Mixture Control

[gismo]

Ummm....I think the last part is what I was saying -- the instrumented cylinder (falsely) says you're OK while the non-instrumented cylinder is toasting itself because it's been leaned too far. In the case where "the instrumented cylinder goes LOP first and non-instrumented remain on the rich side," all you do is use more gas than you have to.

No, my alter ego (the other Lance F) is correct IMO. We're talking about leaning one cylinder till it peaks and then a bit further. If that one cylinder isn't the richest (last to peak) some of the other cylinders will be richer and they may be just enough rich of peak to overheat. In this situation the notion of "leaned too far" is ambiguous. The problem cylinders are too close to peak and worst case are too close to peak on the rich side. While that is certainly "leaned too far" from a ROP persective, it's "not leaned enough" from the LOP way of thinking.

 

[poadeleted20]

I'm not sure if that is completely correct. (Or I'm just reading that wrong). The Temperature/Mixture curve is pretty steep on the lean side and much more 'flat' on the rich side. The temperatures drop off in a hurry on the lean side. If you've got any cylinders that haven't gone over peak, they probably running WAY too close to peak for their own good.

Peak EGT is just fine in cruise at 75% or less. That's just about where you end up if you follow Lycoming's "lean to rough, enrich to smooth" advice.

 

[poadeleted20]

No, my alter ego (the other Lance F) is correct IMO. We're talking about leaning one cylinder till it peaks and then a bit further. If that one cylinder isn't the richest (last to peak) some of the other cylinders will be richer and they may be just enough rich of peak to overheat.

In cruise at 75% or less, even peak CHT mixture (about 75-100 rich of peak EGT) should not overheat the cylinders.

In this situation the notion of "leaned too far" is ambiguous. The problem cylinders are too close to peak and worst case are too close to peak on the rich side. While that is certainly "leaned too far" from a ROP persective, it's "not leaned enough" from the LOP way of thinking.

I'm afraid I don't follow that.

 

[Lance F]

In cruise at 75% or less, even peak CHT mixture (about 75-100 rich of peak EGT) should not overheat the cylinders.quote]

This isn't correct. Peak CHT is going to occur much closer to EGT peak than 75 - 100dF. Even Continental's Operator Manual (IO-550) shows max. CHT at ~35 to 50dF ROP. And that's the problem. Too many POHs (and OWTs) say to operate at 50dF ROP and that's exactly the point that's hardest on the engine.

This also expains my previous objection to the comment on single probe EGT operation. If the only probe you have happens to be on the cylinder that is first to go LOP then the others, which you are not measuring, are slightly on the ROP side where you get highest CHTs (which we'll assume are also not being monitored) and most stress on the engine.

 

[Missa]

Ok, I admit it: I've got all the acronyms except CHT... it's not ringing the bell... someone ring it for me please.

Missa

 

[wsuffa]

Ok, I admit it: I've got all the acronyms except CHT... it's not ringing the bell... someone ring it for me please.

Missa

Cylinder Head Temperature

 

[Missa]

Cylinder Head Temperature

I knew it would be one of those 'hand slapping forhead moments'...

Thanks,
Missa

 

[AdamZ]

No, my alter ego (the other Lance F) is correct IMO. We're talking about leaning one cylinder till it peaks and then a bit further. If that one cylinder isn't the richest (last to peak) some of the other cylinders will be richer and they may be just enough rich of peak to overheat. In this situation the notion of "leaned too far" is ambiguous. The problem cylinders are too close to peak and worst case are too close to peak on the rich side. While that is certainly "leaned too far" from a ROP persective, it's "not leaned enough" from the LOP way of thinking.

Ok now my head is spinning a bit. I was taught if you are running hot to enrichen the mixture a bit and if its cold as a mutha to lean it. ( this of course is a side from normal leaning procdures.) Therefore I has always thought the 1) Lean of Peak gives you best economy at slightly higher heat and the 2) Rich of Peak gave you best cruise at slightly lower heat. (ie lean of peak makes it run hotter than lean of peak) Did I get this wrong?

I recall Rons talk at last years Wings Fly B Q. He discussed the EGT probes and that having one EGT probe isn't really that accurate. To get any accuracy I assume you would need an EGT probe in each exhaust. So then why do so many planes have only one egt gauge? Its gotta be because they only have one probe right? So if it isnt' all that accurate ie it only monitors one exhaust temp then why have them. Would it be better to just lean to rough and enrich to smooth? Are CHT gauges an improvement over EGTs? I have never seen a CHT guague ? monitor that didn't give a reading for every cylinder. I have seen EGTs that only monitored on cylinder.

