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 Materials
Cast iron was used to make piston rings up until the 1970s. The ability of iron to break in easily after assembly or engine rebuild, to conform to cylinder irregularities, and to resist high heat made it an ideal material. Today, cast iron rings are occasionally used for the rebuild of gasoline-fueled engines but never for diesel engines. Ductile iron and steel are the most common ring materials used today TABLE 7-2. Special coatings are applied to the ring face to improve hardness or oil retention.

Turbocharged diesel engines with high temperatures and cylinder pressures require more durable ring materials. Malleable iron was often used as a ring base material because it will bend and not fracture like cast iron rings. Chrome plating was used on the ring face of malleable iron rings to improve wear and heat resistance. This material was first used during World War II to prevent the engines in desert tanks from dusting out as a result of the ingestion of desert sand through the air intake system. Dusting out occurs when dirt is drawn into an engine. This causes premature abrasive wear of the cylinder walls and rings. When cylinder walls and compression rings wear due to dusting out, the ends of the piston ring will become tapered Figure 32.

FIGURE 07-32: The ring face material on this compression ring wore due to dusting out.

Chrome ring face material has one major disadvantage: Its smooth face will not retain oil for lubrication. The consequence of this is higher cylinder wall wear in the ring turnaround area (the area traveled only by the compression ring) because upper cylinder wall lubricant is non-existent above the intermediate ring Figure 33. During the 1970s, ring manufacturers found that a coating of molybdenum (a common, porous, and resistant metal alloy) on the compression ring face improves wear characteristics Figure 34. Wear resistance is improved because moly-faced rings have a porous finish. This porosity permits oil retention on the ring face, which allows oil to be carried to the upper cylinder wall, thus reducing upper cylinder wall wear. After this discovery, a combination of chrome and molybdenum was used for compression rings for decades.

FIGURE 07-33: Most cylinder wear takes place at the top and tapers toward the bottom. A lack of lubrication oil above the oil control and intermediate ring is the primary reason for most wear taking place in the ring turnaround area.

FIGURE 07-34: This illustration of a molybdenum-faced (or “moly”-faced) ring has a dull, porous finish that is used to retain oil to aid upper cylinder wall lubrication. Note that chrome rings are shiny.

A much better material for compression rings than cast iron and ductile iron is steel, which is twice as strong as ductile iron. Because alloyed steel is the best material for withstanding the pressure and temperature loads found inside high-compression turbocharged diesels, this material has been used for over 30 years in many HD diesel applications. Generally, steel compression rings have the following advantages:

• Improved breakage resistance
• Improved heat resistance
• Improved mechanical stress resistance
• Reduced ring-side wear
• Reduced groove-side wear
• Longer service life
• Simpler and less expensive to manufacture (made from coiled wire)

With the introduction of EGR, ring face materials need to be even harder, so combinations of ceramic metal alloys are now being used. Further hardening of ring material is accomplished by nitriding the rings. Nitriding is a hardening process that binds nitrogen to the surface of the steel rings. The hardening extends into the ring to a depth of about 0.001" (0.025 millimeter [mm]). This hardening technique produces rings that have almost 50% greater wear resistance than non-hardened steel rings and 400% greater wear resistance than gray cast iron rings. Nitriding improves the ring resistance to side wear and face wear to the point where ring wear is negligible. Cylinder walls will wear out before these rings do.

Other diesel engine ring face materials use a combination of plasma (electric arc), sprayed moly, and ceramics, such as chromium carbide or silicon carbide. Ceramics are extremely hard and wear resistant but do not conduct heat well. Molybdenum cermet is one such material. It is comprised of 80% molybdenum and 20% chromium carbide ceramic, which makes for durable ring face material in a turbocharged diesel engine with high levels of recirculated exhaust gas.