Ruh Roh

Image: doctor flying a v-tailed Bo.....LOL.
 
As long as everyone understands that is a satire site. I've noticed that some do not.
 
  • Like
Reactions: Ted
And after seeing the picture posted I figured we were going for the gear up landing.
 
Ted DuPuis said:
As an engine guy, I agree 100% with the statement that "most pilots don't understand it."

All of you, I'm looking at... all of you. ;)
Click to expand...

Ted,

What parts do people not understand?

I run LOP... Wish my instrumentation was a bit better, but I do have all-cylinder CHT/EGT, just no lean assist and fairly poor resolution.

Here's what I understand:

1) Every cylinder peaks at a slightly different time during leaning, mainly due to air distribution.
2) If you're running ROP you need to be rich of the leanest cylinder's peak (the first cylinder to peak as you are leaning the mixture is the leanest). If you're running LOP you need to be leaner of the richest cylinder's peak (the last one to peak). There is a range in which you'll have some cylinders ROP and some LOP.
3) The EGT is as hot as it is at peak because the combustion is the slowest, thus there's more heat reaching the sensors.
4) Peak should be just about at the perfect stoichiometric mixture. Rich of peak means there will be unburned fuel coming out of the exhaust, lean of peak means there will be some oxygen remaining in the exhaust that wasn't used for combustion. In theory, if you were perfectly at peak (and all your cylinders peaked at exactly the same mixture), there would be neither fuel nor oxygen remaining, in an ideal world. In reality, there would be some of both because they weren't in the same place at the same time during combustion and thus could not be combined.
5) The absolute value of the EGT is not important, as it will vary depending on conditions. Only the value relative to peak EGT matters.
6) CHT matters because it's an indicator of internal cylinder pressure and cooling, and cylinders get weaker pretty fast above 400ºF CHT. However, it's a lagging indicator and thus is not useful for leaning. Thinking of it like control-performance for instrument indications in IFR, EGT is control, CHT is performance.

There's more, of course, but that's what comes to mind immediately. How am I doing?
 
Last edited:
flyingcheesehead said:
Ted,

What parts do people not understand?

I run LOP... Wish my instrumentation was a bit better, but I do have all-cylinder CHT/EGT, just no lean assist and fairly poor resolution.

Here's what I understand:

1) Every cylinder peaks at a slightly different time during leaning, mainly due to air distribution.
2) If you're running ROP you need to be rich of the richest cylinder's peak. If you're running LOP you need to be leaner of the leanest cylinder's peak. There is a range in which you'll have some cylinders ROP and some LOP.
3) The EGT is as hot as it is at peak because the combustion is the slowest, thus there's more heat reaching the sensors.
4) Peak should be just about at the perfect stoichiometric mixture. Rich of peak means there will be unburned fuel coming out of the exhaust, lean of peak means there will be some oxygen remaining in the exhaust that wasn't used for combustion. In theory, if you were perfectly at peak (and all your cylinders peaked at exactly the same mixture), there would be neither fuel nor oxygen remaining, in an ideal world. In reality, there would be some of both because they weren't in the same place at the same time during combustion and thus could not be combined.
5) The absolute value of the EGT is not important, as it will vary depending on conditions. Only the value relative to peak EGT matters.
6) CHT matters because it's an indicator of internal cylinder pressure and cooling, and cylinders get weaker pretty fast above 400ºF CHT. However, it's a lagging indicator and thus is not useful for leaning. Thinking of it like control-performance for instrument indications in IFR, EGT is control, CHT is performance.

There's more, of course, but that's what comes to mind immediately. How am I doing?
Click to expand...

I think you got that backwards. When Rich of Peak you want to be rich of the leanest cylinder's peak (all the other cylinders are richer still). When running Lean Of Peak you need to be lean of the richest cylinder's peak (since all the other cylinders are leaner still). You want to avoid having one cylinder closer to peak (which can be the dreaded "red box").
 
Kent,

The initial post was satire. I made a (semi-joking) response, which was intended to be semi-satire, but also half true. :)

Honestly, most people don't get LOP. Your understanding is pretty good, but has a few holes.

At risk of turning a joking thread into a serious one.

