Aluminum has been replacing cast iron as the metal of choice for engine blocks for more than 40 years, as automakers seek to reduce vehicle weights and improve fuel economy. However, reducing the use of heavy iron castings comes at a cost – complexity.
Aluminum is lightweight and durable, but it doesn’t have the tribological properties – lubricity and friction resistance – of cast iron. Pistons don’t slide as smoothly across raw aluminum, making the material ill-suited for direct contact between moving engine parts. The solution, from the introduction of the first mass-market aluminum engine blocks in the 1960s, has been the use of cast-iron cylinder sleeves. While they work, those sleeves, also known as cylinder liners, add back some of iron’s weight. Also, powertrain engineers have to increase the radius of each cylinder in the cast design to make room for the liners, a sometimes significant increase in engine displacement because (as my math-teaching mother would constantly remind me), diameter is the figure that gets squared when you calculate area.
So for almost as long as engine producers have been using aluminum, they’ve been looking for alternatives for iron sleeves. The engineers at machine tool producer Gebr. Heller Maschinenfabrik GmbH, Nuertingen, Germany, believe they have the answer – a cylinder bore coating module that melts iron wire and sprays it in a thin layer to line engine cylinders.
The concept isn’t exactly new. Daimler’s engineering department began studying the arc-coating process in 1998, and since 2006 has used a similar process in more than 80,000 engines, mostly high-end large V-8s for its AMG line.
Heller’s innovation, says head of new business and technologies Bernd Zapf, was to create a controlled, repeatable, and reliable crankcase manufacturing system for lining cylinders at high speeds, allowing the technology to move past specialty cars into more mainstream vehicles.
“To make this economically possible, we had to be able to increase speeds and ensure quality at higher production rates,” Zapf says. “It was a major challenge because the process had to be perfect and repeatable. We have no margin for error with this kind of process.”
Heller CBC process
The module that Heller engineers designed for Daimler uses a multi-tasking MC20 machining center to perform fine boring, roughing, coating, rough honing, finish honing to expose pores, and finish machining. Taking unfinished engine blocks, the machining center finely bores the cylinders before roughening them a bit to make the surface better for iron adherence. Then, twin iron and carbon wires are heated to 2,000°C, so a fine coating can be sprayed onto the cold cylinder surface, where the aerosolized metal instantly cools and hardens. Unlike cylinder liners that could be as thick as 5.0mm, the CBC process, which sprays at 60m/s to 80m/s, leaves a 0.1mm thick coating. With the coating in place, the MC20 machining center then hones the sprayed surface and performs finish machining. Total cycle time for V-8 engines is about 6 minutes.
Processing the multiple steps in one machine was a key enhancement to the coating process, significantly speeding finish times.
Another development, Zapf says, is the use of fine boring on the sprayed surface instead of honing. Fine boring is faster and gives a more consistent finish, improving both cycle times and quality.
“Right now, the main use is on engines with larger diameter cylinders, 60mm to 85mm, engines for motor sports or truck engines,” Zapf explains. The spraying rig is too large to fit into the much narrower cylinders of small-displacement engines, but that’s a hurdle Heller’s engineers hope to overcome as they continue to develop the technology.
While spray coating carries some big advantages compared to cylinder sleeves or liners, quality is a big concern. BMW tried using a process in the 1990s called Nikasil that coated engine cylinders with nickel. However, the liners of those high-performance V-8 engines reacted poorly to higher sulfur levels found in U.S. gasoline at the time, and the engines leaked fuel. Since then, U.S. sulfur limits on fuel have gotten much stricter, and Zapf says the carbon-iron coating that Heller’s process uses does not have such problems.
To assure a consistent finish, the MC20 machining center uses several post-processing checks. A Jenoptik IPS 100 bore inspection sensor system measures the coating in a single pass, creating a 360° view of the cylinder surface. The system looks for surface defects such as blow holes, scratches, or improper porosities by steadily advancing the optic probe into the bore. As the probe advances, the image builds in ringed slices, showing high-detail, undistorted views of the inner cylinder surface.
Shrinking, automating, and improving the engine-coating system, a technology that Daimler officials have dubbed Nanoslide, has helped improve the fuel economy and performance of several engines. The V-6 diesel used in the BlueTec ML350 crossover, for example, is 4.3% smaller and 3% more fuel efficient than if the engine still used iron sleeves, Mercedes officials say.
The technology has won Mercedes multiple technology awards since its inception nearly a decade ago, but speed constraints kept it limited to low-volume vehicles. With the addition of the diesel line, the technology has become more widely used in 2014.
Vincent Trampus, vice president of sales at Heller Machine Tools, Troy, Mich., says his company is excited to help develop manufacturing processes for the fuel-saving technology.
“Although the technology has only been used for exclusive low-volume series until now, its application in medium-volume production already provides significant competitive advantages compared to existing cylinder lining technologies,” Trampus says. “The technology complies with the production rules and criteria of the automotive industry. Now, it is only a small step to mass production.”
NANOSLIDE technology uses a twin-wire arc spraying process to melt metal wires and spray the material onto cylinder walls. Final finishing of the resulting ultra-fine to nano-crystaline iron coating creates a mirror-like surface with fine pores, reducing friction and wear between the piston assembly and the cylinder wall.
Heller Machine Tools
IMTS 2014 booth #S-9336
Jenoptik Industrial Metrology North America LLC
IMTS 2014 booth #E-5545
About the author: Robert Schoenberger is the editor of TMV and can be reached at 330.523.5381 or firstname.lastname@example.org.