Oil Systems & Coking
radken
I would both hope and believe that coking on the failed engine was not a related cause given its rather short cyclic life on wing. Coking in the longer engine service life is a problem and with the higher operating oil temperatures, could begin to occur sooner as cyclic life progresses. Everything depends on accurate metal and air temperature predictions for bearings, sumps and surrounding cavities, particularly in hot turbine areas of the engine. Heat transfer analysis is extremely difficult due to the complexity of the oil flow in and around the bearings. Obviously there is prior history to fall back on, but in a new engine design (IMO, the 970 is one), rig testing of the bearing cluster would be performed to determine bearing heat characteristics and instrumented engine test data would be used to determine air and heat in the surrounding areas. Then the heat transfer analysis program would be refined and run to determine if the design is adequate or not. It is important to keep all oil wetted surfaces below 400℉ and that includes anticipated soak back heat generation. If one does not accomplish this, coking can occur in sumps along the walls and in tubing associated with the sump. The big worry in the 970 would be the roller bearings under the IPT rotor.
But, probably the most difficult item to face is to get all the vibrations/harmonics identified and dealt within the oil tube piping system. This usually winds up being iterative process during the engine testing phase, identification and correction.
To give you some ideas on maximum oil temperatures on various engines:
RB211 Series 335℉
Trent 700 374℉
Trent 800 375℉
Trent 900 385℉
Trent 1000 365℉
CF6-50 320℉
CF6-80C2 320℉
PW4000 350℉
GE90 270℉
GEnx 320℉
A main coking generation factor is, what happens when you shutdown a hot engine quickly as barit1 points out and there is no longer oil flow?
Hope this helps...