Pyro (EGT) vs Oil Temp for Turbo Cool Down?

[Axion]

I have been thinking about this "turbo cool down" issue since I got my CTD 9 months ago. I have read about a million posts that debate pre-turbo versus post-turbo pyrometer installation. I understand how EGT can indicate problems and keep one from melting a motor. The part I have an issue with is using this parameter for turbo cooling.



I realize we do not want to just shut down our engines after long and hard use. Ultimately we want to ensure the oil is not baked onto any critical rotating parts. I can see how monitoring EGT will indicate a drop in turbo temp, but if we're worried about baking the oil, why are we not monitoring the temperature of the oil exiting the turbo?



If we can measure the temperature of the oil in the pan, and can measure the temperature of the oil exiting the turbo, we can indicate a differential between the two. With some research and trial and error, we could come up with a differential temperature that we could consider normal. After working the engine hard, monitor this differential until it drops to a "normal" level and then shut it down.



Now, I'm not an electrical or instrumentation engineer, but I would think setting up two sensors in some sort of circuit to indicate a differential should not be that difficult. I'm not advocating ignoring EGT but would the oil temperature differential be considered a reasonable way to monitor turbo temp? Am I out to lunch? I appreciate the wisdom of those that have a more intimate knowledge of this engine than I.

 

[Mundgyver]

Your oil will run 10 to 15* hotter than your water temp. I have seen my oil temp top 245* on a long uphill pull with 23,000lbs. This is at the turbo inlet. The concern over coking is the big reason I run full synthetic. Good to 400*



As long as you let the engine cool down (idle time) and get the temp in the 300* or close you should be fine. Remember you are pulling heat out of the turbo bearings as you idle which is not a problem.



Heat soak is the enemy when shutting down to quick and not allowing time for the heat to exit the tailpipe.

 

[Axion]

Thanks for the info and sorry for taking so long to respond. Been away at work for a while with very little internet access.



So I think what you are trying to tell me is that the relatively cooler exhaust gas passing through the turbo at idle will extract more heat than the oil passing through the bearing section during the same time period. Makes sense to me.



Thanks again.

 

[rbattelle]

I think the oil temp idea is logical but impractical. You'd want to mount a thermocouple or RTD in the drain line close to the turbo without negatively impacting the oil flow to the bearing.



I think we should also remember that there is no air circulation through the bearing. Because of that, the EGT reading is not a "first order" measurement of bearing temperature - it relies on heat transfer from the bearing compartment to the EGT probe via the turbo case and exhaust elbow. So, strictly speaking, it's possible to read 300 degrees post-turbo and still have >300 degrees at the bearing. Always wait until you're below 300 degrees to shut down.



In that sense, then, a measurement of oil temperature leaving the turbo bearing would be a "better" measurement of shutdown temp than any pyrometer. Even better would be to mount a thermocouple directly to the outside of the turbo bearing case! Hmmm... that sounds like an interesting idea.



Ryan