In medium speed engines. How can it be checked if a rise in viscosity is due to poor combustion, lube degradation or residual fuel contamination? I read somewhere that flash point may not be the best indicator since heavy fuels (residual, fueloil, crude) could have a higher FP than diesel and also because the volatile fractions could evaporate.

Could the cause of visc increase be checked through the % variation of oil parameters TBN, pentane insolubles, wear?

Thanks in advance

V and Ni values usualy indicate fuel oil dilution. Increase of TBN would point to evt. cylinder oil.

The question i post is because there are some analysis in medium speed engine oil where viscosity may show an abnormal behaviour other than expected according to pentane insolubles (D893B). an increase in pent insol may lead to an increase in viscosity

IN one engine the lab reported these values:
Analysis 1. 16,7 cSt/ 1,46%
Analysis 2. 18,1 cSt/ 1,50 %

In other engine we received these values:
Analysis 1: 18 cSt/ 1,38%
Analysis 2: 17,78 cSt/ 1,68%

OEM limits are:
Pentane Insolubles: Max 2%
Viscosity 100C: + 25%

The values where confirmed by the lab
What could be causing this results?
Thanks for your support

Ernique,

I would lsuggest 2 things. First have FTIR and spectrographic metals analylsis done on oil in addition to viscosity. The FTIR can eacily sort out the fuel / oxidation issues and the spectrograph will help with other wear related issues. With soot / solids concentration this high I might consider small Centrifuge cleaner See:

http://www.albion.com.gr/mar.htm

Enrique,
First I need to assume the viscosity you reference is for a XXW-50 SAE grade oil. The min/max vis @ 100 is 16.3 to 21.9. So I need to say you appear to within spec. If your past analysis shows that you are consistently at say 17.5 then you are well within specifications (for the viscosity) and should then be looking at the additional oil analysis parameters to determine if you are operating efficiently and without inherent engine problems. Chasing viscosity can be misleading because as you have said fuel dilution will normally LOWER the vis while TBN decrease (acid build up) will increase the vis. So all of these have to be assessed in parellel with the vis to assure your conclusions are supported by the information. Preferentially we want to have the data draw the conclusion and then for the human mind to qualify the data.
Regards.....