"What it Takes to be a Commercial Aviation Jet Engine Lubricant"

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What it Takes to be a Commercial Aviation Jet Engine Lubricant
Debbie Sniderman
FEB 28, 2020

Below is just an excerpt...
Quote
Chemistries: Esters
Performance from today's jet engine lubricants must go well beyond the capability of petroleum-based oils. That's why most of them are based on synthetic polyolesters, which provide the needed thermal stability. Esters are synthetic fluids formed by reacting fatty acids and alcohols, producing water as a byproduct, which is driven off with heat. One of the primary raw materials that serves as the source of fatty acids is palm kernel oil or coconut oil.

Esters are polar molecules and very hygroscopic, having a natural affinity for water and readily absorbing it from the atmosphere. So, it is critical to keep ester-based lubricants away from water. Water contamination of ester fluids can cause hydrolysis, which may form acids, especially in sealed systems. This is a battle the aviation lubrication industry fights continuously.

To keep lubricants safe from this threat, they are packaged in metal, which should always be kept sealed until use. Opened containers should be used quickly to protect it from absorbing water from the atmosphere.


There are two basic types of esters in use today: diesters and polyolesters. Diesters are relatively simple molecules compared to the more complex polyester molecules. Diesters have slightly lower viscosities than polyolesters, and both have good fluidity at cold temperatures compared to other lubricants. This is important because they must routinely work in temperatures of −50° C when aircraft are flying...
 
Noticed that in Tokyo watching a tech top up a GE90 with a Mobil Jet juice …
 
great info for sure! extreme conditions require extremely good lubricants "built" to suit the applications!
 
Have run Rayco 899 in my 2 Eaton superchargers since their birth, never had a failed component.
 
Turbine oils are low viscosity synthetics... no worries about premature wear of their turbine gears because either with a viscosity of 10 or 5 Oil still retains its protective nature...

Shell
AeroShell Ascender_TDS
23cSt @ 40C
5.40cSt @ 100C

AeroShell Turbine Oil 2_
10.5cSt @ 40C

AeroShell Turbine Oil 390
12.9cSt @ 40C
3.4cSt @ 100C

AeroShell Turbine Oil 500
25.2cSt @ 40C
5.1cSt @ 100C

AeroShell Turbine Oil 555
29cSt @ 40C
5.4cSt @ 100C

AeroShell Turbine Oil 560
23cSt @ 40C
5.2cSt @ 100C

AeroShell Turbine Oil 750
36cSt @ 40C
7.5cSt @ 100C

Exxon Jet 254
26.4 cSt @ 40ºC (102 ºF)
5.3 cSt @ 100ºC (212 ºF)
cSt @ -40 C (-40 ºF) 11,500

Understanding synthetics means more refining to remove less desirable
elements from crude until what's left is mostly higher-performing
molecular structures. Waxy stuff had to go because it made oil congeal
at winter temperatures. Aromatics had to go because they lost
viscosity too rapidly when hot. Unsaturates had to go because they
were vulnerable to heat-driven gumming and sludging. And so on.

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Last edited:
Originally Posted by thastinger
Have run Rayco 899 in my 2 Eaton superchargers since their birth, never had a failed component.


Royco899 MSDS... basically a 10 grade synthetic...

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Last edited:
Originally Posted by Barkleymut
I wonder if aviation mechanics have a forum and debate the difference between 10w30 and 0w16?


Viscosity difference between a 30 grade and 16 grade can be as low as 3.2 cSt in kinematic (gravity) flow...

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Originally Posted by Barkleymut
I wonder if aviation mechanics have a forum and debate the difference between 10w30 and 0w16?

A&Ps aren't that stupid.
 
Overall a very good article but a few comments regarding the chemistry.

Polyol esters only absorb water from the atmosphere up to a water content of about 0.03 to 0.05%, and can only hold a maximum about 0.2% in solution. This is not a problem in aviation turbine engines, indeed the industry specifications do not even include a water content requirement. Once that turbine starts any slight water content is quickly removed. The oils are packaged in steel cans because plastic bottles filled with a light polyol ester tend to cavitate and the additives transgress through the bottle causing staining. Hermetic steel cans can preserve the oil literally for decades and are not resealable, so the whole can is used or any left over is discarded. Plastic bottles are more likely to be left open and are more subject to cross contamination.

Fatty acids derived from palm kernel or coconut oils are usually less than 25% of the acids used in jet engine oil esters. The majority of the fatty acids are C5 from a synthetic process, and C7 from either synthesis or castor oil. A few products contain some C9 acid from synthesis or tallow.

Diesters as a family do not have lower viscosities than polyol esters. Both are readily available with viscosities of 2-12 cSt @ 100°C. Diesters are no longer used in either the MIL-PRF 23699 (5 cSt) or MIL-PRF-7808 (3 cSt) oils, only polyol esters, which constitute about 94-96% of the formulation.

For further reading see: Jet Turbine Oils

You can download the MIL-PRF-23699G specification here: MIL-PRF-23699G
 
Originally Posted by Tom NJ
Overall a very good article but a few comments regarding the chemistry.

Polyol esters only absorb water from the atmosphere up to a water content of about 0.03 to 0.05%, and can only hold a maximum about 0.2% in solution. This is not a problem in aviation turbine engines, indeed the industry specifications do not even include a water content requirement. Once that turbine starts any slight water content is quickly removed. The oils are packaged in steel cans because plastic bottles filled with a light polyol ester tend to cavitate and the additives transgress through the bottle causing staining. Hermetic steel cans can preserve the oil literally for decades and are not resealable, so the whole can is used or any left over is discarded. Plastic bottles are more likely to be left open and are more subject to cross contamination.

Fatty acids derived from palm kernel or coconut oils are usually less than 25% of the acids used in jet engine oil esters. The majority of the fatty acids are C5 from a synthetic process, and C7 from either synthesis or castor oil. A few products contain some C9 acid from synthesis or tallow.

Diesters as a family do not have lower viscosities than polyol esters. Both are readily available with viscosities of 2-12 cSt @ 100°C. Diesters are no longer used in either the MIL-PRF 23699 (5 cSt) or MIL-PRF-7808 (3 cSt) oils, only polyol esters, which constitute about 94-96% of the formulation.

For further reading see: Jet Turbine Oils

You can download the MIL-PRF-23699G specification here: MIL-PRF-23699G



Great info Tom, thanks. What sparked my interest in this area was watching translated German documentaries on the Me262 and what oils were used to keep it flying...granted, which wasn't very long since the metallurgy of the turbine blades wasn't as advanced as the overall axial turbofan technology.
 
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