Menu Sign In Contact FAQ
Banner
Welcome to our forums

Engine management / leaning / peak / lean of peak (merged)

A discussion about John Deakin on another thread reminded me that I recently came over this rebuttal about an article by Lycoming which makes “fun” of John Deakin and other experts (without quotation marks) advocating LOP operations.

The funny thing is that the Lycoming article (part #SSP700) has vanished from Lycoming’s website, but it is possible to find it on other sites.

John Deakin’s rebuttal is interesting reading, especially his description of LOP operations on the old, giant radial engines that equipped yesterday’s airliners.

I have been reading John Deakin’s Pelican’s Perch column and Mike Busch’s The Savvy Aviator on AvWeb ever since I started flying in the mid-1990s and have always found their arguments immensely convincing, especially since they are backed up by the tests they have performed on their instrumented engine test bed. Another compelling argument is the TTSOH achieved on Mike Busch’s engines on which he applies what he preaches. Another convincing demonstration I saw was flying a twin with one engine ROP and the other LOP, but with power perfectly symmetrical, and observing the difference in the engine temperatures R/L.

Ever since, I have tried to keep the engines clean and cool by flying 65% at peak EGT (or thereabout) and limiting the CHT to 360°F, or lean of peak (if possible).

The one thing which is however difficult to grasp, even for a believer like myself , is why EGT does not matter, and hence why we can disregard the absolute value of the EGT and focus solely on CHT

LFPT, LFPN

Watch at 07:50.


Last Edited by boscomantico at 14 Jan 15:47
Mainz (EDFZ) & Egelsbach (EDFE), Germany

Aviathor wrote:

The one thing which is however difficult to grasp, even for a believer like myself , is why EGT does not matter, and hence why we can disregard the absolute value of the EGT and focus solely on CHT

As I have read it in their articles, it is because a slight variation in the placement of the sensor would lead to a relatively large change of temperature, of course without a change in the real temperature of the exhaust gases. So even between the 6 (or 4) cylinders you would see differences which are not due to a real difference in the temperature.

Second argument is that it varies so rapidly that any measurement is only an integration of everything that passes by. So you measure an “average EGT”, but what does that tell you if you want to use it as a limit?

Thirdly, at the point where you measure the temperature of the exhaust gases, the absolute value doesn’t matter anymore as it has already cooled off. Contrary to a TIT, which will go into the turbo afterwards and give off its heat there, EGT is already “on the way out”. If you really want to use EGT as a limiting factor for your engine, you would want to measure it inside the cylinder. But then the heat in the exhaust gas only matters so much as it is able to transfer heat to the cylinder itself. And this effect can be directly observed much better by looking at CHT.

Last Edited by Rwy20 at 14 Jan 15:52

boscomantico wrote:

Watch at 07:50.

Yes. I’ve seen this before. Those webminars are absolutely priceless, and the Q&A at the end are often even more enlightening.

But it means that the exhaust valves (when open) are exposed to gases that are at least the temperature reported as EGT. Of course, during the power cycle they are exposed to temperatures that are even higher (4000°F). I looked up the melting temperature of Co-Cr and it seems to be around 2700°F

The question I am getting (from non-believers ) is why limiting EGT (even disregarding the absolute value) isn’t better for exhaust valve life? Of course it does not help when people have seen scientific articles that say that one must limit the exhaust temperature and that the sum of CHT and EGT should be kept below some number.

Last Edited by Aviathor at 14 Jan 16:29
LFPT, LFPN

Aviathor wrote:

The question I am getting (from non-believers ) is why limiting EGT (even disregarding the absolute value) isn’t better for exhaust valve life?

Watch at 24:25.

Bonus answer/question: Why, on a turbo, is TIT always showing higher than EGT? It’s measured further down the exhaust system, so shouldn’t it be cooler?

Last Edited by Rwy20 at 14 Jan 16:40

Turbo powered aircraft indicate an average value of a number of cylinders. Usually 4-6 cylinder exhaust strokes.
On Naturally aspirated aircraft you have an average value of one cylinder, again only during the exhaust stroke. The other 3 colder cycles are not shown seperately.
Therefore a higher collective average temperature is observed on a turbo powered aircraft.
Absolute EGT values are only a relative indication.

How does that work?

Administrator
Shoreham EGKA, United Kingdom

@complex-pilot I didn’t really understand what you’re saying, but it doesn’t sound right. If you only have one EGT gauge, it is mounted in the exhaust of one cylinder, I think. Same goes for CHT, if you have one gauge, it is stuck to one cylinder, not necessarily the hottest.

What I meant was this, where the TIT (1620°) is constantly higher than any of the EGTs (1425° to 1505°) in cruise:

Last Edited by Rwy20 at 14 Jan 19:48

Aviathor wrote:

The one thing which is however difficult to grasp, even for a believer like myself , is why EGT does not matter, and hence why we can disregard the absolute value of the EGT and focus solely on CHT

The valve termperature is determined by three factors: EGT, CHT, and how well the valve is seated. The valve spends very little time exposed to the exhaust gas on its own, most of the time is it seated in its opening; and while the area of the valve that is exposed to the exhaust gas stream is larger, the temperature of the valve is much closer to the CHT than the EGT. So poor valve seating, carbon deposits etc. are more likely to “kill” the exhaust valve than high EGT.

Biggin Hill

Rwy20 wrote:

What I meant was this, where the TIT (1620°) is constantly higher than any of the EGTs (1425° to 1505°) in cruise:

Best theromodynamic guess: higher (static) pressure?

Biggin Hill
Sign in to add your message

Back to Top