Introduction
Engine lubricant viscosity is an essential property that affects fuel economy. Viscosity is a crucial parameter to measure for oil analysis due to the importance of oil conditioning and lubrication [1]. Viscosity enables machines to operate at various temperature conditions. In general, viscosity tends to decrease with increased temperature and vice versa which explains why lubricant oils flow smoother in summer than they do in winter. Therefore, it is important to specify the grade of lubricant oil for proper usage.
Figure 1. SAE J300 Motor Oil Viscosity Chart [3]
The major engine lubricant oils are classified by the Society of Automotive Engineers (SAE) viscosity grading system as shown in Figure 1. These viscosity grades determine which specific engine lubricant oils are suited for different engines. Modern engine lubricant oils are multi-grades which must comply to viscosities for both low and high temperature. The 5W SAE viscosity grade refers to a viscosity at low temperature which thins the lubricant oil and enables it to flow faster during start-up which enhances engine protection and concurrently lowers long-term wear [4]. The 30 SAE viscosity grade denotes another viscosity specification at high temperatures of 100°C. It also has a lower number which corresponds to a thinner oil.
Viscosity is mainly defined into two facets: kinematic viscosity and absolute (or dynamic) viscosity [5,6]. An oil’s kinematic viscosity is defined as the resistive flow under the force of gravity and is a measurement of a fluid’s inherent resistance to flow when no external force is applied. [6]. On the other hand, dynamic viscosity is defined as a fluid’s resistance to flow when an external force is applied. In addition, the density of the fluid can directly affect the value of kinematic viscosity, while on dynamic viscosity it does not [6].
Other than the two main viscosities, high temperature, high shear (HTHS) viscosity directly impacts the fuel efficiency and durability of an operating engine. It is one of the few methods used to efficiently measure the ability of fully-heated oil at a temperature of 150℃, between the narrow openings of fast-moving engine parts such as the piston ring and lining, and gearwheel contact points [3]. As the value of HTHS increases, the protection of engine parts strengthens. In order to determine the lubricating quality of an oil for rings and bearings, the HTHS performance test is intended at 302℉ (150℃) under SAE requirements [7].
Most of the modern petroleum lubricants are formulated with additives for various purposes such as controlling oil oxidation, reducing wear/scuffing, and preventing corrosion from friction and acids [8]. Since it is inherent that viscosity is easily affected by temperature, industries have been trying to use proper additives to produce lubricant oils which refrain from viscosity loss under temperature change. Those additives are viscosity index improvers, also known as viscosity modifiers, which are primarily oil soluble polymers and copolymers [9]. The small, coil-shaped polymer molecules unfold and expand as temperature rises, resulting in a higher viscosity compared to essential oils. It is beneficial to increase friction in the liquid to compensate for the decrease in viscosity caused by higher temperatures [9]. The viscosity index is a dimensionless number that indicates the amount of viscosity loss proportional to temperature increase [7].
Figure 2. Viscosity Index – schematic [9]
The VI scale consists of high and low limit points. The high VI-oils is more desirable because it has minimal viscosity changes with respect to changes in temperature [9]. While many lubricant oils contain additives that are not heavily affected by the change in temperature, these formulated lubricant oils are commonly non-Newtonian, which has viscosities that vary with shear rate and shear stress [8]. At low temperatures, the lubricant oils demonstrate Newtonian behavior which retains constant viscosity. However, viscosity decreases as the shear rate climbs to critical value.
The lubricant oil is exposed to extreme conditions with both high shear rates and temperatures ranging from 105 to 107 sec-1 and 100 to 170℃, respectively [8]. HTHS experiment simulates these real operating conditions to obtain accurate viscosity measurements of the lubricant oils. The two most common viscosity measurements are the rotational and the capillary type viscometers. This paper is going to discuss the capillary viscometer for the high-temperature, high shear experiment. It is simple to construct and operate and has less demanding temperature control requirements [8]. The capillary viscometer requires a capillary with precise specified dimensions such as inner diameter and length. The time would be measured for the flow of a liquid through the capillary.
The capillary viscometer has been designed for measuring the viscosity of the lubricant oils at a given temperature of 150℃ and 106 sec-1 shear rate which meets the specification of SAE viscosity classification J300. The portable computer embedded in the HTHS viscometer offers automatic calculations for viscosity and shear rate [12]. The capillary viscometer is rudimentary in obtaining data for pressure versus flow rate and provides a better understanding of the disturbances in the capillary tube caused by a pressure drop. It is found in literature that excess pressure drops were larger for non-Newtonian fluids compared to Newtonian fluids as both have a similar viscosity [8,14-16].
Measuring viscosity at high temperature high shear, the capillary viscometer is operated complying to ASTM method (D5481). This test method achieves the viscosity of engine oils through a single apparatus at a fixed temperature and single shear rate. It suggests an appropriate shear rate of 1.4*106 s-1 at the wall which leads to diminution in discrepancy between this test method [13]. It is required to make calibrations with Newtonian oils with viscosities from 2 to 5 mPa-s at 150℃ to determine the viscosity of a sample oil [13]. The calibration must be done with at least four different standard oils and the sample is pressurized under direct contact with the driving gas, Nitrogen. [11,13].
