As a measure to reduce the friction force between automobile engine parts, it can be roughly divided into the following three types. One is to reduce the actual contact area between the components; the other is to form a low shear stress between the components; the third is to reduce the input load between the components.
Reducing the surface roughness of the parts helps to reduce the actual contact between the parts. On the basis of not changing the initial surface roughness, the surface of the counterpart can be smoothed by sliding, which can further reduce friction force. At present, most engine cylinder bores are constructed by casting a sleeve made of gray cast iron into an aluminum alloy cylinder block. The thermal conductivity of cast iron (50W/(m·k)) is lower than that of aluminum (100W/(m·k)), and the thickness of cast iron is several mm. Recently, in order to improve the heat dissipation in the combustion chamber, cast iron sleeves are used to replace the cylinder holes, and the cast iron is spray-coated, which can not only improve the sealing performance of the interface, but also reduce the thickness of the tube wall, greatly improving the heat transfer and thermal conductivity of this part. heat transfer. In addition, as a phenomenon peculiar to thermal spraying, a certain degree of voids are introduced into the film, and the voids are exposed on the surface layer, thereby improving the oil retention of the cylinder bore surface. The surface of the cylinder hole can be smoothed because it is not necessary to use the conventional honing meshing of the engine cylinder hole to ensure oil retention. In addition to the above-mentioned advantages of good heat transfer characteristics, it can also effectively reduce the friction between the boundary and the mixed lubrication area.
Regarding the contact between the bumps on the surface roughness of the automobile parts, the entire surface layer including the bumps is used as a low shear stress material, and as a method to reduce the roughness and reduce the friction force alone, a solid lubricant can be used. In particular, when the surface pressure is not so high, the so-called soft coating can be used, that is, granular molybdenum disulfide (MoS2) or graphite, tetrafluoroethylene as a solid lubricant is dispersed in a polyamide resin (PAI) In the coating, or the coating obtained by directly irradiating MoS2 particles to the workpiece at high speed, etc. This kind of coating can be applied to the surface layer of piston rod and crankshaft bearing alloy with large contact area in engine components. On the other hand, for parts with high surface pressure, a DLC coating with a friction coefficient as low as that of solid lubricants is developed in the case of hard and non-lubricated cases, and the same effect can be obtained in the case of lubrication. Coating. This article will introduce the DLC film without hydrogen (hydrogen-free DLC film) and the DLC film with Si (Si-DLC). The difference between the two in terms of surface treatment and design ideas is that the coating itself has no independent characteristics, and the combination with the lubricating oil or the moisture contained in the lubricating oil can significantly reduce the friction.
In order to reduce the friction between the piston ring and the cylinder bore, CrN film has been used in recent years to replace the previous Cr plating or nitriding treatment technology. The thickness of the Cr-plated film is about 100, while the thickness of the CrN film with good wear resistance is as thin as 25, which can reduce the deviation range of the film thickness. As a result, the frictional force between the piston ring and the cylinder bore can be reduced without changing the lower limit value of the piston ring tension, but only by setting the intermediate value smaller. In addition, since the wear amount of the CrN film is small, the initial curvature of the outer surface of the piston ring can be maintained, and the contact range that increases with wear can be suppressed, which also contributes to reducing the frictional force.
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