Piston rings are an important part of the engine yet they tend to be neglected because they are a consumable item. Often they can be found in the corner of a storeroom covered in dirt and rust, and in severe cases they have been painted.

Piston rings should be kept in a dry place and stored flat. A light coating of protective lubricant will prevent them from rusting. If they are covered, make sure they are checked regularly to ensure they are in satisfactory condition.

The functions of piston rings are:

1. Provide a seal to the combustion chamber to prevent gases and combustion products passing the piston.
2. Control the lubricating oil.
3. Conduct heat away from the piston to the liner.

Types of Piston Rings

1. Compression ring – Provides a gas seal.
2. Scraper or Oil Control ring – Distributes oil on the cylinder liner preventing the oil passing upwards into the combustion chamber. These rings are normally found on trunk piston engines.

The piston ring sits in a machined groove, located such that the ring operates at an acceptable temperature. If the rings where fitted too high, the high temperatures would rapidly burn off the oil and the rings would seize in their grooves. The piston ring must be free to move in its groove, therefore, a clearance is required. Ring clearances are shown in the figure below.

Effect of Clearances

Groove Clearance

• Allows pressure to build up behind the ring
• Allows oil to flow into the groove
• Allows pressure to build up behind the ring

This may also be termed gap clearance and is required to accommodate the ring expansion as it heats up.

Too small Groove Clearance

• Ring will stick in the groove. This will result in poor sealing and possible blow by which will burn away the oil and cause scuffing. Insufficient gas pressure behind the ring will affect sealing.

Too large Groove Clearance

• Ring flutter and possible breakage

Too small Butt Clearance

• As the ring expands the butt will come together. This will exert a large radial pressure on the liner, breaking down the oil film and increasing scuffing wear. Ring seizure may occur.

Too large Butt Clearance

• Excessive gas leakage

The piston rings operate in a hostile environment. The load is fluctuating and at top dead center the rings are at their slowest speed and highest temperature. The rings must withstand corrosive combustion products. Piston rings must therefore have high tensile strength to resist breakage, combined with good anti-corrosive properties. Rings must also maintain tension at lower combustion pressures and be compatible with the liner material.

Reference
“Operation and Maintenance of Machinery in Motor Ships” by N.E. Chell