The sample is introduced via a 10mL syringe which is connected to the superior part of a viscometer cell. The sample flows through the capillary freely using gravity and reaches the bottom of the filling tube inside of the viscometer cell for 15 minutes where it reaches equilibrium temperature. The volume of the sample inside the cell is precisely regulated by sucking out an amount of the sample back into the syringe before applying pressure. It has been suggested that it is better to adjust the pressure before running the test. When the “Run” switch is activated, the solenoid valve is simultaneously closed out from the atmosphere and opened for pressurized gas flow into the viscometric cell [12]. The capillary viscometer measures the flow time to 0.01 seconds with an automatic built-in digital timer. When the gas initially flows into the cell, a timer automatically starts and stops after the sample is displaced from the cell [8]. The flow time, pressure, and temperature are displayed on the screen of the portable computer. Generally, the flow time ranges from 20 to 30 seconds at pressures of 100~500 psig [12]. The recorded flow time and pressure are used to calculate the sample’s viscosity and shear rate in 106 s-1.
Table 1. Known values for Calibration
Before the HTHS experiment is operated, calibrations were performed using Newtonian standard oils. According to the ASTM method (D5481), a specific temperature is required to reach the targeted viscosity for each standard oil as shown in Table 1. The density of the standard oil is used to get the volume of the oil and corresponding flow time after completing the calibration test. The calibration is performed at least three times to get the average volume of each standard oil. When three sets of valid data are input in the calculation interface of the portable computer, the apparatus automatically saves the records and calculates the specified flow time using the average volume. Thereafter, calibration constant calculation is proceeded for each standard oil for at least three times as well. This calibration calculation measures the viscosity of the standard oil with the valid pressure and flow time.
Table 2. Measured viscosity and average value of viscosity of each standard oil
Viscosities of each standard oil have been determined as shown in Table 2. The three measured viscosities are averaged to a final viscosity that has a slight discrepancy from the known viscosities in Table 1. Errors between the known viscosity and the measured average viscosity are 1.596%, 0.882%, 0.450%, and 1.382%, respectively. These errors might have contributed to the collection of the standard oil from the outlet and oil weight measurements. Bursting is generated at the end of displacing the oil from the cell, which leads to losing some amount of the oil as seceding from a beaker placed under the outlet of the apparatus. In addition, the oil must be collected until the oil mist is completely gone by repeatedly clicking the start and stop button. This step relies on human judgment which may contribute to the inaccuracies. Furthermore, systematic error occurs while the collected oil is measured with a weight scale since it is not stabilized on reading the scale.
Through the HTHS experiment following ASTM test method D5481, the capillary viscometer is a very useful and simple way to measure the viscosity of the lubricant oil. Identifying the proper HTHS viscosity of lubricant oils is of paramount importance in the industry for determining suitable motor engines. The lubricant oils are categorized into lower HTHS viscosity and higher HTHS viscosity. Each classification has its own advantages. The lubrication oils with lower HTHS viscosity provides better fuel economy to the industry and lower greenhouse gas emission. The high HTHS viscosity accomplishes better wear protection for the engine. The most important facet for industries to focus on is finding the appropriate balance between fuel economy and engine protection for the formulation of lubricant oils.
References
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[2] “The Importance of an Oil’s Viscosity.” Machinery Lubrication, Noria Corporation, https://www.machinerylubrication.com/Read/29185/oil-viscosity-importance
[3] Mesmaeker, David D. “The Challenges of A Low HTHS Viscosity.” Q8-Oils, Kuwait Petroleum, 1 Jun 2017, https://www.q8oils.com/automotive/low-viscosity-challenges
[4] “MULTI GRADE CAR ENGINE OILS EXPLAINED.” OPIE OILS.CO.UK, Opie Oils, TecAlliance, https://www.opieoils.co.uk/t-multi-grade-car-engine-oils-explained.aspx
[5] “Viscosity Grade v. HTHS Viscosity: What’s the Difference and Why is it Important?” LUBRIZOL ADDITIVES 360, The Lubrizol Corporation, 14 April 2019, https://www.lubrizoladditives360.com/viscosity-grade-v-hths-viscosity-whats-the-difference-and-why-is-it-important/
[6] Ranowsky, Amanda. “What is the Difference Between Dynamic and Kinematic Viscosity?” CSC Scientific Blog, CSC Scientific Company, Inc., 15 Jan 2015, https://www.cscscientific.com/csc-scientific-blog/whats-the-difference-between-dynamic-and-kinematic-viscosity
[7] “Oil Viscosity.” ZPlusTM The Brief #13, ZPlus LLC, 29 Jun 2009, https://zddplus.com/wp-content/uploads/2017/05/TechBrief13-Oil-Viscosity.pdf
[8] Palekar, Vivek M. Development of A High Shear Capillary Viscometer. The Pennsylvania State University The Graduate School Department of Chemical Engineering, 1993.
[9] “Viscosity index.” Anton Paar, Anton Paar GmbH, https://wiki.anton-paar.com/en/viscosity-index/
[10] Fitch, Jim. “Don’t Ignore Viscosity Index When Selecting a Lubricant.” Machinery Lubrication, Noria Corporation, https://www.machinerylubrication.com/Read/28956/lubricant-viscosity-index
[11] “How to measure viscosity.” Anton Paar, Anton Paar GmbH, https://wiki.anton-paar.com/en/how-to-measure-viscosity/
[12] Balasubramaniam, Vasudevan. High Shear Rheology Of Multigrade Lubricants. The Pennsylvania State University The Graduate School Department of Chemical Engineering, 1992.
[13] ASTM D 5481-10, Standard Test Method for Measuring Apparent Viscosity at High-Temperature and High-Shear Rate by Multicell Capillary Viscometer, ASTM International, West Conshohocken, PA, 2010, www.astm.org
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