The viscosity of internal combustion engine oil is an important indicator that affects its lubrication performance, including low-temperature starting viscosity, low-temperature pumping viscosity, kinematic viscosity, and high-temperature high shear viscosity. This article introduces the viscosity indicators and test methods of internal combustion engine oils, as well as the situation of classification standards for internal combustion engine oils in China. It interprets the revised content of GB/T 14906-2018 "Viscosity Classification of Internal Combustion Engine Oils" implemented on July 1, 2019. 1. Development of SAE viscosity classification standards for internal combustion engine oils The engine oil viscosity classification system established by the Society of Automotive Engineers (SAE) in the United States began in 1911 and has undergone more than 20 revisions. It establishes the relationship between engine oil viscosity and service performance based on the engine's requirements for lubricating oil viscosity temperature performance. The latest version is SAEJ300-2015. The initial specifications specified the specific gravity, flash point, ignition point, residual carbon value, and viscosity (Saybolt viscosity), and the viscosity code was set based on the average of its low or high temperature viscosity requirements, taking the first two digits. In 1926, the first SAE viscosity classification based on Saybolt viscosity was introduced, which defined six viscosity levels of 130 ℉ (54.4 ℃) and 210 ℉ (98.9 ℃), represented by the numbers 10, 20, 30, 40, 50, and 60. Subsequently, in 1933, low temperature viscosity levels of 10W and 20W were introduced at 0 ℉ (-17.8 ℃), while also adding 70 levels. In 1950, levels 10, 60, and 70 were abolished, and the low-temperature range was increased by 5W. Although this classification is still based on Saybolt viscosity, SAE viscosity classification has begun to approach today's classification. In 1962, the SAE viscosity classification was officially titled SAE J300, and the cold start simulator method was used to measure the viscosity of 5W, 10W, and 20W oils at -17.8 ℃. The high temperature viscosity was measured at 98.9 ℃ using kinematic viscosity. In 1974, an appendix was added to the standard to briefly describe the relevant definition and test process of the engine pumping viscosity, but the cryopump index was not included in the standard. In 1980, the low-temperature pumping performance index was officially included in the standard specifications, and at this time, the modern engine oil viscosity classification system was basically formed. The main work of the 1980s was the study of low-temperature viscosity indicators. In 1980 and 1989, low-temperature starting viscosity indicators, boundary pumping temperature indicators, and low-temperature pumping viscosity indicators were set. Then in 1993, the high-temperature and high shear viscosity index was set. With the advancement of engine technology, the measurement conditions and indicator limits for low-temperature pumping viscosity and low-temperature starting viscosity (CCS) were revised in 1995 and 1999, respectively. The corresponding test temperature was reduced by 5 ℃, and the indicator limits for different viscosity grades of oil were also adjusted accordingly. In 2007, the high-temperature high shear viscosity index was adjusted, and the limit value of high-temperature high shear viscosity index for 0W-40, 5W-40, and 10W-40 viscosity grade oils was adjusted from not less than 2.9mPa·s to not less than 3.5mPa·s. SAEJ300 has added viscosity levels of 16, 12, and 8 in the 2013 and 2015 editions of the standard, and specified the 100 ℃ kinematic viscosity and 150 ℃ high-temperature high shear viscosity index limits for the above three viscosity levels. In summary, revisions to SAEJ300 have been ongoing frequently since the 1980s and 1990s.
2. Situation of viscosity classification standards for gasoline engine oils in China
The viscosity classification standard for internal combustion engine oils commonly adopted internationally is the Society of Automotive Engineers (SAE), and the viscosity classification standard for internal combustion engine oils in China has always been based on SAE viscosity classification. In 1994, China adopted SAE J300-87 to develop the GB/T 14906-1994 "Viscosity Classification of Internal Combustion Engine Oils" and established a viscosity classification system for domestic combustion engine oils.
The internal combustion engine oil product standards GB11121-2006 "Gasoline Engine Oil" and GB 11122-2006 "Diesel Engine Oil" released in 2006 not only need to adapt to the constantly updated requirements of SAE J300, but also combine with the actual situation in China to adopt a segmented standard collection method for the setting of viscosity and temperature indicators of the product. The viscosity indicators for SE, SF grade gasoline engine oils and CC, CD grade diesel engine oils still use the standard indicators before 1995, and SG, CF and above quality grade oils are consistent with the current SAEJ300 standard at that time.
At present, the latest version of the SAE internal combustion engine oil viscosity classification standard is SAE J300-2015, and the technical indicators have undergone significant changes. To meet the needs of modern engine and lubrication technology development, it is necessary to revise GB/T 14906-1994 using the latest SAE J300 standard. The newly revised national standard 14906-2018 Viscosity Classification of Internal Combustion Engine Oil was approved for release on December 28, 2018 (National Standard Announcement of the National Standards of the People's Republic of China, 2018, No. 17), and the implementation date was July 1, 2019. The technical requirements are equivalent to those of SAEJ300-2015.
