I am investigating a case of hydraulic oil (SAE10) blackening on a substantial number of hydraulic excavators. Preliminary oil analysis showed that oil oxidation and deterioration is not the cause of blackening. I have read previous message board discussion on the Dark colored oil and and suspect that we may have a situation of micro-dieseling. Would appreciate if you could help me with the following questions:
1.What other method can we use to test if oil has undergone micro-dieseling in a hydraulic system?
2. What extent of micro-dieseling would need to turn a hydraulic system of 585Ltrs,tank+system capacity (175+410)into black condition?
3. Would using SAE30 or a 15W40 grade oil produce a different outcome?
4. Would present of goldish color on component in the hydraulic system indicate that micro-dieseling is happening at that component?

The blackening of oil is due to coke formation
of oil.Usually inadequate stability of base oil
(Group I)results in dark color of oil.

SAE 10 oil has to be selected only if your
ambient temperature is dropping to sub-zero.
SAE 10 should not be used if ambient temperature
is greater than 40'C.

Monitor cooling system performance.High coolent
temperature increases hyd oil temparature and
subsequent dark color of oil.

Regards

Bala.

I'm also investigating similar problem with mining shovel/excavator fleet, but not sure how much the blackening affect the machine reliability or performance.

Micro-dieseling is caused by the implosion of air bubbles as they move from a low pressure zone (reservoir) to a high pressure zone (pump). When the bubble implodes, a very high temperature is created essentially cracking the hydrocarbon molecule. This typically produces by-products with both light-ends and heavy-ends. The heavy-ends will agglomerate and form deposits, (the golden coating that you referred to) while the light-ends may flash off. This explains why micro-dieseling in hydraulic oil develops a distinct sulfur odor, not unlike rotten eggs. Does your oil have a peculiar smell? If so, micro-dieseling may be the cause.

From an analytical standpoint, FT-IR is a good method to detect micro-dieseling. I prefer to view the used oil graph overlaid on top of a new oil graph to interpret the results. Look for a peak in the 1600-1630cm-1 range.

Finally, switching to a heavier viscosity fluid will not help out your situation, as it will take even longer for air bubbles to escape. Exhaust all efforts to minimize the air in your system. Is it possible to look in your reservoir while your system is running to see if there are any air bubbles? You may want to investigate the “bubble eliminator”, an innovative product designed to remove air from a system.

Best of luck.

Greg,

Compliments to you for the simple but detailed explaination of micro-dieseling.

Hussam Adeni

Hi Greg!
Your explanation is really wonderful!
Regarding

quote:

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From an analytical standpoint, FT-IR is a good method to detect micro-dieseling. I prefer to view the used oil graph overlaid on top of a new oil graph to interpret the results. Look for a peak in the 1600-1630cm-1 range.

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, can u plz. explain it a bit more elaboratly?
TK

Dear Tapa,

A lab will typically report an "oxidation" or "nitration" value. I prefer to look at the entire graphical overlay. An example of a portion of the graphical overlay that is attached. (If the file doesn't appear, I'll e-mail it to you.) Micro-dieseling, a kind of thermal degradtion, produces molecular by-products that fit in the nitration range.

I hope this helps.

FTIR-Thermal_vs_oxidative.gif (20 Kb, 374 downloads) FTIR chart

Would really appreciate if anyone could help me with the following questions:

-what is the exact chemical reaction involved with Micro-dieseling?

-Does micro-dieseling produce NOx nitrated species like in combustion of petrolum products?

-What chemical actually produced the strong odour of oil after micro-dieseling occured?