MAHLE has developed a piston ring with new technology that offers engine manufacturers improvements in weight, quality and overall value. MAHLE’s new “second” piston ring uses chrome-silicon steel to replace cast iron traditionally used for the second of three rings found on gasoline-powered spark-ignition engines. MAHLE’s thinner, lighter-weight steel-wire design for the second ring is based on the company’s own NanoNapier Steel (NNS) technology for which a patent is pending.
Conventionally, there are two types of piston rings: compression and oil control. A piston assembly includes one or more compression rings that generate a seal between the outer surface of the piston and the wall of the cylinder to prevent high-pressure combustion gases and air from escaping the combustion chamber. Today’s engines have three piston rings: two compression rings: top (or first) and second and the oil control ring. The second ring is known as a Napier ring.
In addition to sealing off the combustion chamber, piston rings also dissipate the heat that arises in the piston to the cylinder surface and control the oil balance. As temperatures, pressures, and rotational speeds continue to rise, piston rings are subjected to ever increasing loads.
Too, the second piston ring also may perform an oil scraping function, which entails scraping oil from the cylinder wall on the downward stroke of the piston.
The Napier-style ring includes a generally tapered outer peripheral face and a lower surface having a hook groove. Some second piston rings are produced from metallic wire that is subsequently machined to the desired cross-section.
In its patent applications, MAHLE notes that:
The machining processes may include turning and grinding, which is a costly and time consuming step to providing the desired shape. The machining process also results in burrs and chips from turning or grinding of the material, which results in waste and fine particles that need to be cleaned from the wire prior to use, which is a problem. Known designs have attempted to alleviate these problems by using cast iron second rings, rather than steel from drawn/rolled wires, as cast iron is easier to machine and therefore results in less machining costs and fewer burrs. However, the use of steel wire in second rings is desirable because of its relative lightness and durability when compared to cast iron, and thus a means of reducing machining processes required for forming a steel wire second ring is desirable.
Additionally, previous designs incorporating the Napier-style profile for use in a steel wire second ring required multiple machining operations. The need for multiple grinding steps using a plurality of grinding surfaces results from the need for a generally sharp edge that is required to increase the scrapping effect of the second ring. However, typically drawn/rolled wire cannot generally be formed with sufficiently sharp-edged profiles (generally, corners in profiles of drawn/rolled wire have a rounded shape with a minimum radius of 0.05 mm). Thus, multiple grinding operations to sharpen the edge are required.
More specifically, a first grinding operation must be applied to a bottom surface of the ring, and a second grinding operation must be applied to the outer diameter surface. The standard Napier hook includes an axial offset, which prevents the underside of the hook from being contacted with a single grinding surface, e.g., during a lower surface grinding operation. Thus, at least a second grinding operation is typically required along the outside diameter (O.D.) surface in order to form the relatively sharp edge (in addition to the necessary lower surface grinding operation). Previous micro-Napier type designs similarly include an axial offset, which completely prevents any contact by the grinding surface during application to the lower surface to the outer edge.—US 20130154196
In short, MAHLE has designed a method for constructing an improved Napier-style ring from steel wire.
Rings made from cast iron are heavier and require a considerable amount of machining. Cast iron simply is no longer suitable for most current and next-generation engine applications.—Steven Sytsma, a piston-ring product expert at MAHLE
Cast iron for decades has been the material of choice for the second piston ring because of its wear characteristics and compatibility with other engine-design features. The trend to higher revving, lower-friction engines, however, has resulted in the need to reduce ring size and weight as axial piston heights (distance between a piston’s top and bottom) have dropped.
Sytsma notes that over the past 20 years ring height has decreased by 40% or more—from 2.0 mm to 1.2 mm or less—causing MAHLE to look for cast-iron replacement materials.
MAHLE began development of a first-generation second ring with steel in 2010 using a steel-wire design. The company’s engineers relied on proven suppliers to deliver wire that met MAHLE’s demanding product requirements. The new steel rings have fewer material defects and improved quality overall which in turn provides significantly better customer value.
Benefits of MAHLE’s NNS second-ring include the elimination of foundry-related defects, reduced engine-oil consumption and substantial face- and side-wear resistance. In addition, the thinner axial height and radial thickness provided by the new design has the potential to help make pistons even lighter and engines more capable of operating with less vibration and friction at higher rpms.
Because a NNS second ring is axially and radially smaller, it also has less mass and is dynamically more efficient at its dual tasks of both sealing off oil from the upper portion of the piston chamber and regulating gas pressure within the piston ring belt. Engineers will be able to take advantage of thinner second rings to further reduce weight as well.
MAHLE currently supplies top, second and third rings to vehicle OEMs, engine manufacturers and piston suppliers around the world. Several years ago MAHLE replaced cast iron with steel for its top rings.
Customer interest in our new NanoNapier Steel ring has been exceptionally strong. Production of the new ring for a major original equipment customer will begin later this year at a MAHLE facility in St. John’s, Michigan, and we are actively pursuing contracts for more than 20 additional programs with multiple customers as well.—Scott Ferriman, MAHLE’s vice president for North American sales
Ferriman adds that the company currently is quoting on business with annual production volumes of 15 million NNS rings or more.
Piston ring formed from ring blank US 20130154196