Multi-grade oil and single-grade oil The viscosity range of multi-grade oil can span several viscosity grades. Taking 15W/40CD diesel engine oil as an example, 15W/40 can include 6 viscosity grades such as 15W, 20W, 25W, 20, 30, and 40, so it is called multi-grade oil. The 15W/40 viscosity grade has both the low-temperature viscosity of 15W oil and the high-temperature viscosity of 40 oil. Therefore, one type of oil can be used in winter and summer, and there is no need to change the oil due to seasonal changes. Therefore, multi-grade oil is also a general-purpose oil for all seasons. Multi-grade oil is made up of lighter distillate base oil with viscosity index improver, pour point depressant and other functional additives.
Compared with single-grade oils, multi-grade oils have better viscosity-temperature properties. Small change). It can be seen from <2> that, when the viscosity of 100 is the same, the multi-grade oil has a smaller low-temperature viscosity and a higher high-temperature viscosity. Furthermore, in the specifications of multi-grade oil, two analysis results of low temperature viscosity (CCS) and boundary pumping temperature (MRV) should be provided to ensure that the oil has good low temperature performance during use.
The low-temperature performance of lubricating oil is mainly measured by the low-temperature viscosity and boundary pumping temperature. The low-temperature viscosity of the oil mainly affects the cold start of the engine. If the low-temperature viscosity of the lubricating oil is too large, the torque of the anti-lubrication system when the crankshaft is rotating If it is too large, it may result in failure to reach the specified minimum speed and difficult to start, and because the viscosity of the lubricating oil is too large, it is difficult to form effective lubrication, which will increase the wear. Therefore, the viscosity limit of lubricating oil at low temperature should be stipulated. The boundary pumping temperature of lubricating oil refers to the lowest temperature at which the oil can be continuously and fully supplied to the inlet of the engine pump.
Because the engine’s starting process is very short (especially at low temperatures), the lubricating oil in the crankcase is too late to be pumped to the friction surfaces. In order to ensure that the friction surfaces of the engine can be lubricated in time, it is required that the engine must be lubricated very quickly after starting. The oil pressure of the lubricating oil system can reach normal in a short time. Since the single-grade oil uses a base oil with a heavier fraction and does not add a viscosity index improver, there is no requirement for low-temperature viscosity and boundary pumping temperature in the oil product specifications, so the high and low temperature performance is not as good as that of the multi-grade oil. It can also be seen from <2> that if the viscosity of 100 is the same, the multi-grade oil has a smaller low-temperature viscosity and a higher viscosity of 150 and 230, which will be beneficial to the engine’s low-temperature start and high-temperature wear resistance.
In addition, when the engine is running normally, due to the high shear rate (106s-1), the oil has a temporary viscosity loss, generally not less than 3.5mPa.s. If the viscosity is too small, the oil film is easily damaged, the sealing effect is not good, and the oil consumption increases. , Will also produce wear. The working temperature of each part of the engine varies greatly during use, from atmospheric temperature to 300, so the viscosity-temperature performance of the oil is particularly important. Through the above analysis, it can be seen that the viscosity-temperature characteristics of multi-grade oil are significantly better than single-grade oil.
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