 

[Let'sgoflying!]

Ok now my head is spinning a bit. I was taught if you are running hot to enrichen the mixture a bit and if its cold as a mutha to lean it. ( this of course is a side from normal leaning procdures.) Therefore I has always thought the 1) Lean of Peak gives you best economy at slightly higher heat and the 2) Rich of Peak gave you best cruise at slightly lower heat. (ie lean of peak makes it run hotter than lean of peak) Did I get this wrong?

Someone post a picture, this will answer a lot of questions!

 

[AirBaker]

Someone post a picture, this will answer a lot of questions!

I'll find a graph.

 

[Let'sgoflying!]

And I will add one question.
I understand what the experts say about CHT being controlling in most cases.
But somewhere there we have been able to drop the absolute values of EGTs as being of real importance. The probe placement, the probe accuracy perhaps... but when the burning gases are coming out at 1550, that is darn near acetylene torch temps (even if you are keeping CHTs below your 375 or whatever has been chosen).
So tell me (convince me again) that I am not melting metal when I have a high EGT in the face of reasonable CHTs.

 

[NC Pilot]

I'll find a graph.

Read through these avweb articles and you will get all the graphs you want and more.

http://www.avweb.com/news/columns/182179-1.html

http://www.avweb.com/news/columns/182176-1.html

http://www.avweb.com/news/columns/182583-1.html

http://www.avweb.com/news/columns/183094-1.html

or talk from this graph:

 

[AirBaker]

or talk from this graph:

That was what I was looking for. 50 degrees rich of peak EGT is the same temp as 50 degrees lean of peak. (A pound of feathers = a pound of lead). However the CHT temps drop off like a rock. Like Lance F said, running just a little rich of peak is much worse than running a little lean of peak in that sense.

 

[gismo]

In cruise at 75% or less, even peak CHT mixture (about 75-100 rich of peak EGT) should not overheat the cylinders.

That's OK for some small bore low compression engines, but higher power ones come with a recommendation to avoid 50 ROP above 65% not 75% (YRMV). Granted the original post was likely WRT a small bore 150-160 HP 4 banger but this discussion appears to have wandered beyond that limit. Deacon and co. also recommend avoiding anything closer than 100 ROP above 65% but at this point I think that's awfully conservative.

In this situation the notion of "leaned too far" is ambiguous. The problem cylinders are too close to peak and worst case are too close to peak on the rich side. While that is certainly "leaned too far" from a ROP persective, it's "not leaned enough" from the LOP way of thinking.

I'm afraid I don't follow that.

I'm trying to say that the worst possible (likely) outcome of leaning a single probe EGT to the lean side is that the single probe is supposed to be on the cylinder that peaks first as you pull the mixture (aka the leanest cylinder). And when that (leanest) cylinder is 20-50 LOP, the richest cylinder (last to peak going rich to lean) could be somewhere in the 50 ROP to peak range which can be abusive of the valves if the power level is high enough.

 

[gismo]

Ok now my head is spinning a bit. I was taught if you are running hot to enrichen the mixture a bit and if its cold as a mutha to lean it. ( this of course is a side from normal leaning procdures.) Therefore I has always thought the 1) Lean of Peak gives you best economy at slightly higher heat and the 2) Rich of Peak gave you best cruise at slightly lower heat. (ie lean of peak makes it run hotter than lean of peak) Did I get this wrong?

LOP is always cooler (lower CHT's and valve temps) than ROP for the same temp diff from peak EGT and the same power output. And since the power output falls off as you lean beyond 100 ROP, the temps are even lower as you go LOP if you don't increase RPM and/or MP to compensate the power lost to the lean burn. The whole notion of "Lean is Hotter" was born of the thinking that leaning beyond peak EGT produces the same thermodynamic response as leaning on the rich side[/quote]

I recall Rons talk at last years Wings Fly B Q. He discussed the EGT probes and that having one EGT probe isn't really that accurate. To get any accuracy I assume you would need an EGT probe in each exhaust. So then why do so many planes have only one egt gauge? Its gotta be because they only have one probe right? So if it isnt' all that accurate ie it only monitors one exhaust temp then why have them. Would it be better to just lean to rough and enrich to smooth? Are CHT gauges an improvement over EGTs? I have never seen a CHT guague ? monitor that didn't give a reading for every cylinder. I have seen EGTs that only monitored on cylinder.[/quote]

A single EGT setup was pretty much the norm in the 70's and 80's, even on highly stressed engines. There were aftermarket multi cylinder EGT's that used a manual rotary selector switch to pick one cylinder at a time and even a multi meter setup but true all cylinder monitoring was born when Insight launched their GEM (Graphic Engine Monitor) a number of years ago. This was the first widely accepted device that allowed the user to monitor EGT and CHT on all the cylinders at the same time with enough resolution to actually see which EGT peaked first.

