Measuring Viscosity

An experiment was conducted to model the effect of temperature on the viscosity of motor oil and compare the results to commercial values established by the Society of Automotive Engineers (SAE) for SAE 30, SAE 40 and SAE 5W30.

The viscosity of the oil sample at eight temperatures between 20 °C and 55 °C was determined using a rotary Brookfield DV-II+ viscometer. The temperature of the oil sample was controlled by the circulation of water from a Brookfield TC-502 water bath temperature controller through a water jacket assembly around the oil chamber. This type of viscometer measures the torque required to rotate an immersed spindle in a fluid.

Overall, the experiment yielded appropriate results: the viscosity of the oil sample decreased as temperature increased. However, upon research, none of the three SAE motor oil viscosity ratings were within the uncertainty of ±1% of the full-scale viscosity value of the oil sample at the given temperature. The motor oil that did match the tested sample within ±1% was SAE 10W40. The next closest was SAE 30 at ±10%. If the possible motor oils are restricted to SAE 30, 40, and 5W30, it can be concluded that it was likely SAE 30.

Introduction

Motor oil is crucial for reducing excess heat due to friction in internal combustion engines and ensuring optimal performance and minimal wear. As
engine lubrication is a highly important and relevant subject, efforts were made to explore how motor oil viscosity varies with temperature. To this end, a rotary Brookfield DV-II+ viscometer was implemented to measure the dynamic viscosity of a sample of motor oil at eight different temperatures between 20°C and 55°C. The device calculated the viscosity by measuring the torque required to rotate a spindle immersed in the oil at a given RPM, taking into account the geometry of the spindle and the type of container used for the oil. The viscometer was accurate to within ±1% of the full-scale viscosity value for a given spindle geometry and RPM, while the uncertainty for the temperature probe immersed in the oil was ±1°C.

The temperature of the motor oil was regulated by passing temperature-controlled water through a water jacket assembly that encased the oil container. The oil viscosity, the torque and RPM of the rotating spindle, and the full-scale viscosity value were recorded for each of the eight temperatures. A plot of motor oil viscosity versus temperature was then made. Dynamic viscosity values for SAE 30, SAE 40, and SAE 5W30 were tabulated and compared to the data collected during the experiment.

It was predicted that the oil viscosity and the oil temperature would have an inverse relationship. The objective of the experiment was to not only prove that this was the case, but also to examine the specific behavior of the oil under different conditions. The importance of this experiment was the identification and understanding of the relationship between viscosity and temperature and how this can affect systems that use motor oil in real world environments.

Data Tables/Graphs

Table 1. Commercial Values for Dynamic Viscosity [1]

SAE 30| SAE 40| SAE 5W30|
85.9 @ 40°C| 136.2 @ 40°C| 61.98 @ 40°C|

Table 2. Experimental Values for Temperature, Full Scale Viscosity, and Absolute Viscosity

Temperature (°C)| 20.1| 25.4| 30.1| 35.2| 40.2| 45.0| 50.0| 55.0| Absolute Viscosity (mPa·s)| 321.26| 226.80| 168.49| 125.06| 95.11| 74.08| 57.75| 46.23| Full Scale Viscosity (mPa·s)| 333.3| 230.7| 176.4| 130.4| 96.75| 74.98| 58.81| 47.61|

Figure 1. Illustration of Inverse Relationship between Temperature and Absolute Viscosity

Discussion

The effect of temperature on the viscosity of a given oil sample was observed during this experiment and the values compared to various dynamic viscosity values of different motor oils such as SAE 30, SAE 40, and SAE 5W30. During testing, the temperature of the oil was increased from 20 °C to 55 °C in increments of 5 °C. The objective of the experiment was to analyze the change in viscosity with temperature. As expected, the viscosity of the oil decreased as temperature increased. The intermolecular force of the molecules in the oil decreased as heat energy was added, which required that the spindle operate at a higher RPM in order to maintain approximately the same torque value. Since the experiment was heavily dependent on the equipment used, no possible changes to the procedure could be made to better the quality of the collected data.

The experiment produced the expected results with little room for human error. A Brookfield DV-II+ Rotational Viscometer with Rheocalc software was used along with a general purpose water bath. The equipment and software used provided sufficient data for the purposes of the experiment. However, there were a few factors that may have reduced the accuracy of the data. The water bath fluctuated in temperature, which caused a corresponding fluctuation in the viscosity readings. As observed during the experiment, small fluctuations in temperature affect the readings on the Rheocalc software.

Differences in percent-torque readings for different temperatures could also have hindered accurate data collection. The percent-torque was a ratio of the actual torque exerted on the spindle and the full-scale torque for the given RPM. The goal was to keep this ratio between 10% and 100%, but as close to 100% as possible. This was achieved by changing the RPM of the spindle; however, the inability to control the RPM in small increments gave different possible torque percentages at different temperatures. At a percent-torque greater than 100%, the equipment could sustain damage, while at a percent-torque less than 10%, the data would become unacceptably inaccurate due to the increased difference between the actual torque and the full-scale torque. Since the full-scale torque represents the ideal torque value for a given RPM, the accuracy of the data is directly proportional to the percent-torque.

The Society of Automotive Engineers (SAE) has set a standard rating system for different grades of oil; 10, 20, 30, 40, 50, etc., all denote the amount of time for a sample of oil to flow through a capillary viscometer at a constant temperature of 100 ˚C, the ideal engine operating temperature [2]. For instance, oil with a grade of 30 would take between 25 and 34 seconds at the constant temperature of 100 °C to flow through this type of viscometer. The oils designated with a W such as the SAE 5W30, represent multi-grade oils that have been tested at colder temperatures. The main difference between multi- and single-grade oil is that multi-grade oil contains a viscosity index improver that meets both warm and cold temperature specifications [3]. In theory, SAE 5W30 should act the same at a specified colder temperature as SAE30 would at 100 °C, thus reducing friction and engine wear due to dry running during cold starts. The first number and the W designate the grade of the base oil without the viscosity index improver, tested at colder temperatures, while the second number designates the grade tested at running temperature.

The results of the experiment indicated that viscous properties of SAE 10W40 were the closest to those of the oil used in the experiment. The viscosity of SAE 10W40 motor oil matched that of the oil sample to within ±1% of the full-scale viscosity value of the sample at 40 °C [1]. However, SAE 10W40 was not one of the oils in question. At a temperature of 40 ˚C, the recorded absolute viscosity of the oil sample was approximately 95.11 mPa·s. The dynamic viscosity of SAE 30 oil was given to be 85.9 mPa·s. Though the difference is nearly 10%, it is still reasonable to conclude that the oil in the viscometer was SAE 30 if it is assumed that the sample was restricted to being SAE 30, 40, or 5W30.

References

1. Oil Selection:
http://www.ratwell.com/technical/OilSelection.html

2. Understanding the SAE Motor Oil Viscosity Standard: http://www.ideas4aged.com/uploads/3/7/0/4/3704787/stan_toepfer_understanding_motor_oil_viscosity.pdf

3. Automotive Fluids:
http://www.kewengineering.co.uk/Auto_oils/oil_viscosity_explained.htm