As a tribologist, we use a tool called the Stribeck curve to engineer low friction piston rings.
The Stribeck curve utilizes the formula Viscosity X Speed / Load to generate the lubricant film parameters. Thinking back to our water-skiing analogy, the viscosity was the water. The speed was the velocity of the boat, and the load was the weight of the skier.
Following the logic of the Stribeck curve, as the piston nears top dead center on the compression stroke, the piston rings experience the highest load (from increased cylinder pressure) and lowest speed (as the piston comes to a stop). As a result, this is a source of high friction and higher wear.
As the piston begins to move down and accelerate (like the skier coming up from the water), both friction and wear decrease as the lubrication regime transitions from boundary to mixed film and then to hydrodynamic.
Near mid-stroke the piston reaches maximum velocity (in some engines, this can be in excess of 100 feet per second). Due to those high speeds, friction begins to increase again as the lubrication parameter moves to the far-right end of the Stribeck curve.
Taking all of this into consideration, Total Seal develops piston rings designed to overcome these frictional challenges.
First, utilizing PVD applied thin film coatings on the face of the ring lowers friction in the boundary lubrication regime. The low friction coatings not only reduce friction, but they also reduce wear in boundary lubrication conditions.
Next, smoother surface finishes increase load carrying capacity, which speeds the transition from boundary to mixed and then to full film lubrication.
Finally, thinner ring profiles provide less hydrodynamic drag.
As you can see, tribology and piston rings go hand in hand, and ultimately, that means Tribology effects engine efficiency. So the next time you pick up a piston ring, take a moment to consider what a remarkable tribological device it is.