If It Takes A Piston Ring, We Can Build It

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TECH ARTICLE

What is Tribology?

And what does it have to do with piston rings?

Tribology, this strange word is the study of friction, wear and lubrication. You also might be wondering what Tribology has to do with piston rings? Beside the fact that piston rings operate on a thin film of oil, there’s quite a bit of tribology related to piston rings. Let me explain.

The highest source of friction in an engine comes from the contact between the cylinder wall and the piston ring. Research from both Ford Motor Company and the US Department of Energy concluded that roughly 45% of all engine friction comes from the piston ring to cylinder wall interface.

Because a piston has to change direction with each stroke of the engine, the piston rings accelerate and decelerate 250 times each second at 7,500 RPM. The constant changes in speed and load (due to the changes in cylinder pressure with each of the 4 engine cycles) create a unique and interesting phenomenon - a piston ring experiences all three forms of lubrication during each engine cycle.

The three forms of lubrication are Hydrodynamic, Mixed Film and Boundary. Let me explain each in more detail.

If you have ever been water skiing, you know that the skier is down in the water when the boat is not moving. As the boat beings to accelerate, the skier comes up partly out of the water. As the boat comes up to speed, the skier comes fully out of the water.

The change in speed effects the relationship between the skier and the water. The same is true for engine lubrication.

When the boat is stopped, there is no oil film, which is boundary lubrication. Flat tappet camshafts and pushrod tips experience this form of lubrication. There is no oil film to protect the parts, so the additives in the oil provide the protection.

When the boat is accelerating, that is like mixed film lubrication. Because the wheel on a roller follower can trap a small amount of oil between the wheel and the cam lobe, roller camshafts operate in the mixed film condition.

When the boat is at full speed, that is like the hydrodynamic state or full-film lubrication. Engine bearings operate in hydrodynamic lubrication. The oil film separates the bearing from the journal of the crankshaft.

Again, you are probably wondering what this has to do with piston rings!

Well, as the piston rings cycle back and forth in the cylinder, each cycle takes the piston ring through all three stages of lubrication.

This cycling produces those high levels of friction that rob horsepower from your engine. Fortunately, understanding how this occurs unlocks the secrets to reducing that friction, which, in-turn, unlocks hidden horsepower.

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, thisis 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 of100 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.