3. Meaning and test methods of viscosity index for internal combustion engine oil
3.1 Low temperature start-up viscosity
Low temperature viscosity is an important indicator for determining the starting performance of internal combustion engine oil. In cold weather conditions, the power required for engine starting is less than the power provided by the motor, and the engine can only start. Pure mineral oil (single-stage oil) conforms to the viscosity characteristics of Newtonian fluid. On the paper with double logarithmic coordinates, its temperature and viscosity show a linear relationship. The low-temperature viscosity of single-stage oil has always been determined by extrapolation.
With the wide application of Viscosity Index Improver, the viscosity of internal combustion engine oil is not only related to temperature, but also affected by shear rate. At high shear rates, the structure of polymer polymers can be distorted or even destroyed, resulting in a loss of oil viscosity as a non Newtonian liquid that cannot be obtained using extrapolation at low temperatures. In response, the United States conducted methodological research and officially introduced the Cold Start Simulator (CCS) method into the SAE J300 standard in 1993. The method has been improved multiple times to expand the applicable temperature range. The cold start simulator consists of a pair of closely matched stators and rotors, which are driven by a DC motor. The test temperature is maintained by adjusting the coolant flowing through the stator to investigate the rheological properties of the engine oil at low temperature and high shear rate (104~105s-1), and to simulate the state of the engine oil in the piston and cylinder liner during low-temperature starting. The latest standard is ASTM D5293-17a, and the testing temperature has been expanded to -10~-35 ℃. The current method standard in China is GB/T6538-2010 "Determination of apparent viscosity of engine oils - Cold start simulator method", developed using ASTMD5293-04.
3.2 Low temperature pumping viscosity
Low temperature pumping viscosity is a measure of the ability of engine oil to flow into the oil pump and supply sufficient oil pressure during the initial operating phase of the engine under low temperature conditions. The low-temperature pumping performance of internal combustion engine oil is very important. If the oil cannot be successfully pumped, even if the low-temperature starting viscosity is reached, the engine will still fail to start. The smooth pumping of the oil pump requires two conditions: firstly, the oil needs to flow into the filter screen through the pressure head between the oil surface and the filter screen, which is affected by the critical yield stress of the oil. The second is to be able to continuously enter the inlet pipe from the filter at a sufficient flow rate, which is related to the critical viscosity of the oil.
Low temperature pumping viscosity is measured by a small rotary Viscometer (MRV). The domestic methods and standards include GB/T 9171 Measurement of Boundary Pumping Temperature of Engine Oil and NB/SH/T 0562 Measurement of Yield Stress and Apparent Viscosity of Engine Oil at Low Temperature. The current standard GB/T 9171-88 is developed using ASTM 3829-79 to predict the boundary pumping temperature of internal combustion engine oil within the range of 0~-40 ℃. It is still used for the determination of low-temperature starting performance of low quality oil grades such as SE, SF, CC, CD, etc.
NB/SH/T 0562-2013 was formulated by ASTM D4684-08, still using a small rotary Viscometer, improving the cooling rate to TP-1 cooling cycle, and measuring the yield stress and apparent viscosity at a shear stress of 525 Pa and a shear rate of 0.4~15s-1. This is because in the winter of 1980-1981, the United States and Northern Europe still experienced a large number of pump failures in oil passing ASTM 3829. Research suggests that the cooling cycle of this method only takes 2 hours to pass through the cloud point range, resulting in small wax crystalline particles and low viscosity, which cannot predict the low-temperature pumping performance in certain situations. The TP-1 cooling cycle requires the internal combustion engine oil to cool at a controlled rate for at least 45 hours, ultimately reaching the test temperature. Using low-speed cooling (0.33 ℃/h) in the wax crystallization formation zone can provide sufficient growth time for wax crystallization, thus predicting pumping failures that cannot be predicted by ASTM D3829.
3.3 Kinematic viscosity
Kinematic viscosity is an important indicator of internal combustion engine oil, which is not only related to some form of oil loss, but also often serves as a guide for users to select oil at normal engine operating temperatures. The kinematic viscosity is determined by GB/T 265 Petroleum Products - Determination of Kinematic Viscosity and Calculation of Dynamic Viscosity. At present, some other automatic viscosity measurement methods have also been standardized.
3.4 High temperature and high shear viscosity
During the operation of internal combustion engines, the bearings are subjected to high impact loads, with the temperature of the lubricating oil at the bearings reaching over 140 ℃ and subjected to shear at a high rate of 106s-1. It is necessary to set high temperature and high shear viscosity project aggregation indicators to predict the effective viscosity of high shear parts of the engine under harsh operating conditions.