1) Your #2 point is one that has some level of debate. Ultimately, you want to keep all cylinders out of the 25-50F ROP range if you're trying to actually keep things in a happy range. Now, if your cylinders have a close spread (the "GAMI-spread" as the GAMI folks like to call it) then really this shouldn't be a big deal. If you have such a wide spread that at an appropriate LOP setting you've got some cylinders ROP, then your engine probably can't run LOP effectively without running rough. But I like to keep everything on the lean side.

2) Your #5 point I disagree with. Metal melts at particular temperatures. While most LOP zealots disagree, I will say that people who run high EGTs are more prone to burned exhaust valves (especially on Continentals) and turbo issues. I still try to keep at or below 1550F if I can. On turbo engines sometimes 1600F EGT is all you can do. A lot of people disagree with me on this point and say "But the POH doesn't have an EGT limit!" Right, because per the POH's performance charts and the factory instrumentation, you weren't going to end up in an area above limits.

3) CHTs aren't useful by themselves for leaning, but a lot of times they influence how you lean. And on a Continental (at least on the D rocker Continentals) I found that leaning to 365F would tend to actually produce the same airspeed at a lower fuel flow vs. 380F because the engines ran more efficiently. This was on the 310 which had very good baffling (after I spent a whooooole lot of time and RTV on it).
 
No.. most people don't get it.. as evidenced by a lot of threads here

Unfortunately a lot end up running at peak or just barely rich of peak when they think their LOP
 
Initial Fix said:
Was this published on April 1?
Click to expand...
That was my guess. Why does the FAA care? Continental pretty much says once you're at max continuous (or below) you can set the mixture anywhere from FULL RICH to so lean that the engine is misfiring without any worries as to engine damage. Lycoming doesn't go that far, but their "red zone" is actually RICH OF PEAK not on the lean side.

I have my doubts about a "news site" that uses a poorly formated Wordpress web site that is populated only with random adsense ads (i.e., no real relevant sponsorship). It looks like a so-called "parody" site. It's pretty obvious from the other inane articles (banning pilots from cockpits, etc). It's obvious that the morons involved are oblivious to the fact that the point of parody is to be entertaining (much like the scum at duffleblog).
 
jsstevens said:
I think you got that backwards. When Rich of Peak you want to be rich of the leanest cylinder's peak (all the other cylinders are richer still). When running Lean Of Peak you need to be lean of the richest cylinder's peak (since all the other cylinders are leaner still). You want to avoid having one cylinder closer to peak (which can be the dreaded "red box").
Click to expand...

You are correct... I did not say that right. I was thinking about the first cylinder to peak vs. the last and got them backwards. I'll go back and fix it.
 
Ted DuPuis said:
Honestly, most people don't get LOP. Your understanding is pretty good, but has a few holes.

At risk of turning a joking thread into a serious one.

1) Your #2 point is one that has some level of debate. Ultimately, you want to keep all cylinders out of the 25-50F ROP range if you're trying to actually keep things in a happy range. Now, if your cylinders have a close spread (the "GAMI-spread" as the GAMI folks like to call it) then really this shouldn't be a big deal. If you have such a wide spread that at an appropriate LOP setting you've got some cylinders ROP, then your engine probably can't run LOP effectively without running rough. But I like to keep everything on the lean side.
Click to expand...

Yeah, it's hard enough to get people on the right side of peak, I'm not gonna even start on how far rich or lean of peak they should be. ;)

As I understand it, 25-50º LOP is good enough, while 50º ROP where a lot of old POHs tell you to run is about the worst spot to be. 100-125º ROP is about where best power is.

2) Your #5 point I disagree with. Metal melts at particular temperatures. While most LOP zealots disagree, I will say that people who run high EGTs are more prone to burned exhaust valves (especially on Continentals) and turbo issues. I still try to keep at or below 1550F if I can. On turbo engines sometimes 1600F EGT is all you can do. A lot of people disagree with me on this point and say "But the POH doesn't have an EGT limit!" Right, because per the POH's performance charts and the factory instrumentation, you weren't going to end up in an area above limits.
Click to expand...

