As a measure to reduce the friction between automobile engine parts, it can be roughly divided into the following three types.
One is to reduce the actual contact area between components;
The second is to make low shear stress between parts;
The third is to reduce the input load between 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 roughness of the surface, the surface of the mating material can be smoothed by sliding, which can further reduce the friction force.
At present, most of the engine cylinder bores are constructed by casting bushings made of gray cast iron into the 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 instead of cylinder bores, and the cast iron is sprayed. This can improve the sealing of the interface and thin the tube wall, which greatly improves the heat transfer and heat transfer in this part. Heat transfer.
In addition, as a phenomenon unique to thermal spraying, a certain degree of voids are introduced into the film, and the voids are exposed on the surface layer, which can improve the oil retention of the surface of the cylinder bore.
Since it is not necessary to use the honing mesh of the conventional engine cylinder bore to ensure oil retention, the surface of the cylinder bore can be smoothed. In addition to the above-mentioned advantages of good heat transfer characteristics, it can also effectively reduce the friction from the boundary to the mixed lubrication area.
Regarding the contact between the protrusions on the surface roughness of the parts, the entire surface layer including the protrusions is used as a low shear stress material. As a method to reduce the roughness and reduce the friction alone, solid lubricants can be used.
In particular, when the surface pressure is not so high, a so-called soft coating can be used, that is, granular molybdenum disulfide (MoS2) or graphite, and tetrafluoroethylene as a solid lubricant are dispersed in polyamide resin (PAI) The coating in the medium, or the coating obtained by directly irradiating the MoS2 particles against the workpiece at a high speed. This kind of coating can be applied to the surface layer of piston rod and crankshaft bearing alloy with large contact area in engine parts.
On the other hand, for parts with high surface pressure, we have developed a DLC coating that has the same low friction coefficient as a solid lubricant when it is hard and non-lubricated, so that the same effect can be obtained under lubrication. Coating.
This article will introduce the DLC film that does not contain hydrogen (hydrogen-free DLC film) and the DLC film that contains Si (Si-DLC). The two are very different from the past in terms of surface treatment and design thinking in that the coating itself has no separate characteristics, and the combination of the lubricating oil or the moisture in the lubricating oil can significantly reduce the friction.
In order to reduce the friction between the piston ring and the cylinder bore, in recent years, CrN film has been used to replace the previous Cr plating or nitriding technology. The thickness of the Cr-plated film is about 100, while the thickness of the CrN film with good wear resistance is only 25, which can reduce the deviation range of the film thickness. As a result, without changing the lower limit of the piston ring tension, and only setting the intermediate value to be smaller, the friction between the piston ring and the cylinder bore can be reduced.
In addition, since the amount of wear of the CrN film is small, the initial curvature of the outer surface of the piston ring can be maintained, and the increased contact range due to wear can be suppressed, which also helps to reduce friction.
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