WHEN EVERYTHING RUNS SMOOTHLY ...
How do you prevent too much friction from occurring between the crankshaft and its main and conrod bearings in a combustion engine? It’s quite simple: use hydrodynamic bearings. This means that the oil pump forms a load-bearing, lubricating oil film between the journal and the bearing. Because of the rotation of the crankshaft, the oil is distributed evenly in the gap: the crankshaft “floats” in the bearings without actually touching them, despite the enormous cylinder pressures and the physical forces acting upon it. The service life of the crankshaft and bearings is almost indefinite in this operating condition—or at least it is in theory. Reality often looks a little different. The protective lubricating film is by no means permanently present.
... AND WHEN IT DOESN’T
What happens when the engine was just started up? The static weight of the crankshaft, flywheel, and coupling or transducer causes the main bearing journal on the flywheel side and the lower bearing shell to make contact. On the control side, the crankshaft is pulled against the upper bearing shell. This is due to the fact that the timing belt or timing chain is preloaded. When the crankshaft begins to rotate under these conditions, the conrod and main bearings are strained—because metal is now rubbing against metal. The oil pump simply did not have enough time to create a load-bearing, lubricating oil film in the bearings. Known as mixed friction, this causes wear and tear, even if the abrasion is rather limited when taking into consideration the entire service life of the engine.
STOP AND GO
The lights are on red, the engine turns off. The lights turn green and, with just a little pressure on the gas pedal, the engine springs back to life. Nowadays, more and more vehicles are fitted with a stop-start function. And this is a good thing. Shutting off the combustion engine—even for only a few seconds—saves more fuel than is required to restart it.
In hybrid vehicles, the engine control system may even turn the engine off completely while the vehicle is in motion, provided the electric drive generates enough power. And even in overrun mode, the combustion engine is not in operation if the kinetic energy is being stored in the batteries as electric power.
By frequently shutting off the combustion engine, fuel savings in the region of 20 to 25% are possible. Moreover, the corresponding drop in CO2 emissions means that the burden on the environment is reduced considerably. However, it does put engine components, especially bearings, under considerable stress. When the engine is shut off, the mechanically driven oil pump also stops running, the oil pressure drops, and the crank pins touch the bearings. The metallic contact between the journal and the bearing when the engine is restarted leads to increased friction and therefore to wear on the bearings.
In theory, the oil pump could build up sufficient oil pressure before the crankshaft starts moving. Such an oil pump, however, has yet to be installed in passenger car or commercial vehicle series production. Due to the generally insufficient oil pressure before the crankshaft begins its motion, we at MAHLE have developed an extremely effective solution: our self-lubricating bearings.
MAHLE HAS MORE UP ITS SLEEVE
Along those lines, we apply an innovative polymer coating to standard aluminum two-component bearings or bronze bearings in several micro-thin layers. At the end of the process, this layer is only a few thousandths of a millimeter thick—but the result is amazing thanks to the solid lubricants and aluminum flakes embedded in the polymer matrix. This special chemical combination ensures that there is no more metallic contact between the crankshaft and the bearings. As a result, mixed friction on start-up is no longer a concern.
LESS HEAT, LESS FRICTION
The aluminum flakes in our coating are good heat conductors and prevent a thermal overload in the bearings. At the same time, embedding solid lubricants allows for a significant reduction in friction and higher mechanical strength during start-up. The coating is also extremely resistant to chemical stress. In summary, bearings can enjoy a much longer life with the MAHLE coating!
Whether in today’s engines or the technology trends of tomorrow: our polymer-coated bearings meet current and future requirements. But our innovation is capable of so much more! Thanks to the outstanding frictional properties of our bearings, recently developed low-viscosity engine oils can now be used. Power loss due to friction and fuel consumption both continue to decrease. In addition, the polymer coating can be used as an alternative to high-strength sputter coating. Initial field tests have already reported good results.
NUMBER ONE IN ORIGINAL EQUIPMENT
Since the beginning of 2013, we have been supplying our polymer-coated bearings for series production—to renowned German, French, and American engine manufacturers.