The pressures on heavy-duty diesel engine development are steadily increasing both figuratively and literally. In October 2002, stricter tailpipe emissions standards will go into effect in the U.S., and over the next decade progressively tougher legislation in both America and Europe will mandate emission levels be a fraction of what they are today. The primary technology being applied to reduce diesel emissions is exhaust gas re-circulation (EGR), which returns between 15% and 40% of exhaust gases to the combustion chamber. There they are ignited as part of a fresh fuel charge, and largely eliminated from the pollution equation. According to Keri Westbrooke, director of Heavy-Duty Product Technology–Pistons, Federal Mogul (Southfield, MI), "EGR has a "magical" effect on reducing some of the gaseous emissions, particularly NOX, but this solution also creates another series of problems." Namely, that the increased cylinder pressure and heat needed for EGR to operate properly places greater stress on the piston, liner and rings. And this leads to greater potential for in-cylinder wear. Also, the re-circulation process brings in what Westbrooke calls "a lot of nasty stuff" like soot, water and metal oxides that can contribute to corrosion and abrasion.
To combat these negative side effects, diesel engine makers are upgrading their materials and processes. Rings are receiving exotic chrome coatings to enhance wear resistance and all liners are having their running surfaces hardened via heat treatment (some currently are not heat treated). But the most radical change is taking place with the piston, which is undergoing a complete redesign.
Tough But Cool
Federal-Mogul's new diesel engine piston is dubbed "Monosteel" because, unlike current diesel pistons with their forged steel crown and cast aluminum piston skirt, the Monosteel is a single-piece, all-steel construction designed primarily for increased strength and cooler running. To that end, it features a closed oil gallery–up to 70% larger than that found on traditional designs–that forms a box section which provides greater resistance to firing loads, and allows use of a greater amount of oil to cool the piston. Westbrooke says this configuration keeps the upper part of the piston, which must endure the combustion flame front, 50°C cooler, and the lower pin bore area 30°C cooler. The lower running temperature of the pin bore, combined with greater piston rigidity, allowed Federal-Mogul engineers to eliminate the costly and trouble-prone pin bore bushings.
The Monosteel piston starts life as two separate forgings that are pre-machined and then friction welded together. (Using two forgings allows Federal-Mogul to vary the material composition of each part based on cost and application. For example, the top portion can be spec'd from a higher-grade steel well-suited to the higher heat and greater corrosion of an EGR environment, while the larger lower portion can be made with a less exotic and costly grade.) After friction welding, the pistons are heat treated to create a uniform molecular structure across the weld before machining.
Though the Monosteel piston can be machined using existing equipment, Westbrooke says that, "As part of this whole Monosteel development, we are also developing a parallel machining module concept for use anywhere in the world." The modules would employ a common architecture and common equipment, thereby simplifying worldwide operations. And since the Monosteel design eliminates aluminum, on-site aluminum foundries would no longer be necessary, significantly reducing both equipment and facility investment.
Ready for Cars?
Federal-Mogul has signed a multi-million dollar contract to supply its new pistons to "a global heavy-duty truck engine manufacturer," and the company is focusing all of its development efforts in the heavy-duty arena on the Monosteel. As for smaller displacement applications, Westbrooke says, "This technology can certainly be applied to the light vehicle diesel market, which predominantly uses aluminum pistons." His team is currently in the concept stage of a Monosteel piston for light-duty vehicles, but they have not yet achieved the competitive cost and weight results necessary.