Powertrain engineers know well the mechanical, thermal, and lubrication challenges presented by engine downsizing, particularly as displacement and cylinder count migrate to what used to be motorcycle levels. Cylinder bore distortion, as well as its potential for affecting performance and causing excessive oil consumption, is a major concern in higher-output engines with small interbore bridges in their lightweight aluminium blocks.
Working with its various OEM customers, Federal-Mogul has developed a solution for the bore-distortion problem in downsized gasoline ICEs. It is a patented dual-material cylinder liner technology the company calls Hybrid Liner. According to Gian Maria Olivetti, Vice President for Technology and Innovation, the new liner system “increases the strength and stiffness of the combined block and liner assembly, allowing significant weight reduction without compromising engine performance and durability."
Designed for volume production
The Hybrid Liner is comprised of a conventional cast-iron sleeve that is coated on the exterior surface with an aluminum material, using a proprietary Federal-Mogul process. The surface topography of the outer layer mechanically locks the liner into the aluminum block casting more effectively than alternative technologies, both structurally and thermally, Olivetti claims. The ferrous-metal sleeve provides good tribological properties and is designed to be compatible with cost-effective piston-ring packs.
The liner coating is an AlSi12 alloy with a melting point below that of the aluminum engine block. This helps to provide excellent intermetallic bonding between the surfaces of the two materials. The coating is applied using a process that uses wire-arc thermal spray technology to ensure that coverage, thickness, and bonding strength are uniform around the complete height and circumference of each liner. The application process was designed to meet volume-production cost and robustness requirements, according to the company.
The roughness of the coating surface also is controlled to provide undercuts and micro-porosities needed to form a strong intermetallic transition zone between the coating and the cast-aluminum block material.
Olivetti noted that compared to alternative technologies, the Hybrid Liner “reduces bore distortion in a running engine by two-thirds.” He added that maximum second-order bore distortion can be as low as 11% of that seen in conventional cast-in liners. Cylindrical distortion under operating loads is up to three times better, and as a result, oil consumption is reduced by up to 40%, he said.
Surviving 200-bar pressure test
The new Hybrid Liner, which is produced at Federal-Mogul's Friedberg, Germany, facility, also results in up to 30% higher heat-transfer rates, which in turn have the potential for reducing cylinder wall temperature by up to 40°C (104°F), compared to alternative designs. The design also offers increases in dynamic strength. Federal-Mogul’s cyclic pulsing pressure test showed no failure in the Hybrid Liner at pressures up to 200 bar (2900 psi), whereas a standard liner block design under test cracked at 100 bar (1450 psi).
The Hybrid Liner also allows engine OEMs opportunities to improve overall engine packaging by reducing the space between adjacent cylinders. Olivetti said a 3-mm (0.118-in) wall thickness between bores is sufficient on pressure-die-cast blocks with Hybrid Liners.
"If a water channel is drilled between two cylinders, the Hybrid Liner technology limits the formation of cracks, allowing coolant to penetrate only as far as the liner coating,” he explained, while conventional liners can allow the coolant to seep down the joint between liner and casting, reaching the crankcase and contaminating the lubricating oil.