The current high temperature and high shear viscosity measurement methods in China include SH/T0618-1995 "Method for Measuring the Dynamic Viscosity of Lubricating Oils under High Temperature and High Shear Conditions" (formulated with reference to CECL-36-T-84) SH/T 0703-2001 Apparent Viscosity Measurement of Lubricating Oil at High Temperature and High Shear Rate (Multiple Capillary Viscometer Method) (equivalent to ASTM D5481-96) and SH/T 0751-2005 Viscosity Measurement at High Temperature and High Shear Rate (Cone Plug Viscometer Method) (modified to ASTM D4741-00).
4. Main revisions of GB/T 14906-2018
Compared with GB/T 14906-1994, the newly revised standard has added viscosity levels, revised indicator items, test methods, and indicator settings, and added normative Appendix A.
4.1 Increase viscosity level
With the increasing demand for automotive emissions and energy efficiency in various countries, the fuel economy of automobiles has attracted much attention. During engine operation, only 40% of the energy generated by fuel combustion provides effective power, with 25% of the energy lost due to mechanical friction. Reducing friction loss is an effective way to improve fuel economy. In the case of fluid lubrication, the friction coefficient decreases with the decrease of viscosity. Under the condition of ensuring normal lubrication of engine parts, reducing the viscosity of internal combustion engine oil can reduce the hydrodynamic resistance, thereby reducing energy consumption and achieving the purpose of energy saving. The latest API diesel oil specification FA-4 aims to improve engine fuel economy by reducing the high temperature and high shear viscosity of the oil, and the internal combustion engine oil is showing a trend towards low viscosity. The viscosity classification of internal combustion engine oil has also added viscosity levels of 8, 12, and 16.
4.2 Revision of low-temperature starting viscosity
This revision changes the original standard "low-temperature viscosity" to "low-temperature starting viscosity", resulting in changes in test conditions and indicator limits. The rapid development of modern engine technology has made it easier for engines to start at low temperatures. Therefore, the revised standard has reduced the measurement temperature of low-temperature starting viscosity by 5 ℃, and the indicator limits for different viscosity grades of oil have also increased to varying degrees.
4.3 Revision of low-temperature pumping viscosity
SAE J300 set the low-temperature pumping viscosity index in 1989 and revised it in 1995. Reduce the test temperature by 5 ℃ and modify the indicator limit from no more than 30000mPa · s to no more than 60000 mPa · s. The low temperature pumping performance index in GB/T 14906-2018 has been modified from "boundary pumping temperature" to "low-temperature pumping viscosity", and the limit value of the index is consistent with the current SAE standard. In GB/T 14906-1994, grades 0, 20, and 25 W oils were determined using GB/T 9171, while grades 5, 10, and 15 W oils were determined using SH/T 0562. The revised standards were all determined using NB/SH/T 0562.
4.4 Revised kinematic viscosity
The kinematic viscosity index of low-temperature viscosity level remains unchanged; The lower limit of the 100 ℃ kinematic viscosity of 20 viscosity grade oil has been modified from "not less than 5.6mm2/s" to "not less than 6.9 mm2/s"; Add 100 ℃ exercise index limits for viscosity levels 8, 12, and 16.
4.5 Increase high-temperature and high shear viscosity
For high-temperature viscosity levels, GB/T 14906-2018 has added high-temperature and high shear viscosity index items and testing methods to ensure that polymer containing internal combustion engine oil can meet the needs of the engine under high temperature and high shear conditions. There are two regulations for 40 viscosity grade oil based on the type and requirements of the engine used. The high-temperature high shear viscosity requirement for 0W-40, 5W-40, and 10W-40 grade oil is not less than 3.5mPa · s, and the requirement for 15W-40, 20W-40, 25W-40, and 40 grade oil is not less than 3.7mPa · s.
4.6 Add normative Appendix A
GB/T 14906-2018 is in line with advanced standards to meet the research and use needs of high-end oil products in China. At the same time, it is necessary to coordinate with the specifications of domestic fuel oil products. A normative appendix has been added, retaining the viscosity classification of the original GB/T 14906-1994 and providing viscosity classification for SE, SF quality grade gasoline oil, CC, CD quality grade diesel oil, and agricultural diesel oil.
5. Conclusion
In recent years, the rapid development of China's automotive industry and the continuous tightening of environmental regulations have put forward higher requirements for engine lubricants. Timely revision of the viscosity classification standards for internal combustion engine oils is beneficial for adapting to the needs of modern engine and lubricant technology development, and promoting the development and upgrading of engine oils in China.