I agree that metal melt at particular temperatures - But that's what CHT is for. A high EGT will certainly keep certain parts of the engine hotter than they'd otherwise be, and without getting too far into the thermodynamics I would guess that the cylinder walls and exhaust valve would both be subject to temperatures somewhat higher than the EGT reading. However, since that's a convective heat transfer while the cooling from there is going to be conductive (until it gets to the fins and is convectively cooled) the metal isn't getting as hot as the exhaust gases. We don't really know what the temperatures at the individual points inside the engine are, so I just go with the research GAMI/Deakin did WRT CHT, because that's what I can measure anyway.

I guess It's worth knowing the EGT as a measure of the amount of heat being added to the system, but the temperature of the metal is what's really important, and we just don't know what the surface temp inside the cylinder is.

3) CHTs aren't useful by themselves for leaning, but a lot of times they influence how you lean. And on a Continental (at least on the D rocker Continentals) I found that leaning to 365F would tend to actually produce the same airspeed at a lower fuel flow vs. 380F because the engines ran more efficiently. This was on the 310 which had very good baffling (after I spent a whooooole lot of time and RTV on it).
Click to expand...

OK, I'm a big fan of efficiency... So I have questions about this.

Which Contis are the "D rocker" ones?
When you were at 365 CHT vs 380, roughly where would you be on the EGTs relative to peak?
What do you think was the cause of the extra efficiency?
Wouldn't those values change with OAT?

Thanks! And lest we get too serious, when are you coming out with those fuel line magnets that will save us a gallon an hour by aligning the (non-ferrous) hydrocarbon molecules? :rofl:
 
flyingcheesehead said:
I agree that metal melt at particular temperatures - But that's what CHT is for.
Click to expand...

Like EGT, CHT is just a measurement in a single spot. Thing is, you've gotten that EGT probe up to 1550+F (which is metal), so that means that there's exhaust gasses of at least that temperature coming out of the exhaust port, going over the exhaust valve (which is what burns), etc. GAMI/Deakin/whatever are quick to point out all the variances in EGT, and that's sort of correct, but I spent a lot of hours running engines on dynos with EGTs on each cylinder, the same engines over years and multiple installs/removals, and I didn't observe a whole lot of variance in EGT based on each installation. There's still value in that number. How do you operate turbines? EGT is a big part of that.

So if you're running hotter, that 1600+F that you're seeing on EGT is translating into more heat going into the exhaust valves, exhaust system, turbo... The turbos all have limits on temperature, but those might not make it into the POH if the OEM decided that in cert testing it wasn't necessary based on the prescribed power settings. Don't believe that has an effect on things? That's fine, I don't care, I'm not paying the bills on your plane. Incidentally that's why I don't teach this stuff anymore or respond to questions about engine operation on here or other places, because everyone responds with "But GAMI and Deakin say..." and then decide they like the GAMI/Deakin answer better than mine because mine would prescribe more limits on operation, even if it's being nicer to the machinery.

In which case, if those are the opinions you care about, why ask me? This goes back to the half-serious side of my original post above. I've paid enough maintenance bills (about $500k worth for aircraft I've been in charge of) and spent enough time operating engines under all conditions to see results. If you don't care about the machinery, that's fine. I won't say that everything I do is based on being kind to the machine - it's not - but I also can state outright when I'm doing something because I care about going fast or there's a bigger picture involved. I don't even participate in engine debates on the MU-2 group and aside from working for a piston aircraft engine manufacturer as an engineer, I worked for a turbine aircraft engine manufacturer as an engineer. Operate however you want.

OK, I'm a big fan of efficiency... So I have questions about this.

Which Contis are the "D rocker" ones?
Click to expand...

The ones for which the rocker box covers look like the letter "D". ;)

s-l640.jpg


When you were at 365 CHT vs 380, roughly where would you be on the EGTs relative to peak?
What do you think was the cause of the extra efficiency?
Wouldn't those values change with OAT?
Click to expand...

Don't remember what the EGTs were relative to peak. Something on the LOP side. Once I learned how my engines ran I selected a fuel flow that I would target, and then trim based on CHT. Remember that I haven't flown the 310 in over 4 years, and the 414 was a different beast (haven't flown that in over 2 years).

