As the thermodynamic efficiency of combustion engines increases, so do the engine and exhaust gas temperature levels. Sealing off the hot side of a turbocharger has therefore become a challenge.
Multilayer steel gaskets are typically used to avoid a loss of exhaust gas and uncontrolled emissions. Yet the use of steel as gasket material is limited. Even a temperature-resistant steel such as 1.4828 (X15CrNiSi20-12) will show a change of its metal matrix when the object temperature rises above 600°C to 700°C (1112º to 1292°F) . Once the steel is exposed to this amount of heat, some of the material’s properties are lost.
When the steel loses tensile strength, for example, it can begin to slip and creep at increased temperatures, which can cause gas leakage. So the most critical factor is the springback that is essential to maintain a good high-temperature sealing performance over the vehicle lifetime.
At the 2011 Frankfurt Motor Show, ElringKlinger presented an alternative approach to turbocharger sealing. Called metal-profile gaskets, they're made from a selection of nickel-chromium-based alloys. As the hardening or strengthening temperature of the different types of Ni alloys extends to above 1000°C (1832ºF) the axial gaskets resist gas temperatures of up to 1100°C (2012ºF) and object temperatures in excess of 800°C (1472ºF) without thermally induced changes to the material matrix and properties.
“Metal-profile gaskets have quickly established themselves as a turbocharger trouble-shooting technology,” said Dr. Oliver Göb, head of special gasket development at ElringKlinger, based in Dettingen/Erms, Germany. He said production started last year.
Nickel alloy gaskets have been in use in jet engine applications for many decades but have always been considered too costly for automotive applications. “To overcome this hurdle we had to develop a new gasket design," Dr. Göb told AEI.
He explained that because the nickel material is so precious, the stamping process typically employed to manufacture multilayer steel turbocharger gaskets is ruled out. "The amount of scrap is way too high, and the stamped surface quality is insufficient," he noted. "Instead we have developed a special rolling-shaping process that transforms laser-welded flat rings into a gasket with a V-shaped cross section.”
Sealing and damping in one component
In comparison to multilayer steel gasket designs, the metal-profile gaskets are lightweight and filigree components. Dr. Göb explained that to ensure the gasket maintains its springback, it is ideally installed in a groove. When the turbocharger components are screwed together the groove depth will act as a force-shunt, which ensures that the gasket is sufficiently preloaded but not compressed all the way.
In the installed state, the gasket not only acts as a sealing element, it is in essence also a spring with limited damping properties.
“If designing-in a groove is not an option, the groove function can be replaced by some sort of stopper geometry,” he said. Compared to a flat seal that consists of up to seven layers of high-grade steel, the metal-profile gaskets are made from a much smaller amount of material, which compensates for some of the higher material cost, he added.
The technical benefits of the nickel alloy gaskets include a higher level of dynamics facilitated by the material strength. As a consequence, the gasket is also a clamping element that can adapt to the pseudo-static movements of the sealing gap during turbocharger operation much faster than flat conventional steel.
“This can help to improve the overall sealing integrity and to reduce an uncontrolled loss of exhaust gas,” Dr. Göb adds. “Considering the strict Euro 5 and 6 emissions regulations, it will be even more important to avoid any leakage along the exhaust path. Also, a loss of exhaust means that there is less energy available for the turbocharger.”
When the filigree gasket is installed in a groove, it is not exposed to the aggressive, hot gas environment. But even if the gasket gets exposed, it is protected by a material property of Ni alloys that makes them a good choice for exhaust applications. The Ni alloys have an excellent oxidation and corrosion resistance, which is the result of a thick passivating oxide layer that forms on the surface during use.
Within the range of Ni alloys used by ElringKlinger to manufacture metal-profile gaskets, the temperature strength is stepped. If the object temperature of the sealed components can be lowered by design measures, for example, a less expensive version of alloy can be used. After the shaping process, the metal-profile gaskets are slightly ovalized to make assembly easier.
This step of the production automatically puts the laser weld to a quality test. The surface roughness requirements of the gasket are the same that apply to multilayer steel gaskets. While the gaskets will typically be used to seal off steel or cast iron materials, they can also be used with aluminum or other materials, according to Dr. Göb.