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Piston Ring: Function, Design & Types (Explained)

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  • 5 min read

Last Updated on: 7th September 2023, 12:36 am

Piston rings have three main functions.

  1. Creating an airtight seal in the combustion chamber, reducing loss of gases and pressure to the crank case.
  2. Facilitating better heat transfer from the piston to the cylinder liner.
  3. Regulating the amount of oil between the piston and cylinder wall and scraping it back down to the oil sump.

In this article, we’ll go over what the piston ring is, how they work, their design, history and more.

diagram showing the piston rings

Piston Ring Functions

Piston rings are circular metal rings that encircle the piston inside an engine.

These rings are situated in grooves on the outer surface of the piston, sealing the gap between the piston and the cylinder liner.

They have the following roles in an engine.

1. Creating an Airtight Seal

One of the major roles of piston rings is to create an airtight seal between the piston and the cylinder walls.

This prevents the escape of gases and pressure from the combustion chamber into the crankcase.

diagram of how the piston ring maintains an airtight seal

2. Oil Regulation

Piston rings also help manage the distribution of lubricating oil along the cylinder walls.

They prevent excessive oil from reaching the combustion chamber, which could lead to undesirable outcomes such as fouled spark plugs and increased emissions.

They also scrape oil from the cylinder walls and return it to the oil sump.

diagram showing how a piston ring regulates oil

3. Heat Transfer

Piston rings help in the heat transfer from the piston to the cylinder walls. This controlled heat transfer prevents the piston from overheating and maintains its structural integrity.

diagram showing how a piston ring helps heat transfer

Importance of Piston Rings

The importance of the piston ring lies in their contribution to engine performance, longevity, and overall efficiency.

Without them, engines would experience poor compression, increased oil consumption, reduced power output, increased emissions, and a higher likelihood of overheating and damaged or cracked pistons.

Piston rings are important components of an engine that allow engines to operate smoothly, efficiently, and with minimal emissions.

Design

Piston rings are designed with extreme precision in order be effective.

Typically, an engine employs multiple rings per piston, each one serving a specific purpose. The common types include compression rings and oil control rings.

  1. Compression Rings: These form a tight seal between the piston and the cylinder walls. They withstand the high pressures and temperatures generated during combustion, ensuring efficient power generation.
  2. Oil Control Rings: These rings regulate the distribution of lubricating oil along the cylinder walls. They prevent excessive oil from entering the combustion chamber, improving efficiency and reducing emissions. They also scrape oil from the liners and return it back to the oil sump.

The number of rings used depends on the engine design and its intended purpose. Modern engines often use multiple to achieve optimal performance.

piston diagram

Ring Construction

The rings are precisely machined to fit the grooves on the piston and cylinder walls, ensuring a secure and effective seal.

  • Compression rings typically have a rectangular or keystone shaped cross-section.
  • Upper compression rings typically have a barrel profile, while the lower compression ring usually has a taper napier facing.
  • Some engines also have a taper facing for the top ring.

Oil control rings are commonly made using either a single cast iron piece, several steel pieces, or a combination of steel and iron with a helical spring to create the necessary tension for a secure seal.

When it comes to cast iron oil rings and those accompanied by a helical spring backing, they feature two distinct scraping lands.

In contrast, oil control rings from multiple sections of steel typically comprise a pair of steel rings, often referred to as “rails”.

These rails are separated by a spacer-expander spring, which serves to maintain distance between the two rails and exert radial pressure simultaneously.

The gap in the piston ring compresses to a few thousandths of an inch when inside the cylinder bore.

Ring gap shapes include:

  • square cut
  • angle cut
  • tite joint
  • step cut
  • hook step
  • mitre step

Wear and Tear

Piston rings operate in a harsh environment of extreme temperatures, pressures, and friction. As a result, they experience wear and tear over time.

Proper engine maintenance, including regular oil changes and tune-ups, is essential to prolong their lifespan.

Worn or damaged rings can lead to decreased engine performance, increased oil consumption, and emissions issues.

  • Andy Lewin

    Andy Lewin is a senior mechanic, ASE qualified master technician, and an experienced automotive engineer. He's passionate about serving the automotive community with the highest-quality and trustworthy information on all things automotive. He loves to write about car repairs, maintenance, car modifications and tuning, faults, and much more.

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