A Cast of Thousandths

An SwRI-developed method of casting diesel engine cylinder heads with greater precision
wins an R&D 100 Award     image of PDF button

 
image of Hybrid Ceramic-Sand Core Casting tech development team

SwRI members of the Hybrid Ceramic-Sand Core Casting Technology Development team are (from left): Principal Designer Doug McKee, Assistant Director Marc Megel and Program Manager Barry Westmoreland, all from SwRI’s Engine, Emissions and Vehicle Research Division.


image of CAD drawing

A CAD drawing shows models representing the sand cores required to produce a cylinder head casting.


image of traditional, reddish sand core

A traditional, reddish sand core contains white ceramic material at its center (left photo) to produce the finished product (right photo).

image of finished product

A novel method that combines sand and space-age ceramics to cast precision metal engine parts was recognized as one of the 100 best innovations of the year by R&D Magazine.

Southwest Research Institute (SwRI) has won 37 R&D 100 Awards, sometimes called the “Oscars of Innovation.”

The new process, called Hybrid Ceramic-Sand Core Casting, combines aerospace ceramic and automotive sand core casting processes, enabling precision casting of extremely small passages in automotive cast iron or steel components. This product was developed in a joint effort with United Kindom-based Grainger and Worrall, Ltd., as part of a three-year, multi-phase research and development program. The goal was to develop a new generation of heavy-duty diesel engine architecture with significantly higher peak cylinder pressure (PCP) capability than current state-of-the- art engines. This is needed to enable future exhaust emissions-reduction and high-efficiency combustion technologies without sacrificing engine performance, size or weight characteristics.

Cylinder head manufacturing

The conventional method for manufacturing iron cylinder heads for internal combustion engines is to use a sand-casting process, because the internal fluid passages are geometrically complex and sand casting is inexpensive. With its intricate shape capability, ease of extraction from finished castings and low material cost, sand casting is well-suited for the functional and economic requirements of making engine cylinder heads. However, newer geometries that allow higher peak-cylinder-pressure operation, as well as high cooling velocity and efficiency, require internal passages that are too small to manufacture reliably using conventional sand casting.

The critical limiting factor of sand casting is the minimum achievable size of internal passages. As cross-sectional dimensions are reduced, the ability to resist premature breakdown in the presence of molten metal is also reduced. Thus, there are limiting dimensions below which a sand core will disintegrate during casting because of factors such as thermal shock, evaporation of chemical binder agents and physical penetration from the molten metal.

By contrast, ceramic cores, such as those used in the aerospace industry to cast cooling passages in turbine blades, do not break down in the presence of molten metal, even at very small sizes. However, because they are relatively expensive they are not commonly employed in the automotive industry. They have never before been used in a hybrid ceramic-sand core design.

A ceramic-sand hybrid

For the new hybrid ceramic-sand core product, the ceramic portion forms the coolant passages between the engine’s gas exchange port walls and fuel injector or spark plug boss in the lower water jacket core. The ceramic section is used to form valve bridge passages as well as the annulus around a cast injector sleeve. Using ceramic for this part of the core allows much smaller passages to be formed than if the core were made entirely of sand.

To demonstrate this technology, samples were cast using a core profile manufactured using both a conventional 100 percent sand core technology with representative minimum cast passage diameters of ~5 mm, and the new hybrid ceramic-sand technology employing varying section thicknesses through the valve bridge area to quantify its benefit with a minimum width on the order of 1.5 mm. Casting trials using the conventional sand core technology resulted in extensive sand burn-in that blocked passages through the injector bore and valve bridge regions. The ceramic-sand core casting trials resulted in completely clean bridge areas free of burn-in, even at the smallest bridge width. Additionally, no finning or defects of any kind were observed at the sand-ceramic interface, indicating a sufficient bond between the core materials.

The hybrid process is extremely scalable, unlike the conventional technology. This allows it to be considered for future downsized, high-output engine concepts designed for optimized cooling and high PCP structure rather than having to design around the limitations of sand cores.

