston Ring Materials:

Generally, the material used for piston rings is fine-grained alloy cast iron. It contains silicon and manganese. It has good heat and wearing-resistance qualities. Besides, it also has a good hardness. Manufacturers use chromium plating for top rings. It is because they expose themselves to combustion’s highest working temperature and corrosion. Chromium plating also helps the rings to resist scuffing. Also, it is because it’s difficult to weld or ‘butt’ them to the cast-iron cylinder. However, one should not use chromium-plated rings if the cylinder bore itself has plating with chromium or any such hard material.

Generally, manufacturers provide piston rings with a porous phosphate coating. It reduces the scoring of the surfaces during the running-in period. The porous surface has cavities for the worn particles. It also acts as an oil reservoir, which remains even after the coating wears out. Thus, it reduces the friction between the rings and cylinder walls.

Manufacturers also use molybdenum-filled rings in modern engines. Again, it’s because the molybdenum surface has a larger oil carrying capacity. Thus, it provides a better cylinder wall lubrication, resulting in longer engine life. Molybdenum also withstands higher temperatures (2620⁰C). Therefore, it resists scuffing.

Furthermore, manufacturers also use thermo-chemically treated chromium for the top ring application where minimal lubrication is available. Besides, manufacturers also use alloy steel and stainless steel for their anti-pitting and anti-corrosion qualities. It also resists carbon clogging and excessive tension loss at higher engine temperatures.

Type of Piston Rings:

There are mainly two types of piston rings.

  1. Compression rings
  2. Oil control rings

Number of Rings:

Furthermore, the number of rings used on a piston varies depending upon the requirements. For example, earlier engines used two to four compression rings and one to two oil control rings. However, manufacturers restrict the number of rings to three for modern engines with lower heights. Out of these, two are compression rings, and one is the oil control ring.

A minimum of two compression rings is necessary. It is due to the high-pressure difference between the combustion chamber and the crankcase. This pressure difference could be as high as 70 atmospheres. A single ring cannot take such high pressure. Hence, manufacturers use two compression rings instead of one. Increasing the number of rings also reduces the pressure between the rings and the cylinder walls. Besides, it also decreases wear and increases engine life.

Mahle, Diamond, CP Carrillo, Ross, and Arias are some of the world’s manufacturers of pistons & piston rings.

Watch How to Measure Piston Ring Gap here:

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