Short Circuits Cut Down on Friction: Liquid Iron
|
The introduction of electric arc wire spraying in engine production has made it possible to build very low-friction cylinder running surfaces in aluminum engines. The new 6.3-liter V8 engine developed by engineers at Daimler's AMG subsidiary is the world's most powerful eight-cylinder naturally aspirated engine. It delivers an output of 386 kW (525 hp), and 630 Nm of torque at the crankshaft, and is now being used in various AMG models, most recently the Mercedes-Benz CL 63 AMG high-tech coupe. The fully aluminum engine has 32 valves, a cylinder bore of 102.2 millimeters, and a stroke of 94.6 millimeters. It achieves its impressive power not only from its large displacement and favorably streamed intake and exhaust system, but also by means of a unique innovation deep in its interior: The running surfaces of the light-metal cylinders consist of an "EAS coating" that ensures extremely low-friction operation. Thin layers instead of liners EAS stands for "electric arc wire spraying" - a thermal coating system that involves spraying liquid iron onto the interior walls of a cylinder to create a thin layer. This coating replaces the liners that serve as robust cylinder-bore running surfaces in a conventional aluminum engine block. "Electric arc wire spraying is a well-known technique that involves bringing together the ends of two conducting metal wires," explains Patrick Izquierdo of the Surface Treatment and Forming department at Daimler's Production and Materials Technology unit. "An electrical short circuit is created as soon as the wire ends make contact, and this in turn generates a large amount of heat that causes the ends of the wires to melt. This molten metal can then be sprayed like paint through a nozzle." Together with his colleagues from the Component Production Planning and Flexible Component Production departments, Izquierdo modified EAS technology for automotive series production applications and built a first series production facility for the process at the Mercedes plant in Untertürkheim. Thinner running surfaces reduce weight EAS units contain a magazine in which two copper-plated iron wires are uncoiled from spools and fed into a spray head at a predefined speed by carrier rollers. During this process, they pass through contact wire feed tubes in which a current is applied to them. When the two wires touch, this large current creates a permanent short circuit whose high-energy arc immediately causes the 1.5 millimeter-thick wires to melt. Behind this melting zone is a nozzle that releases either pressurized air or an inert process gas, such as nitrogen. This streaming gas atomizes the molten metal in a spray, thereby accelerating the particles (which are at nearly 2,000 degrees Celsius at this point) and discharging them onto the surface to be coated, where they then cool and harden. Depending on the spraying distance and jet nozzle system used, the particles can reach speeds of between 50 and 150 meters per second. The current that creates the electric arc is generated by a device similar to the one used for electric welding. In other words, the EAS technique utilizes proven standard technology, which means it can be put into practice rapidly, and at relatively low cost. Daimler engineers were the first in the world to use EAS for engine production, and the technique has enabled them to achieve a range of improvements that benefit vehicle developers and customers alike. "One of the great advantages is that the thin EAS coatings have such good tribological properties, which have led to significant reductions in friction. And that means less wear, of course," says Karl Holdik of the Tribology department at Daimler Research in Ulm. Holdik and his colleagues address fundamental issues regarding the electric arc wire spraying technique. The researchers in Ulm have also discovered that about five percent of the surface of a thin EAS coating consists of fine pores that can store oil, which means engines with such coatings have excellent emergency running properties as well. Another advantage of using EAS is that it makes an important contribution to lightweight construction techniques that improve fuel economy. That's because conventional cylinder liners made of gray cast iron have a wall thickness of approximately three millimeters and can weigh several kilograms, depending on the engine they're used with. EAS-coated running surfaces, on the other hand, have thicknesses ranging from 0.1 to 0.15 millimeters, which means they add practically no additional weight. This is why engines that are produced using EAS technology are between seven and 12 percent lighter than other engines. High pressure with nanomaterials Forgoing the use of cylinder sleeves also saves space - to the tune of six millimeters per cylinder, which means the crankcase can be shortened by several centimeters, depending on the engine in question. This is particularly good news for series development engineers, who have to fit a large number of components into an engine compartment. Experts also have been very positive in their assessments of the thermal behavior of the thin EAS coatings. The reason for their high praise is that the coatings are more effective than cylinder liners when it comes to transferring the heat generated in the combustion chamber, and that means using EAS contributes to more efficient cooling of pistons and piston rings. The electric arc wire spraying project was launched at the end of 2000, when researchers in Ulm began studying the structure, adhesive strength and frictional properties of various EAS coatings in order to determine how they would behave in engines. They also began working with spray nozzles and application techniques as a means of demonstrating that EAS could in fact be used in automobile production. "After we established that the concept was feasible, we began working closely with colleagues at Process Technology, and we ultimately advanced the wire spraying system to the series production stage at Production and Materials Technology's technical facility in Untertürkheim," Holdik reports. These days, the Ulm Research Center and the Untertürkheim plant are operating fourth-generation EAS facilities that can coat a cylinder in just 30 seconds. After roughening the cylinder walls with a high-pressure water jet, a robot takes the engine block and positions it under the spray head in a manner that enables the latter to be inserted vertically into one of the cylinders. The spray head rotates on its longitudinal axis during the spraying procedure, so the cylinder walls receive a uniform coating all round. The series production facility put into operation at the Untertürkheim plant in 2006 has a capacity of 25,000 engines per year. The researchers in Ulm are now working on the further