And Rings or Fries
Let’s now get into some of the functional considerations in the understanding of both pistons and rings.
There are five basic design features of a piston. It should (1) reduce operating friction, (2) transfer maximum heat to the cylinder walls, (3) be of low net weight, (4) provide necessary piston pin support (to cylinder pressure loading), and (5) prevent oil passage into the combustion chamber (inasmuch as piston design can accomplish this).
The consideration of how well a piston lubricates a cylinder wall should include the fact that its oil is partially “carbonized.” This is the result of oil clinging to the cylinder wall above the piston during combustion, resulting in exposure to the burning air/fuel mixtures. This condition also affects piston ring design and material selection. As mentioned in a previous Shop Series, there is a relationship between combustion surface
and combustion volume. And the greater the combustion surface-to-volume, the lower the temperature of oil above a piston during combustion.
All this nice theory may seem of little value right about here, but it relates directly to how combustion-efficient an engine actually is and how long it can run before pistons and rings require replacement. And that relates to something called dollars.
Basic piston design can be categorized as follows: closed type, open type, and closed slotless type (which form the basis of many high-performance piston designs). The closed-type design incorporates a machine-turned (or sawed) slot located in the oil ring groove, separating the skirt area from the area in which all ring grooves are cut (the so-called “ring belt”). Such a cut allows good heat transfer from the ring belt to the piston skirt and aids operational bore-to-skirt clearance.
The open-type design of piston incorporates a slot located just below the oil ring. This gap separates the ring belt from the piston’s skirt. It is placed and designed to reduce piston skirt distortion and help control piston-to-bore clearance. It also helps free the oil ring from distortion caused by piston skirt loading. All this aids oil control and improves the retention of cylinder pressure, which is both power and fuel economy.
The closed slotless design relates to the high-performance type of piston. Here there is no separation slot between the ring belt (piston head) and skirt. This method provides very good resistance to high cylinder pressure and works well with high horsepower and rpm levels.
C. Additional compression ring pressure against the cylinder wall can be accomplished by the drilling of small (usually 0.025-inch or thereabouts) holes down through the tops of pistons. Such holes are located to intersect top compression rings in the “back clearance” area (behind the ring) so that ring-to-wall pressure is increased. This improves cylinder pressure seal and aids net engine output. Although usually associated with race-type engines, this particular piston modification can also improve an engine’s fuel economy by virtue of increased cylinder pressure and more net work on pistons. D. Cylinder pressure paths typically follow down onto the top compression ring (forcing ring movement against the cylinder wall) and then onto the second compression ring (for similar pressure movement). It is this type of pressure/ring relationship that leads to low-tension rings (pressure against the cylinder wall) that maintain good pressure seal during an engine’s power stroke. And in most cases, reduced ring pressure (other than during the power stroke) against the cylinder walls increases net power output. E. These are typical compression and oil scraper ring designs. Varations on these types have been developed, but the basic concept of each remains unchanged. And while we are not able to show all variations of piston rings currently in use, these represent the basic types.