It's an industry first for Hyundai: mass production of a one-piece casting that integrates a four-cylinder exhaust manifold with a twin-scroll turbine housing and an internal wall to precisely split exhaust gas flow between cylinder pairs (1-4 and 2-3). The design offers (and delivers on) the promise of fast turbocharger spool-up to effective speed, resulting in greater low-end torque. But because this had never been done before, Hyundai initially found no takers when it sought a casting supplier who could do the job in production quantities of tens of thousands. As a result, it first had to develop and validate a production procedure, which it has patented.
The result, an option on the new Hyundai Sonata, is a 2.0-L direct-injection turbo that not only produces 274 hp (204 kW) but also 269 lb·ft (365 N·m) torque from 1750 rpm up. The shapes of the internal passages—which provide a fast, smooth flow of exhaust gasses from the manifold into the twin-scroll turbocharger housing—are covered by one of the patents held by Hyundai for the casting.
The casting is of austenite stainless, a steel alloyed with nickel and chromium. It is easy to fabricate, has high tensile strength and corrosion resistance, and withstands both very high and very low temperatures. The basic production process is by shell molding, using resin-impregnation as a binder for the sand molds, which must have compression strength sufficient to withstand casting forces.
Although shell molding is a long-used, well-known mass-production process, there are obvious difficulties with properly executing so complex a one-piece casting, particularly the formation of the precisely split internal passages for the cylinder pairs. A key difficulty that had to be solved at the start was making the sand molds so they not only form the required shape but also become a stable assembly when put together.
Multiple-section plain steel boxes carry the patterns to form the sand mold for the external shape and for the sand core that defines the precise shape of the internal passages. It is the method of producing the core for the internal passages that was the major challenge, explained Dr. Donghee Han, Hyundai senior research engineer on the project. The steel core patterns for the internal passages had to be shaped not only for the desired turbo performance, but so they can be removed without damage to the sand core. And this was the basis for another patent, he noted.
Each section of the sand mold is designed so that when the complete mold structure is assembled, it is very stable—a characteristic that is another enabler for high-volume production and that also carries a Hyundai patent. There are four outer mold sections and four internal cores needed to make the one-piece casting.
Another issue is ensuring consistent quality in mass production, including uniform, bubble-free metal thickness throughout the casting.
For the casting process, the entire shell mold assembly must be heated and design temperature maintained, and also filled very quickly with molten stainless steel. The object is to complete the operation before any premature cooling that could result in an imperfect fill of the mold. In the Hyundai turbo casting process, it's done within 8 s solely with gravity feed—no pressure or vacuum, thanks to optimized location and shape of the gate (through which the molten steel flows into the mold cavity).
Also very important is the location of a "rising," to keep air bubbles from being trapped in the casting as it solidifies, Han said. The "rising" is a vertical passage (at the top of the mold cavity) that holds additional molten steel that can flow down into the cavity to compensate for any shrinkage during solidification.
Once the casting has cooled, the sand mold is removed first by shaking, then finally by shot-peening. The casting then goes through required machining, and the housing for the compressor/turbine wheel and wastegate is installed. The Hyundai turbo uses a computer-controlled motor drive for wastegate control.
After Hyundai had developed the process, it went to Samshin Ltd., a supplier of exhaust manifolds, to produce the casting, using a manufacturing operation that the companies co-developed. The casting is sent to Keyang, Hyundai's turbocharger supplier, which installs the compressor/turbine housing and ships the completed assembly to Hyundai.
The turbo engine itself has a casting upgrade to the "spiny"-type cast-iron cylinder liners. Spiny liners, which are widely used, have a corrugated-like exterior surface, so that when the aluminum block is cast around them, the liner-to-block adhesion is improved, for superior heat transfer. However, Hyundai found that gaps in the block casting could form within the female sections of the corrugations, which both reduced heat transfer and created the potential for corrosion that eventually could result in perforation of the liners. So on the turbo engines (to this point), the thickness of the liners was increased slightly, from 2.0 to 2.5 mm (0.08 to 0.10).
The liner's heat-transfer capability is enhanced with an aluminum spraying of the external corrugations, to help fill areas where gaps could be left in the subsequent casting of the block. The aluminum spray coating on the liners blends with the cast aluminum of the block so there's a continuous heat transfer surface.