development of EAS technology. As part of a project funded by the German Ministry of Education and Research, which also includes other automakers and several universities, the researchers are examining new types of coatings whose particle sizes measure in the nanometer range (one nanometer equals one millionth of one millimeter). "The nanolayers can withstand higher pressures in the engine because they are harder than other EAS coatings, yet they are no more brittle," Holdik explains. In addition to their iron content, they also contain small amounts of boron and molybdenum. These dopants make them hard and also improve their tribological properties even further. Use in other engine parts In the aforementioned project, which is known as "NaCoLab" (Nanocrystaline Composite Coatings for Cylinder Running Surfaces with Nanostructured Surfaces and Wear Predictability), Daimler researchers are also seeking to coat aluminum cylinder running surfaces directly with a nanomaterial based on iron carbide and iron boride. Their colleagues at the Surface Technology and Production Management laboratory are simultaneously developing new calculation procedures for engine components that must hold up under particularly high stress loads, as well as wear models and simulation tools. The researchers and developers are also considering other engine areas where it might be possible to use nanomaterials. Here they are focusing on components such as cylinders, which are subjected to high levels of pressure and friction. So it's possible that valve-seat rings and camshaft slide surfaces will also be getting the thin, robust EAS coatings in the future. Today's engines need to be dynamic, fuel efficient and environmentally friendly. That's why development engineers are attempting to boost engine performance, operating temperatures, and cylinder pressures, and also to achieve a more compact design. EAS technology can help them do all this. Karl Holdik sums up the overall benefit: "Electric arc wire spraying enables Daimler to cover a larger portion of the engine production value chain with its own components." Thermal spray coating Thermal spray coating is a process in which a powdered or wire-shaped material is melted by a heat source and then accelerated and discharged onto a component surface, where it hardens and gradually forms a firm layer. In principle, any material can be used that forms a stable "molten liquid" in ambient conditions, or under an inert gas atmosphere - in other words, materials that don't decompose or chemically react with their surroundings. High rate of application with EAS Metal materials processed in such a manner undergo electric arc wire spraying (EAS), a procedure which has become the norm in industry due to its technical robustness and reliability. Also known as TWAS (twin wire arc spraying), EAS can be used to process all electrically conducting wires. Depending on the material used and the melting ability of the spray system, the application rate can reach a rate of up to 20 kilograms of material per hour. An affordable coating procedure Electric arc wire spraying can produce more molten mass in a short period of time than other procedures, such as plasma spraying. It thus offers a big advantage in terms of its high rate of application and the possibility of creating coatings of several millimeters thickness, which in turn makes for a very attractively priced coating system. Five questions for Prof. Heinrich Flegel, head of Production and Materials Technology at Daimler's Vehicle Body and Drive Systems research unit Materials technology plays a major role in automaking, but it's hardly noticed by the public. What benefits does the technology offer customers? Materials are selected on the basis of the functional advantages they offer. This means they are always relevant to customer requirements with regard to safety, for example, which is very important, but also in relation to fuel economy, which is closely linked to the issue of lightweight construction. Material selection is also a key factor in ensuring comfort, especially when it comes to reducing noise and vibrations. In most cases, however, the particular vehicle properties a specific material results in are not emphasized in product advertisements. Which general vehicle development trends have a particular impact on your field of work? That would be the key development issues that automakers need to address: enhancing fuel economy, increasing safety and - not to be forgotten - conveying a sense of high quality and value. This impacts powertrains and the vehicle body. With regard to the former, we're seeing a trend toward downsizing, which means greater demands are being made on the materials used. In terms of the bodyshell, automakers have to at least compensate for the increased weight due to comfort features and other functions. Will plastics and fiber composites largely replace metal materials? The selection of materials to be used in specific components is always based on functional and business criteria and the best possible compromise that can be achieved between them. This is why materials are constantly "competing" with one another, which has led to continual improvement in the quality of individual materials. Nevertheless, I don't think we're going to see a reversal of trend where we will have plastics and fiber materials replacing metals. What role will natural fibers play in future vehicle production, and what advantages do these materials offer? Natural fibers are a key element in enabling us to convincingly present our strategy for sustainable mobility, and we already have natural fiber components in some of our series production vehicles. The current A-Class, for example, has 26 components containing renewable raw materials such as abaca, flax, and hemp; the share of such natural materials is significantly higher than in the predecessor model. Nevertheless, using such natural fibers wouldn't be justified if they didn't offer technical and economic benefits. How will nanomaterials be used in vehicle construction in the future? Nanomaterials are already being used in series production vehicles. The "Ceramiclear" paint used in all Mercedes-Benz model series is more scratch-resistant thanks to nanoparticles. In addition, graphite nanoparticles are used in tires to improve their adhesive properties and reduce wear. In the future, we expect to see nanoparticles used as an additive for plastic formulations as a means of directly integrating plastic coverings into conventional painting processes. They will also be used to raise the resilience - and resistance to wear - of plastic friction bearings, as well as to reinforce cylinder running surfaces, increasing their wear resistance. © 2008 Daimler AG. All rights reserved. Mercedes-Benz engine oil recommendations
|
| EAS |