Piston Ring Science


THE EXPANDING FLAME and pressure of combustion needs to be captured in the engine’s cylinders with an absolute minimum of leakage to create the most power. This makes the job of the piston rings extremely difficult. At just 6,000 rpm, the piston rings will be racing up and down the cylinders at a mind-bending 100 times per second. The rings need to accomplish several distinct jobs in mere microseconds. Not only must the rings seal against high pressure during the combustion stroke, the strength of the piston’s vacuum-pull during the intake stroke is also dependent on a tight ring seal. In addition, they mustn’t create any more friction than absolutely necessary.

They must also prevent the critical oil film bathing the walls of the cylinders from entering the combustion chambers, where it would cause destructive detonation. Finally, the rings need to provide an effective heat path from the hot piston to the cylinder walls, so the engine’s cooling system can absorb the excess.

Ring Types

Modern pistons are commonly fitted with three ring grooves that support five separate parts. The top ring is traditionally called the compression ring, and as the name implies, its primary function is to seal against combustion pressure and prevent its escape into the crankcase— called “blow-by.” With peak combustion pressures in the range of 700 to 1,400 psi, even the tiniest leak path will be exploited, which reduces working pressure on the pistons and pollutes the crankcase oil with combustion byproducts. The ring end gap, a feature of all but the most expensive “gapless” racing rings, is potentially a major leak path. Four-inch diameter automotive pistons typically use a ring end gap of about 0.024 to 0.030 inch, which is considered “safe” (unlikely to be too tight), but not as tight as it could be.

Pro engine builders therefore specify rings slightly larger than stock, so the gaps can be filed to fit individual cylinders, typically with about half the OEM clearance (about 0.016 to 0.018 inch). This tiny detail can make a huge difference in power; as much as 10 percent or more, according to the Sealed Power Corporation. And, as engines accumulate miles, wear causes their end gaps to enlarge continuously. Because the top ring is subjected to a rocking motion as the piston reverses direction at TDC, the top ring will usually have what’s called a “barrel face,” or a slightly curved outer edge that doesn’t lose its seal when misaligned slightly.

The second ring, which is often called the second compression ring, does two jobs. One is to back up the top ring’s sealing job, and the other, perhaps even more important, is to aid the oil ring’s function. To assist with oil control, the second ring will usually have a slight bevel on its face, forming a chisel shape with its sharp end facing the crankshaft, although a number of different detail designs have evolved over the years.

The oil ring, or oil scraper, is a three-piece unit. It fits in a wider groove that contains a series of drain-back holes that pass through the side of the piston, so that oil scraped off the cylinder walls can easily return to the crankcase. Between two thin steel rings is positioned an expander, a wavy spring steel separator that keeps the upper and lower oil rings apart, to provide a generous path for excess oil. Ring Sealing You might imagine that piston rings seal simply by creating a strong radial pressure against the cylinder walls, but this isn’t really the case.

If you closely examine the fit of a first or second compression ring in its groove, you’ll notice that the ring has a slight vertical clearance and that, even when pressed flush with the side of the piston, it still has some significant space behind it. In a running engine, as the piston rises, these clearances provide pathways for pressure above the piston to get behind the rings, forcing them against the cylinder walls.

Because this effect is so important to the rings’ function, anything that might prevent a perfectly flush fit between the rings’ bottom sides and the ring grooves’ lower surfaces will seriously compromise sealing: carbon buildup, scratches, or less than perfectly flat rings or less than perfectly cut ring grooves. In fact, when checking a piston ring’s vertical clearance, builders are warned against pushing feeler gauges between the ring and its bottom groove, to prevent scratching the critical mating surface. If this precaution is necessary, imagine how the shade tree practice of scraping carbon buildup from a ring groove with a bit of broken piston ring filed into a chisel shape could mangle the ring groove’s surface!

Ring Materials

Cast iron has been the favored ring material for decades, as it’s relatively inexpensive and its natural porosity allows it to hold oil, giving it good lubricating properties—the same reason it was also favored for cylinders and cylinder liners for so long. Unfortunately, cast iron cylinders don’t expand as quickly with heat as an aluminum piston, potentially causing “cold seizures” if the engine is warmed-up too quickly (the piston expands too fast for the bore). Also, plain cast iron is brittle and it can lose its tensile strength if badly overheated, “collapsing” the ring. A step up from simple cast iron is nodular iron, which has better flexibility or “ductility” as well as greater resistance to high temperatures.

Due to its higher cost, approximately 1.5 to 2 times as expensive as plain cast iron, nodular iron is sometimes used just for the top ring. Because both forms of cast iron rings require considerable labor to cast and finish machine, steel rings are gaining in popularity, as they can be made fairly simply from coils of steel wire with little waste. The wire can be made from carbon steel, stainless steel or tool steel. Also, as rings get ever thinner to reduce sliding friction to the lowest possible degree, steel gains in attractiveness, as it offers perhaps 20 percent greater tensile strength and fatigue resistance than ductile iron.

Ring friction is a major issue, and can constitute from one-quarter to one-third of total engine friction. Automotive top rings once averaged 2.0mm thick (0.078-inch), but have gradually halved to a motorcycle-sized 1.0mm (0.039-inch). F1 and other topflight race teams are said to use rings just 0.5mm thick (0.0197-inch). An important consideration is that lighter rings also reduce the tendency for “ring flutter” at high rpm, where the rings bounce in their grooves, which kills the combustion- pressure, ring-sealing effect. As rings get thinner, the per-unit loading on the ring face will increase, so thinner rings can use a fraction of the mechanical tension thicker rings did in the 1970s. However, to take full advantage of thinner, low-tension ring packages, the engine’s cylinders must stay very accurately round in use, or excessive blow-by will result. The performance advantage from reduced ring friction is not as great as you might imagine, perhaps only 1 to 2 percent.

Ring Coatings

Electroplated chrome was an early favorite, commonly applied only to the sealing face of cast iron top rings. Chrome is hard but it didn’t provide much lubrication-holding ability and could cause scuffing. Molybdenum is currently the ring coating of choice. Moly is inherently slippery and it has a structure that holds oil well. It can be applied by several methods, including particle vapor deposition (PDV), which attaches strongly to steel. Alternatives include Titanium Nitride (TiN), Diamond Like Carbon (DLC), chromium nitride and ceramic.

Care and Handling When installing piston rings, a ring’s upper side will always be engraved with a dot or other mark to indicate the correct direction, except perhaps in the case of steel oil scraper rings that can be inverted without problems. When fitting cast iron rings, carefully expand them radially with your thumbnails, just enough to slip over the piston.

Never try to spiral them onto the piston, by inserting one end into the groove and twisting the ring into place, as the ring can be permanently deformed, never to seal properly. Also, when installing rings, you’ll be advised to keep their gaps from aligning, and service manuals will typically have a diagram to show the ideal gap relationships. But, don’t think they will stay in that alignment. The crosshatching created by cylinder honing will typically cause the rings to rotate in a running engine, at maybe 1 rpm or less, depending on engine speed. MCN


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