These engines are more efficient at lower CHTs (to a point). That effect is clearly shown on a dyno. I won't pretend to understand all of the specifics as to why, but I imagine it probably has something to do with optimal clearances.

Yeah, they'd probably change somewhat with OAT. That was mostly a summer number - basically in the summer if I was at 380F CHT and leaned a bit further (stabilized at 365F or so) then I'd find the same speed. In the winter I'd probably be at 365F anyway. Again, this was on the 310 which I spent a lot of time optimizing all around, so I won't claim that will apply on your Mooney anyway (which as I recall has a cross-flow head design anyway, and may respond differently).
 
Last edited:
Ted DuPuis said:
Like EGT, CHT is just a measurement in a single spot. Thing is, you've gotten that EGT probe up to 1550+F (which is metal), so that means that there's exhaust gasses of at least that temperature coming out of the exhaust port, going over the exhaust valve (which is what burns), etc. GAMI/Deakin/whatever are quick to point out all the variances in EGT, and that's sort of correct, but I spent a lot of hours running engines on dynos with EGTs on each cylinder, the same engines over years and multiple installs/removals, and I didn't observe a whole lot of variance in EGT based on each installation. There's still value in that number.
Click to expand...

Cool. Yeah, I dug around a little before and it sounds like the actual temperature at combustion goes up to around 1000ºC/1832ºF. I'm not sure of the melting points of the exact alloys used on the cylinder sleeves, nor the actual temperature that would be seen inside the cylinder with the cyclic nature of the combustion in addition to friction. The exhaust valve is a fairly small mass part, though, which is likely why it seems to be so susceptible to abuse.

So if you're running hotter, that 1600+F that you're seeing on EGT is translating into more heat going into the exhaust valves, exhaust system, turbo... The turbos all have limits on temperature, but those might not make it into the POH if the OEM decided that in cert testing it wasn't necessary based on the prescribed power settings. Don't believe that has an effect on things? That's fine, I don't care, I'm not paying the bills on your plane. Incidentally that's why I don't teach this stuff anymore or respond to questions about engine operation on here or other places, because everyone responds with "But GAMI and Deakin say..." and then decide they like the GAMI/Deakin answer better than mine because mine would prescribe more limits on operation, even if it's being nicer to the machinery.
Click to expand...

Sorry I hit a nerve. ;) I'm not saying Deakin/GAMI are all-knowing, but they are the source of most of the collective knowledge that we piston pilots have about our engines today. I'm asking you more detailed questions because you know your ****. So thank you for continuing the discussion. :)

Honestly, I don't know what my CHTs are. My crusty old GEM doesn't read anything resembling an accurate absolute value for EGT. I don't think the DA40 was even capable of cracking 1550 on the EGT even if you abused it, which may be why those IO-360s last seemingly forever if kept running.

In which case, if those are the opinions you care about, why ask me?
Click to expand...

Like I said - You know your stuff. Deakin himself was a pilot, not an engineer, and his writing was mostly based on George Braly's research, but I bet you've blown up as many engines on a stand as Braly has. So, I value your opinion too - I mention those guys to give the background of where my existing knowledge came from, so that you know where to poke holes in it.

The ones for which the rocker box covers look like the letter "D". ;)

s-l640.jpg
Click to expand...

Oh, OK. LOL Not what I have - Mine are the individual covers.

These engines are more efficient at lower CHTs (to a point). That effect is clearly shown on a dyno. I won't pretend to understand all of the specifics as to why, but I imagine it probably has something to do with optimal clearances.
Click to expand...

Interesting. My CHTs run pretty low already, but again, my instrumentation isn't as precise as I'd like. Looking forward to getting glass with EIS installed at some point so I have a little better visibility into what's going on. I guess that getting 175 KTAS on 12 gph, I probably don't have a whole lot of efficiency left to be gained, though!

Good stuff... Thanks!!
 
Ted DuPuis said:
As an engine guy, I agree 100% with the statement that "most pilots don't understand it."

All of you, I'm looking at... all of you. ;)
Click to expand...

I am offended! I think I understand it perfectly for my purposes. My plane has no engine monitor, so I don't run lean of peak.
 
You must log in or register to reply here.
Back
Top