Piston ring

Piston Rings: Design, Functions & Construction


What is the function of Piston Rings?

The piston rings are a vital part of the internal combustion engines. They move up and down alongwith pistons. The purpose of the piston rings is to provide a seal between the piston and cylinder wall. 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, they should be able to withstand high temperatures and pressures in the cylinder. They should also be strong to 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 film of oil between the surfaces.
  5. Prevent the lubricating oil from entering the combustion chamber where it will burn eventually; leaving the carbon deposits.

Piston Rings Construction:

Generally, the piston ring is cast individually. It is machined carefully so that it exerts uniform pressure against cylinder walls when in its position. A gap needs to be cut at the ends so that you can expand, slip over the piston head and 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. Due to this, the ring is able to exert pressure on cylinder walls which is a must for better sealing. Besides, the end gap also ensures any circumference-wise expansion of the ring at higher operating temperatures. The piston rings always operate at temperatures higher than the cylinder walls. Hence, the heat simply flows from the rings to the walls.

Ring Gap:

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

Types of ring end gaps
Types of ring end gaps

There are three types of piston ring ends 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 but they are costly.

Piston Ring Materials:

Generally, the material used for piston rings is fine-grained alloy cast iron that contains silicon and manganese. It has good heat and wearing-resistance qualities. It also has a good hardness. Manufacturers use chromium plating for top rings because they are exposed to the highest working temperature and corrosion due to combustion. Chromium plating also helps the rings to resist scuffing because it is very difficult to be welded to the cast-iron cylinder. However, chromium-plated rings should not be used if the cylinder bore itself is plated with chromium or any such hard material.

Generally, manufacturers provide piston rings with a porous phosphate coating. This 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. The molybdenum surface has a larger oil carrying capacity. Hence, it provides better cylinder wall lubrication which results in longer engine life. Molybdenum also withstands higher temperatures (2620⁰C) and thus, 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 its 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

The number of rings:

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

A minimum of two compression rings is necessary 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. It decreases wear and increases engine life.

Mahle, Diamond, CP Carrillo, Ross, and Arias are some of the manufacturers of pistons in the world.

Watch here how to measure piston ring gap:

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