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

1. Piston Rings

Piston Rings

Piston rings are among the most critical components for engine durability and efficient operation. Worn rings cause high oil consumption and a loss of compression that results in low power and excessive blowby Figure 27. Rings, along with cylinder walls, will wear faster than other internal engine parts, which limit engine life. In fact, one of the primary determinants of engine life is the ability of the piston rings to work efficiently. Long ring life is a function of ring design, maintenance practices, and proper engine operation.

Piston Ring Functions Piston rings must perform the following jobs under extreme temperatures for the life of the engine: Form a gas-tight seal between the piston and the cylinder wall: Gas-tight sealing of the cylinder is necessary to provide adequate compression pressure. It also minimizes blowby, which reduces power and leads to oil contamination and excessive crankcase emissions. An oil film on the cylinder walls enhances the sealing between the ring and the cylinder wall. Assist in cooling the piston: Piston rings transfer heat to the cylinder wall, which helps keep pistons cool. Apply a film of lubrication oil on the cylinder wall: Piston rings must apply a film of oil that provides adequate lubrication properties. However, the film of oil cannot be excessive and thus cause high oil consumption and increased emissions. Form a compatible, replaceable wear surface with the cylinder wall: Piston rings must be compatible with the cylinder wall. Compatibility is a property that allows two metals to slide against one another with minimal friction or wear.

FIGURE 07-27: Piston rings provide a gas-tight seal in the cylinder to minimize loss of compression and blowby.

1.1. Video: Piston Rings

1.2. Exhaust Gas Recirculation and Piston Ring Longevity

Exhaust Gas Recirculation and Piston Ring Longevity Refinements to ring design, cylinder bore finishing techniques, and engine oils have helped produce engines that can operate for thousands of hours and more than 1,000,000 miles (1,609,344 km) of use before the piston rings may need to be replaced. Ring design and the performance of engine oil are even more critical with the use of EGR-equipped engines. Higher cylinder temperatures and pressures, combined with the reintroduction of soot and corrosive exhaust gases found in recirculated exhaust gases, can affect ring longevity Figure 28. Diesel EGR rates are more than three times that of gasoline- fueled engines. For this reason, ring technology has changed to compensate for this newer engine wear factor. New materials (such as alloyed steel or ceramic-faced rings) and manufacturing techniques (such as plasma-sprayed ring faces) have produced some of the most sophisticated and durable rings ever made.

FIGURE 07-28: The use of EGR reintroduces soot, an abrasive compound, back into cylinders.

1.3. Ring Classification

Ring Classification

Three basic types of piston rings are found in all diesel engines: compression, intermediate (a combination of compression and oil control), and oil control Figure 29. Each ring has a unique shape and is made from specialized materials to suit the ring’s particular function Figure 30. The top ring is the compression ring and helps seal gases in the cylinder. This ring is sometimes called the fire ring because it is directly exposed to combustion heat. The second ring is the intermediate ring and has both compression and oil control functions. The third ring is the oil control ring.

FIGURE 07-29: A piston with two compression rings, one intermediate ring and one oil control ring.

FIGURE 07-30: Each piston ring has a unique shape and is made from specialized materials to suit the ring’s purpose.

1.4. Compression Rings

Compression Rings

Compression rings are located nearest the piston crown. Their primary job is to prevent gases from leaking past the piston during the compression and power strokes. Gas leakage past these rings, or blowby, robs the engine of power because it reduces compression pressures. It may even allow combustion gases to leak into the crankcase.

Compression rings perform a sealing function in a number of ways. One way is through the static force of ring tension against the cylinder wall. Because rings are manufactured to have a larger relaxed diameter than the diameter of the cylinder they are to be installed in, sealing is accomplished by the ring’s spring tension expanding out against the cylinder wall. In older rings, static ring force could easily be over 20 lb (9.1 kilogram [kg]). However, to minimize friction losses in an engine, high ring tension is not desirable. Thinner rings can seal better by using their knifelike edge. A thinner ring directs more pressure against a small surface area. Today, ring tension may be as low as 4–8 lb (1.8–3.6 kg). These low-tension rings significantly improve fuel economy by decreasing internal friction and cylinder wear.

Another way compression rings seal is through the use of gas pressure during the compression and power strokes, which pushes the ring out with more force against the cylinder wall. To accomplish this, the thickness of the ring is made slightly less than the height of the ring groove. With this arrangement, gas pressure is able to get behind the ring and push it out against the cylinder wall more forcefully than ring static tension Figure 31. Gas pressure also pushes the ring down against the bottom of the groove for better sealing. This means that the higher the cylinder pressure, the greater the sealing capability of the ring. During the power stroke, the compression and intermediate rings seal more effectively (this is also the reason why most blowby occurs during the compression stroke). Piston speed and size are factors in determining how many compression rings are used on a piston. Engines operating at slower speeds require more rings to minimize the loss of compression pressure because more time (during the compression stroke) is available for gases to leak past the rings. Compression rings, like all piston rings, are split for installation and to accommodate thermal expansion. Because gases can escape through the ring end gap, two compression rings are used on some pistons. Even more compression rings are added to slow-speed diesels, such as those found in locomotive and ships. Again, the additional rings help minimize the loss of compression pressure due to increased blowby that can occur at low piston speeds. <.p>

FIGURE 07-31: Gas pressure enhances ring sealing and twists the intermediate ring. The ring notch can be either on the top or on the bottom to enhance the compression function oil control.

1.5. Compression Ring Materials

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.

1.6. Compression Ring Shapes

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.

1.7. Oil Control Rings

Oil Control Rings
The last ring type found on a ring pack is the oil control ring. This ring is usually cast iron with scraper rails that remove excess oil from the cylinder wall while leaving a metered quantity of oil for lubrication. This oil film thickness is typically no more than 0.000004" (0.000102 mm) thick.

The oil control ring is usually a low-tension type of ring and has no significant gas pressure acting upon it. For this reason, a metal expander is placed behind the ring to maintain a constant, uniform pressure against the cylinder walls by the scraper rails Figure 37. The expander ring may be corrugated, slotted, or a wire coil Figure 38. Like the intermediate and compression rings, the oil control ring is vulnerable to premature wear from dirt ingestion into the engine. Scraper rails will show premature wear if abnormal amounts of dirt contaminate the oil through the intake air.

FIGURE 07-37: Oil control rings consists of two scraper rails plus an expander ring. This coiled expander maintains consistent ring pressure against the cylinder wall.

FIGURE 07-38: A corrugated ring expander helps maintain consistent scraper ring pressure against the cylinder walls, since less gas pressure is available to help ring sealing.