Originally Posted By: FowVay
Even with polyol ester oils coking is a major concern.
FowVay, as you probably know, there are different grades of jet turbine engine oils with the differences mostly related to coking tendency. The older 2nd generation products such as Eastman 2380 (formerly Air BP and Exxon), Mobil Jet II, and AeroShell 500 often created coking issues in certain engines, such as the Pratt JT8D-200 series, RR Trents, and some others depending on operating conditions. Mobil Jet 254 and AeroShell 560 are 3rd generation oils with improved coking tendencies, and today the 4th generation oils are even better. These include Eastman 2197, Mobil Jet 387, and AeroShell Ascender. All of these generations use polyol esters exclusively, and even though the additives play the major role in coking, there are coking differences among the various polyol esters as well.
The Eastman 2197 led the way and set the standard for 4th generation oils, utilizing both a low coking polyol ester and an advanced anti-oxidant. This oil was developed by Exxon in the 1990s, and I worked closely with them in the development of the patented polyol ester. It is the largest selling 4th generation oil. I also developed the base ester for Shell's Ascender.
The high temperature capacity of modern jet engine oils is probably maxed out and I wouldn't expect a 5th generation oil any time soon as even some the 4th generation oils are still in the approval process. Since it takes a good ten years to develop and approve a new oil, probably a lot more with a different chemistry base oil, polyol esters will continue to be the exclusive base oil for many years. Current engines are designed around the performance of current oils, not the other way around.
If you are using older generation oils in your CF6 engines you may want to consider changing to a 4th generation oil. If you are using military spec oils instead of branded products, the 23699 HTS oils are also low coking.
Tom NJ