All cylinder CHT is more important than EGT IMO, because the most common problem WRT mixture management is operating too close to the peak EGT at high power and that will normally manifest itself in excessive CHT's. Another benefit is that you can easily see when the baffling and/or airflow over a cylinder is sub-optimal and needs fixing.

 

[gismo]

And I will add one question.
I understand what the experts say about CHT being controlling in most cases.
But somewhere there we have been able to drop the absolute values of EGTs as being of real importance. The probe placement, the probe accuracy perhaps... but when the burning gases are coming out at 1550, that is darn near acetylene torch temps (even if you are keeping CHTs below your 375 or whatever has been chosen).
So tell me (convince me again) that I am not melting metal when I have a high EGT in the face of reasonable CHTs.

The reason is that much of the heat transfer to the valves comes during the combustion stroke before the exhaust valve opens and most of the cooling comes from valve to seat contact when it's closed. That means the CHT has a huge effect on the valve temp, and according to GAMI the EGT's direct effect on valve temps is rather limited. IMO that's why a poor seat on the valve will quickly lead to valve damage as the real "blowtorch" comes from having combustion gasses passing the valve during the power stroke. Don't forget that the downward motion of the piston causes/allows the cylinder pressure to drop significantly during the downstroke and that cools the gasses adiabatically (this cooling of the gasses is really where the power at the crankshaft comes from).

 

[poadeleted20]

In cruise at 75% or less, even peak CHT mixture (about 75-100 rich of peak EGT) should not overheat the cylinders.

This isn't correct. Peak CHT is going to occur much closer to EGT peak than 75 - 100dF. Even Continental's Operator Manual (IO-550) shows max. CHT at ~35 to 50dF ROP. And that's the problem. Too many POHs (and OWTs) say to operate at 50dF ROP and that's exactly the point that's hardest on the engine.

When you're talking about the big bore Continentals, then yes, in the 65-75% range, the engine may in some circumstances overtemp the heads at peak CHT (i.e., 75 or so rich of peak EGT). Those engines are a case unto themselves. But for the light trainers originally under discussion (e.g., C-172), if the CHT busts when cruising at 75% RPM even with mixture at peak CHT, there's a cooling deficiency -- it just should not happen. I can't count the number of planes I've seen with rotten baffle seals -- gaps, cracks, failure to cut to fit, etc, especially the seals behind the nose bowl where nobody ever looks. Invariably the folks who complain about high CHT's despite "book" operation turn out to need significant baffling work, after which the problem goes away.

So the real bottom line is to know what you're flying, and fly it by the applicable book. And there is no book that simply says "lean one inch."

 

[poadeleted20]

I recall Rons talk at last years Wings Fly B Q. He discussed the EGT probes and that having one EGT probe isn't really that accurate. To get any accuracy I assume you would need an EGT probe in each exhaust. So then why do so many planes have only one egt gauge? Its gotta be because they only have one probe right?

Not necessarily, as you can put multiple probes through a rotary switch and read each cylinder on one gauge, but you don't see that very often.

So if it isnt' all that accurate ie it only monitors one exhaust temp then why have them.

Good question, to which I do not have a good answer.

Would it be better to just lean to rough and enrich to smooth?

For cruise leaning, my experience is that even all-cylinder EGT gauging is no better than that for normally-aspirated engines, especially with fixed pitch props.

Are CHT gauges an improvement over EGTs? I have never seen a CHT guague ? monitor that didn't give a reading for every cylinder. I have seen EGTs that only monitored on cylinder.

CHT gauges aren't any more of an improvement over EGT than an oil temp gauge is over oil pressure -- it's just a gauge of a different parameter. Personally, I want both.

 

[Steve]

The 177B I owned had a single cylinder EGT and a single cylinder CHT gauge. The EGT was an ALCOR item, the CHT was OEM. I just used them as a reference point after leaning using the out to rough, back in to smooth method in cruise. Leaned to best rpm for taxi. Maybe good, maybe bad technique, but the plugs never fouled as I recall.

