According to their sources, lubricating oils are divided into four categories: animal oil, vegetable oil, petroleum lubricating oil and synthetic lubricating oil.
The consumption of petroleum lubricating oil accounts for more than 90% of the total consumption, so lubricating oil often refers to petroleum lubricating oil. It is mainly used to reduce the friction between the surfaces of moving parts. At the same time, it has the functions of cooling, sealing, anti-corrosion, anti rust, insulation, power transmission, cleaning impurities and so on. It mainly uses lubricating oil fraction and residue fraction from crude oil distillation unit as raw materials. The main properties of lubricating oil are viscosity, oxidation stability and lubricity, which are closely related to the composition of lubricating oil fraction. Viscosity is an important quality index reflecting the fluidity of lubricating oil. Different service conditions have different viscosity requirements. High viscosity lubricating oil shall be selected for machinery with heavy load and low speed. Oxidation stability refers to the antioxidant capacity of oil products in the service environment due to temperature, oxygen in the air and metal catalysis. After the oil is oxidized, fine asphaltene based carbonaceous substances, viscous paint like substances or paint films, or viscous aqueous substances will be generated according to the service conditions, so as to reduce or lose its service performance. Lubricity indicates the antifriction performance of lubricating oil. The concept of lubricant additive is to add one or more compounds to the lubricant to obtain some new characteristics or improve some existing characteristics of the lubricant. Additives are mainly divided into functional categories, such as antioxidant and metal deactivator, extreme pressure antiwear additive, friction modifier (also known as oiliness agent), detergent dispersant, foam inhibitor, antirust agent, antioxygen preservative, flow point improver, viscosity index enhancer, anti emulsion and so on. The additives sold in the market are generally the composite of the above single additives. The difference is that the composition of the single additive is different and the proportion of several single additives in the composite additive is different.
When the engine is running, if some friction parts are not properly lubricated, dry friction will occur. Practice has proved that the heat generated by dry friction in a short time is enough to melt the metal and cause damage or even jamming of parts (this is the main reason why many cars with water leakage or oil leakage have cylinder pulling, axle holding and other faults). Therefore, the friction parts in the engine must be well lubricated. When the lubricating oil flows to the friction part, it will adhere to the friction surface to form an oil film to reduce the resistance between friction parts, and the strength and toughness of the oil film are the key to play its lubrication role. However, do not use too much, because too much will produce square resistance, which will have a great impact on the speed, so pay special attention to the amount.
Only a small part of the heat generated by fuel combustion in the engine is used for power output, friction resistance consumption and driving of auxiliary mechanisms; Most of the remaining heat is not only discharged into the atmosphere with the exhaust gas, but also taken away by the cooling medium in the engine. The excess heat in the engine must be discharged from the engine body, otherwise the engine will burn out due to excessive temperature. On the one hand, this is accomplished by the engine cooling system, on the other hand, the lubricating oil absorbs heat from the surfaces of cylinder, piston and crankshaft and then carries it to the oil pan for dissipation.
When the engine works, a lot of dirt will be generated. Such as sand and dust brought into the air, carbon deposit formed after the combustion of the mixture, colloid formed after the oxidation of lubricating oil, metal chips produced by friction between parts, etc. These dirt will adhere to the friction surface of the parts. If they are not cleaned, they will increase the wear of the parts. In addition, a large amount of colloid will cause the piston ring to stick and jam, resulting in the abnormal operation of the engine. Therefore, these dirt must be cleaned in time. This cleaning process is completed by the circulating flow of lubricating oil in the body.
There is a certain clearance between the cylinder and piston, piston ring and ring groove, and valve and valve seat of the engine, so as to ensure that there is no stagnation between each moving pair. However, these clearances can cause poor cylinder sealing and combustion chamber leakage, resulting in reduced cylinder pressure and engine output power. The oil film formed by the lubricating oil in these gaps ensures the tightness of the cylinder, maintains the cylinder pressure and engine output power, and prevents the exhaust gas from flowing downward into the crankcase.
When the engine is running or stored, the moisture in the atmosphere, lubricating oil and fuel oil and the acid gas generated by combustion will cause corrosion and rust to the parts, thus increasing the damage of the friction surface. The oil film formed by lubricating oil on the surface of parts can avoid direct contact between parts and water and acid gas, and prevent corrosion and rust.
Eliminate impact load
At the end of the compression stroke, the mixture begins to burn and the cylinder pressure rises sharply. At this time, the lubricating oil in the bearing clearance will alleviate the impact load on the piston, piston pin, connecting rod, crankshaft and other parts, make the engine work smoothly, prevent direct metal contact and reduce wear.