September 12, 2017
Rheinmetall Automotive
Today's strategies for minimizing friction losses
Under the squeeze of ever tighter CO2 constraints, engine developers are nowadays exploring every conceivable avenue for lowering fuel consumption on internal combustion engines. One option is to minimize friction, since friction losses erodeefficiency and have an adverse effect on fuel consumption. Among the challenges facing the auto-industry supplier is to fine-tune each and every component for minimum friction. The chief sources of friction are to be found in the piston assembly, the piston rings and the piston itself, the cylinder working surface and the main, conrod and countershaft bearings. Also meriting attention are the interacting friction surfaces, especially on those parts with mixed-friction contact. Over the past years, Rheinmetall Automotive AG’s Hardparts division has steadily broadened its expertise in tribology and nowadays offers highly efficient, consumption-reducing solutions for pistons, crankcases, and plain bearings.
"The newest gasoline engine pistons, the Liteks-4, are currently ready for series production and offer a substantial reduction in friction," reports Jochen Müller, Head of Development & Technology at KS Kolbenschmidt GmbH. "The basis for this improved friction behavior is, first, the optimized fundamental structure of the piston and the advanced piston clearance geometry. On the pressure and counterpressure sides, differing ovalities are used, these being variable through the piston skirt." While taking into account the thermal deformation of the piston and the working surface of the cylinder, this variability permits low-friction, low-noise design of the piston’s external contour.
"To reduce the mixed-friction effects in cylinder working surface contact, we have resorted to our Nanofriks piston skirt coating," says Müller. This consists of a blend of nanoparticles, binders, solid lubricants, and additives. Tribometrical research has shown that such a blend, when compared to conventional coatings, will reduce the dry coefficient of friction as well as the amount of wear by more than one-half.
Propriety friction test bench
Another contribution toward friction abatement is rendered by the piston rings co-developed with alliance partner Riken. Besides low-friction pistons, smooth, wire-sprayed cylinder surfaces are used in series production with a view to complying with the latest CO2 legislation. Combined with piston ringswith a hard coating, such as diamond-like carbon (DLC), this facilitates a significant friction reduction within the engine operating parameters. Altogether, with the same low oil consumption, these optimized piston assemblies achieve around 25 percent less friction than current series-produced pistons.
"The tribology of the individual components and fine-tuning their interaction is a complex challenge with a host of reciprocal effects. So we’ve developed our own friction test bench to enable us to accurately measure the relevant variables during engine operation," explains Müller. The initial prototypes of the new generation of pistons have also been put through their paces on this test bench and were able to verify the simulated results. Under full-load conditions, the friction of the new Liteks pistons was lowered by up to 28 percent. Under part-load, the reduction was seven percent.
KS Kolbenschmidt’s lineup also includes steel pistonsfor car diesel engines. Compared with their aluminum cousins, the steel variety with their much smaller skirt surface and the reduced lateral forces of the steel piston also play a major role in friction reduction. These pistons, too, feature the low-friction Nanofriks skirt coatingas standard. At operating temperatures, the low thermal expansion of steel pistons also results in a significantly larger clearance between piston and cylinder for an effective minimization of friction. Even given the added clearance, the perfectly coordinated design ensures smooth running of both diesel and gasoline engines .
Cylinder working surface coatings
The cylinder working surface offers further opportunities for limiting friction. Friction measurements indicate a significant advantage of coated surfaces over conventional ones. Because of this, KS HUAYU AluTech GmbH relies on its proprietary RSW technique (Rotating Single Wire), which has been used since 2015 on engine blocks delivered to a premium German automaker.
With RSW, a wire material is melted by a plasma arc and the molten particles, accelerated by a separately infused atomizing gas, are deposited on the cylinder surface.The coating parameters also play an important role. The technology has meanwhile advanced to such a degree thatindividual customer specifications regarding coating properties can be realized. Besides by honing the surface, it is now also possible to selectively influence the surface texture of the cylinder and hence adapt the tribological system (piston, piston ring, cylinder working surface) to customer-specific emission reduction requirements.
"Generally, with its variability regarding coating material and setting options for the process parameters, RSW technology possesses vast potential for fulfilling the requirements regarding reduced engine friction and wear," explains Dr. Christian Klimesch, Chief Developer. "The coating also allows gray-cast bushings to be dispensed with, thus permitting improved heat dissipation on top of weight savings."
Stiffer demands on today's plain bearings
"Besides resilience and robustness, abrasion and wear have always been among the issues of major concern for us as plain bearings manufacturers," is howAthanassios Skiadas, Chief Engineer Engine Bearings & Simulation at KS Gleitlager GmbH, sums up the subject of tribology.However, particularly with new engine designs, a further increase in friction is a critical factor, and one that poses completely fresh challenges. As hybridization progresses, so does to a significant degree the number of start/stops from, for example, previously 400,000 to over one million. Skiadas: "Belt-driven starter-generator engines raise the tribological requirements in terms of wear resistance of the crankshaft bearings. In addition, low-viscosity oils increase the share of mixed friction in the conrod bearings." Hence, a completely new generation of polymer bearings with improved wear resistance and the customary low friction has been developed specifically for hybrid engines. Here, the new L1 layer system comes into play.
For the main bearings on friction- and hence wear-critical engines, wear resistance under difficult conditions such as heavier loads and low-viscosity oils has been enhanced and low-viscosity oils and, one solution is the KS R53L1 polymer bearings. The development work was targeted at maintaining both the robustness and the low coefficient of friction of the series-production bearing.
Materials for conrod bearings
For conrod bearings exposed totoday's more intense mixed-friction conditions, the manufacturer's programoffers the new L1 polymer layer system combined with the more resilient R55 bimaterial substrate. When used in medium- to high-duty gasoline engines, the new material provides the wear resistance required under heavy hybridization and low oil viscosity conditions with no loss in robustness and adaptability. "Dirt tolerance is an important criterion for all lead-freebearings," says Skiadas. "The tolerance of R55 is very good due to the interaction between the polymer layer and the aluminum substrate."
KS Gleitlager offers further proprietary layer systems for its bearings. These include heavy-duty yet adaptable galvanic bearings and sputter bearings coated by a PVD process.
Coefficient of friction as required
There are some applications, specifically in the vehicle's interior, where friction may be required to a specific degree. For such applications, KS Gleitlager offers its KS P150 with a low coefficient of friction and optimized stick/slip properties. Also in the program is the KS P209 with extreme wear resistance yet a slightly higher coefficient of friction. On adjusting mechanisms, this combination ensures best-possible sliding and damping properties along with a moderate self-locking effect. Seat backrest adjusters, for example, can be set to different frictional torques at two mounting points for optimum interactive functions and minimum tooth wear so that the teeth remain securely anchored even in the event of a crash. This example clearly shows how important it can be to be able to generate friction as, when and where required.