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Cylinder Bore Coating: Ready for Prime Time

By SME Media Staff

Heller helps bring arc spraying technology for engine blocks to a wider automotive market.

While lightweight materials now provide excellent solutions to the requirements of modern combustion engine construction, the quality of the cylinder surfaces has become the Achilles heel of this type of powerplant. Surface hardness, roughness, and texture are some of the determining factors for fuel consumption as well as for the durability and performance characteristics of the engine.

To date, lightweight engine blocks use cylinder liners, but there is now a practical alternative in twin-wire arc spraying, an extremely cost-effective technology for coating cylinder bore surfaces of combustion engines. In cooperation with Daimler AG (Stuttgart, Germany) and other partners, Gebr. Heller Maschinenfabrik GmbH (Nürtingen, Germany; Troy, MI), is taking on the task of integrating the process into volume engine production.

Daimler AG is using the newly developed arc spraying technology for coating cylinder bore surfaces and aluminum crankcases of automobile engines under the name of Nanoslide.

The technology uses a twin-wire arc spraying process to continuously melt iron/carbon wires and spray them onto the cylinder surfaces of the aluminum crankcase with the help of a nitrogen gas flow. Compared to other thermal coating processes, the technology is considered extremely cost-effective and technologically advanced—a reliable, repeatable, highly controllable economical process that vastly improves the cylinder bore surfaces, reducing wear and promoting long-life. Spray particle velocity is 60–80 m/s and spray particle temperature is up to 2000 °C.

Principles and Advantages

All thermal spray processes rely on the same principles:

  • Feed stock material is melted by heating
  • Particles are atomized and sped up to a high velocity by a gas
  • Particles will deform and freeze on the cold surface

The coating is formed when millions of particles are deposited on top of each other. A common feature of all thermal spray coatings is their lamellar grain structure with pores, unmelted particles, and oxides resulting from the rapid solidification of small globules, flattened from striking a cold surface at high velocities.

Advantages of the twin-wire arc spray process include:

  • A high amount of material deposit
  • Wires as the feed stock material (easier to handle than powder)
  • It is considered a ‘simple process’ as only a few components are necessary
  • Coating without process control is possible
  • No extra cooling of the torch is necessary

However, the coating process requires optimal calibration of quality-determining parameters such as current, voltage, wire feed and process gas flow.

Specific advantages of the spray coating for combustion engines are:

  • Weight and space advantages over liners
  • Favorable frictional conditions between the liner, the piston and the ring
  • Fuel consumption advantages for diesel and gas engines
  • A very high wear resistance in-service
  • Manufacturability in a reliable process and at favorable costs

Due to their thermally coated cylinder liners, Nanoslide-designed engines are superior in terms of displacement and torque, especially when compared with similar aspirated engines incorporating conventional lining technology. Daimler AG started development of the coating technology in 1998 with small, series production of high-performance AMG engines, including the 6.3L V8. Since 2006, Daimler has used the treatment on more than 80,000 engines.

Moving from Prototype to Series Production

To provide a practical, automated process for higher production applications, Daimler AG has entered a partnership with Heller. For its part, Heller will provide automation of processes and equipment, and develop improved processes stability strategies. The development and demonstration of technology will be conducted at Heller’s Cylinder Bore Coating (CBC) Technology Center at its global headquarters in Nürtingen.

With its global range of systems and services to support wider application of the process beyond the limited-edition, high-performance AMG engines, Heller in cooperation with Daimler and other interested parties will automate all steps of the process and develop it into a reliable application for global markets under the name Heller CBC.

The goal of the CBC Technology Center is to:

  • Coat trial parts for future coating cells
  • Enhance all process steps and equipment for future requirements
  • Adapt the process to different crankcases
  • Produce a constant surface quality with guaranteed properties

The CBC machining process at Heller includes premachining, fine boring, coating, finish machining and finish honing the coated cylinder bore—all accomplished on the Heller MC20 four-axis machining center. Total cycle time, including part load/unload, for an eight-cylinder engine is five–six minutes. The engine blocks may be direct-loaded into the machining center or mounted on a swiveling exchanger.

A key contribution of Heller to the process is to fine-bore the arc-sprayed cylinder coating to impart a final finish. Previously this was accomplished with honing, but the Heller fine boring process takes about half the time and assures a much more consistent finish and form to the cylinder due to the machining center spindle axial orientation to the bore. Tooling cost is also much lower compared to honing.

Measuring the Coating Assures Positive Results

The applied coating is measured with a Jenoptik IPS 100 bore inspection sensor system which provides a rapid, single pass automatic surface inspection of bores from 75 to 110-mm diameter, instantly providing a 360° panoramic view.

The sensor permits detection of surface defects such as blow holes, scratches, porosities and more on bores from 75 to 110-mm diameter, to any depth. Utilizing state-of-the-art CMOS imaging technology, the IPS100 continuously scans the bore surface at high speed, creating a viewable image of the bore interior.

In operation, a 360° circular optic is advanced into the bore and generates an image of the surface. Circumferential lines in the bore are seen as rings in the image. As the sensor advances into the bore, circular scanning allows for the generation of an undistorted image of the inner surface.

“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,” said Vincent Trampus, Heller vice president of sales. “The technology complies with the production rules and criteria of the automotive industry. Now, it is only a small step to mass production.”

This article first appeared in Motorized Vehicle Manufacturing 2013-2014. It was edited by Yearbook Editor James D. Sawyer from material provided by Heller Machine Tools.

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