LO 7-5 Identify and explain the purpose and functions of piston rings, their construction features, and their ring shapes according to ring function

Compression Ring Shapes
Rectangular rings were once the standard piston ring shape, but they had a problem with sticking when oil and fuel residues carbonized in the ring groove. The keystone-shaped ring is the ring of choice for most engine manufacturers today Figure 35. Keystone ring action helps keep the ring land clean and prevents the ring from sticking. To obtain good sealing and minimize friction, a barrel face profile is often ground on the face of compression rings. The barrel face forms line contact with the cylinder wall for tight and relatively friction-free contact. Prior to packaging compression rings, the manufacturer laps rings in a cylinder identical in diameter to the one in which the rings will run. A witness line left by the lapping process shows the contact with the cylinder. This same line will grow wider as the ring wears in the cylinder. The width of the witness line can be used as a visual indication to determine the amount of ring wear Figure 36.

FIGURE 07-35: The keystone ring shape is the most popular choice for manufacturers today.

FIGURE 07-36: Ring contact with the cylinder wall is observed by the compression ring witness line and the shiny lower edge of the intermediate ring.

Intermediate Rings
Intermediate rings may also be called compression rings , even though they perform more than just a compression and sealing function. While intermediate rings seal compression gases that escape past the top rings, they also assist in oil control. A small, metered quantity of oil is required on the cylinder walls for the piston and rings; any excess quantity will lead to high oil consumption and emissions. The ring shape of choice for the intermediate ring is a taper-faced ring. This shape allows the ring to glide over the oil film on the upstroke, enhancing the sealing effect of oil between the ring and the cylinder wall. On the downstroke, the ring will scrape oil from the cylinder back into the crankcase. A variation of this design is to allow the ring to twist in the groove. Using a ring with a groove machined in the bottom or top inside edge of the ring permits gas pressure to cause the ring to twist during the compression and power strokes. Either compression sealing or oil control can be enhanced, depending on the direction of twist. An intermediate ring, if installed upside down, will pump oil into the combustion chamber rather than scraping it from the cylinder wall.