Originally Posted By: Gokhan
Originally Posted By: chemman
I am going to defer to Molakule for a more detailed explanation, but I do not see how inverse cP divided by % NOACK (the units of the equation) can be an indicator of base oil quality.
This is a very good question! I didn't put the answer to it in the original post because I wanted to keep it brief.
My base-oil quality index (BOQI) was inspired by the following presentation:
Global perspective on lubricant specifications and how it affects base-oil quality (link)
John Rosenbaum, Chevron Global Base Oils
If you look at the two key figures of the presentation, the curves for the NOACK vs. CCS viscosity of a given base oil are approximately in the form:
(NOACK) x (CCS viscosity) = constant
In other words, they are inversely related to each other for a given base oil.
Moreover, the "constant" here is inversely proportional to the base-oil quality given by the API Group numbers (I, II, III, IV, GTL, etc.). Smaller the constant is, higher the API Group or higher the quality of the base oil is. Therefore, the inverse of the constant directly gives the base-oil quality. Therefore, I define the base-oil-quality index (BOQI) as follows:
BOQI = 2,000,000/(NOACK x CCS viscosity)
Here 2,000,000 cP was chosen to make the BOQI approximately between 10 and 100.
As I mentioned in the original post, it's crucial to compare the BOQI using the CCS viscosity at the same temperature for all oils, 5W-xx oils being reported at -30 °C for example. This makes it hard to compare a 5W-xx oil to, say, a 0W-30 oil because chances are very high that you won't find the CCS viscosity for them reported at the same temperature.
One caveat that I didn't mention in my original post: Viscosity-index improvers (VIIs) in the finished oil may somewhat alter the CCS-viscosity numbers for a base oil but for comparison purposes this doesn't seem to be an issue.
Note that NOACK is not really dimensionless but has the units of inverse time, as it represents the evaporation rate (as in percent mass loss per unit time). However, when it's expressed, the time period in which it's measured (1 hr = 3,600 s) is omitted (or set to "1") for simplicity. So, the BOQI has actually the units of not inverse dynamic viscosity but
inverse pressure. It's therefore an intrinsic physical property of the base oil.
Perhaps this pressure can be thought of as a tiny pressure, which is about 1 - 2 nanoatmosphere for a typical base oil, caused by the defects in the base oil, and the smaller the pressure is, less the number of defects in the base oil is and higher the quality of the base oil is.