lml@lancaironline.net Mailing List Archive

Gentlemen,

This is my comments in answer to both Walter's and George's response to my posting regarding LOP on a TSI0-550E

I first want to thank both of these gentlemen for there informative participation in the LML. I also want to state that these guys are by far the experts on engines and I am merely an old ME engineer with enough knowledge to get myself into trouble. I have posted my credentials at the end.

Walter wrote;

I don't know upon what data you are relying to make that assumption, but it is incorrect. The hottest mixture for exhaust valves is found at 25dF ROP. LOP mixtures run the exhaust valve MUCH cooler than ROP mixtures. That was first proven by a NACA report in 1943, subsequently reported by Lycoming in 1966 and again confirmed by GAMI's test facilities in 2004. Raw EGT value has essentially nothing to do with exhaust valve temperature.

I agree with what is said here. However, I run at least 100F ROP not 25F ROP and sometimes much more, like at full power I think I'm about 300F ROP. Therefore, I believe that the ROP has excess fuel which should cool the burn and just maybe leave MORE lead on the valve stem than while LOP.

The major factor in guide wear is related to MANUFACTURING fit, and not mixture management.

I think this comment fits my background and I can only confirm that my New cylinders that were provided by TCM came from the factory out of spec. I had all the valve's removed, measured, and hand lapped into the seat to insure a full wide seat and correct valve to stem clearance. On two cylinders the exhaust valves would not clean up to provide a uniform seat all around with just lapping and had to be reground. This is unbelievable and therefore would have had a short life if put into service that way. I suspect that the original cylinders that came on the new engine had this problem also.

In all fairness to TCM on my original installation, which used Lancair's baffling and a New Dukes fuel pump, I had very hot #2 cylinder all the time and was my gauge for temp management. When we removed all the cylinders this same cylinder was Fine=Perfect when three others had less then 40/80 compression. Does this say High CHT's doesn't matter?

Also, after having a TCM rep. go over all my early problems he told me to replace the Electric fuel pump. I almost thought this was a joke and a real diversion to their engine cooling problem. Much later I changed the fuel pump and he was correct. ( longer story).

George wrote;

Ah… that is the conventional theory and wisdom - - that is - - that HOT EGTs translate into HOT exhaust valves. It just turns out that isn’t true. The value of the exhaust gas temperature flowing past the exhaust valve is often inversely proportional to the temperature of the exhaust valve.

I ************* KNOW *********** that comment appears to be completely crazy and verging on the area of silly.

But there is good hard data on this subject going back to the 1940s’ That data shows that the exhaust valve temperatures follow the CHTs - - NOT the EGTs.

And by advancing timing, as just one example as to why this is true, one will LOWER THE EGTS - - but that will make the exhaust valves run very much hotter (along with the CHTs).

By retarding the timing a lot, the EGTs go VERY HOT - - but the exhaust valves get very cool - - along with the CHTs.

In short, EGTs do not correlate with exhaust valve temperatures very well at all.

I do understand this because the exhaust valve is on the closed seat for 3 or the 4 cycles, and is only heated during the one exhaust cycle. In the real world the condition of the seat has a very large effect on the cooling thermal time constant. And we can't ignore the EGT temperature effect to the Oil film on the exposed valve stem. Please, note that on my second set of cylinders that after 82 hrs the Stem's and Guides were wore out of spec. only on the first 1/3 of the valve guide length. This says that the guide is not lubricated enough and/or the stem diameter is too small in diameter, making a smaller bearing surface. This could be solved by having sodium cooled valves which require a larger stem, and using better lubrication.

BTW, I talked to 4 different engine builders about the above and they all commented "probably ALL IO and TSIO TCM engines have the same valve ware at 82 hrs but don't show compression loss until 400 hrs or more" For the record my engine was overhauled after 82 hrs because of a lifter disintegration problem which replaced everything that sees oil.

>> My second set of cylinders had stem and guides wore beyond limits at 82 hrs and I hadn't gone LOP for the first 40 hrs and then maybe half the time for the next 42 hrs. This is the mechanical fuse, so to speak, of the Continental engine, the exhaust valves. Now I run ROP and let the fuel save my exhaust valves and save money. I use about 4 or 5 gal. more on a 540 nm flight and go faster and quicker.<<

I suspect your problem was the dimensional fit of the exhaust valves/guides before you ever started your engine the first time. George

George, I hope I answered the fit issue with respect to the second set of cylinders? I also did that same routine on the present set of cylinders. Also, after this present overhaul I have done major baffling sealing similar to what George has shown in the past plus more between cylinders, and this time around I can now keep full power on up to what ever altitude I need without CHT's going over 405F. I, however, would like to swap #6 and #2 injectors to get a better distribution but then I should probably just fly to Ada and get it really balanced?

Sincerely,

Jim Hergert, BSME with manufacturing option, 40 yrs. in Tool design, plastic molds, die casting, Sheetmetal mfg. with State of the art, NC machines, Lasers, automation and Solid modeling CAD systems. Material's and Just In Time specialty. 250 employees. All in the past. So I built a plastic airplane.
L4P, N6XE
(An Sexy)
409 hrs.