Change in evaporation rate over time?

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5
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Pacific Northwest, USA
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Let's say an oil has a published Noack value of 6% (tested on a virgin sample). At the end of the oil's service interval, say 20,000 miles, would the used oil's Noack value still be 6%? Higher? Lower? We know that most, if not all, lubricant properties change during use. I haven't see any discussion or data regarding how the evaporation rate changes, though. Just something I'm curious about in an academic/hypothetical sense. smile I expect the answer is that there is some difference, but not significant enough to be meaningful.
 
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14,885
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NE,Ohio
probably depends on specific application. if you have one of those honda 1.5turbo engines that can get into double digit fuel dilution.. that could be totally different from a port injected 2L elantra for example.
 
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177
Location
FL, USA
I would expect volatility to either not change or go down a hair as oil picks up soot and other wear metals.
 
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177
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FL, USA
Originally Posted by Rand
If you have one of those honda 1.5turbo engines that can get into double digit fuel dilution.. that could be totally different from a port injected 2L elantra for example.
Yeah I didn't consider fuel dilution. Would expect that to increase volatility of the mix, but you would really hope it would flash off and be carried into combustion.
 
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15,146
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N.H, U.S.A.
I had a similar question. Easy to do an experiment - OUT of Doors on a hot plate. In a graduated beaker. Its HAS to improve after an initial boll down. The upper volatile solvents will "off" first. So an oil may show 10% off in the first hour and then maybe 6% off in the next hour.
 
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1,130
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California
I agree that evaporation should decrease over time. But I suppose that the shearing effect over time also has an effect on volatility... shearing causes reduced viscosity / increase in lighter components? Maybe wrong, just a thought.
 
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3,763
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down in the park
No, as it's not the oil molecules that shear, but the viscosity improvers that are mixed in the oil. Shear should have only a minimal effect on volatility.
 
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