Piston ring

Piston Rings: Design, Functions & Construction


 | 5 minutes read

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. 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, the rings should be able to withstand high temperatures and pressures in the cylinder. They should also be strong enough 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 oil film 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. Thus, the ring can 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 flows from the rings to the walls.

Ring Gap:

The manufacturers carefully determine the amount of end gap. If the end-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 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 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, but they are costly.

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