Integrating ceramic and sand

The ceramic insert is a preformed shape designed to fit into the same corebox into which the sand core is blown, thereby consolidating the insert. The insert is manufactured from a ceramic slurry, using an injection molding process that involves relatively low-cost tooling and processing. Ceramic inserts are mechanically and chemically stable, which translates to long shelf life and the capacity for high-volume repeat usage.

The interface design between the sand core and the ceramic insert is designed to provide maximum grip to maintain overall mechanical integrity of the hybrid sand core and offer maximum surface area for subsequent dissolution in caustic media during the clean-out process.

The ceramic material, silicon carbide (SiC), has extremely low thermal expansion which in all trials thus far has enabled the core to survive the thermal loading during pouring of the casting at 1,300 ° C. This survival ensures that liquid metal is unable to penetrate the core and obstruct the eventual passageway. By comparison, conventional sand cores are a mixture of sand grains bound by a thermosetting resin binder, neither of which is immune to extreme thermal loading, even when coated with a protective barrier known as “core wash.” The eventual failure mode of a conventional sand core will be initiated by a fracture of coating or core under the influence of thermal expansion and or buoyancy loads in the liquid metal such that penetration occurs and the passageway is obstructed.

The relatively small volume of the ceramic insert does not affect in any measurable way the solidification dynamics of the casting. Removal of the insert is mostly via dissolution in a caustic bath, although some mechanical removal is considered possible as well.

The final shape of the passageway formed by the ceramic insert is extremely accurate and smooth. This confers the benefit of highly repeatable dimensions and pressure drop rates for optimized coolant flow. This innovation has been successfully used at dimensions smaller than 2 mm, a value unobtainable by sand core alone.

Improvements over conventional products

Zircon sand with core wash in a hot box configuration can approach the performance of the ceramic-sand hybrid design, but zircon sand is expensive and also is currently in short supply. A key benefit of the Hybrid Ceramic-Sand Core Casting technology is its scalability to smallerbore diameter engine components. As the engine bore becomes smaller, so does the bore spacing and the available space for cooling and air passages. Most conventional casting technologies have minimum section thresholds that limit their ability to implement features for high cylinder pressure and high cooling efficiency on engines with bores below ~110 mm. Finally, high-velocity targeted cooling in cast components can be achieved using machined drillings. This is possible where access allows, and may not be feasible when considering the more complex diesel porting and six-bolt pattern configurations imagined for high-PCP engines. The issue with employing the machined drilling approach is that it requires line-of-sight access to drill the passages and as such cannot be packaged within concealed regions, unlike the hybrid core technology.

Principal applications

Principal applications of this product include complex cylinder head and block castings for high-efficiency, low-pollutant/ greenhouse gas emission, high-power density diesel, gasoline and gaseous fuel engine applications, especially smallerbore diameter (<120 mm) applications, because space to package all necessary features for high-cylinder pressure, good cooling and good volumetric efficiency becomes significantly limited. High cylinder pressure is critical to high power density as well as ultra-low-pollutant emitting, high-efficiency, low CO2 emission diesel, natural gas and gasoline combustion technologies. Additionally, this technology can be used in any industry currently using sand-core metal casting techniques where small diameter passages may be advantageous but unattainable or experiencing high scrap rates due to the process limitations of conventional sand casting.

The innovation also is realistically “future-proof” in terms of scalability, both larger and smaller, where the latter permits similar concepts to be incorporated in passenger-car-size engines, for example. Financial benefits to the consumer are derived from overall combustion efficiency and fuel economy improvements, and the potential to position such technology in the premium sectors confers pricing opportunity to manufacturers as well.

Questions about this article? Contact Marc Megel at (210) 522-3079 or marc.megel@swri.org.

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Southwest Research Institute® (SwRI®), headquartered in San Antonio, Texas, is a multidisciplinary, independent, nonprofit, applied engineering and physical sciences research and development organization with 10 technical divisions.
04/15/14