Now that I have a multi-channel EGT/CHT I futz with the mixture a lot more looking for the optimum setting. We have the technology. We must use it.

 

[Let'sgoflying!]

Someone still needs to develop, post (and circulate widely) the basic steps for How To Lean Your Engine:
a) for each; small bore, large bore
b) for all phases of flight
c) for those with less than all-cyl monitoring, and those with
d) NA and TC
in a document we can all agree upon (!) and feel comfortable that others, including student pilots can easily do.
Otherwise, imho, we are no further ahead with this latest round of leaning discussion and the rumors, owt's, innuendos, and half-truths will persist! (J/K!)
Until we have this Master Leaning Document to refer to, in April we can expect another fine 3-page discussion of the topic, with the same old debates.
Anyone want to start it? I am no technical writer and don't have enough knowledge of the topic to do it justice. I would use such a document though.
:0)

 

[James_Dean]

Someone still needs to develop, post (and circulate widely) the basic steps for How To Lean Your Engine:
a) for each; small bore, large bore
b) for all phases of flight
c) for those with less than all-cyl monitoring, and those with
d) NA and TC
in a document we can all agree upon (!) and feel comfortable that others, including student pilots can easily do.
Otherwise, imho, we are no further ahead with this latest round of leaning discussion and the rumors, owt's, innuendos, and half-truths will persist! (J/K!)
Until we have this Master Leaning Document to refer to, in April we can expect another fine 3-page discussion of the topic, with the same old debates.
Anyone want to start it? I am no technical writer and don't have enough knowledge of the topic to do it justice. I would use such a document though.
:0)

1. Small Bore Non-TC
a. Above 75%
1. Push little red knob all the way forward
b. Below 75%
1. Pull little red knob until engine vibration felt
2. Push little red knob until vibration gone


2. Big Bore without all cylinder monitoring of CHT and EGT
a. Buy more cylinders and get to know your A&P


3. Small Bore with Turbos without all cylinder monitoring of CHT and EGT
a. Buy more cylinders and get to know your A&P


4. Big Bore without Turbos with monitoring of CHT and EGT
a. Takeoff fuel flows need to be at least "redline"
b. Above 75% power run at least 250* ROP
c. Below 75% power run anywhere on either side of +80 or -10 of
peak on ALL CYLINDERS with a maximum of 375* CHT's. If CHT's
are higher than this value
1. Increase airspeed
2. If ROP go richer
3. If LOP go leaner



5. Big Bore with Turbos with monitoring of CHT and EGT
a. Takeoff fuel flows need to be at least "redline"
b. Above 70% power run at least 250* ROP
c. Below 70% power run anywhere on either side of +100 or -25 of
peak on ALL CYLINDERS with a maximum of 375* CHT's. If CHT's
are higher than this value
1. Increase airspeed
2. If ROP go richer
3. If LOP go leaner
d. Buy more cylinders and get to know your A&P




James Dean

 

[tonycondon]

heh, james isn't too far off if you ask me. especially the part about buying more cylinders and getting to know your A&P

 

[AirBaker]

I've been looking at the 'advanced pilot seminar' stuff. They've got all the data on what you're doing to your engine at any one point in time. Fascinating stuff. It makes sense, but just counter-intuitive sometimes.

From that perspective, I'd disagree in regards to the big bore / little bore engine stuff.

If you REALLY want to get more than enough info on ROP/LOP discussions, hop on the Beech list. You'll start getting formulas, charts, etc from those that are providing the Advanced Pilot programs.

 

[tonycondon]

i like it too, simple, easy, no voodoo.

 

[Lance F]

I've been looking at the 'advanced pilot seminar' stuff. They've got all the data on what you're doing to your engine at any one point in time. Fascinating stuff. It makes sense, but just counter-intuitive sometimes.

I wonder, Chris, is it really counter-intuitive or counter OWTitive?

 

[AirBaker]

I wonder, Chris, is it really counter-intuitive or counter OWTitive?

Touche!

 

[Steve]

That technique was shown to me by the CFI that checked me out in the IAR. Works for me. The gauges confirmed it.

I mentioned it to my mechanic, who works on radials as well as the horizontally opposed, and he said doing that on a radial would blow a jug.

From the Red Board:

I like it. -- Mike

 

[Let'sgoflying!]

James Dean may have something there, do the experts have any amendments before we pass it into law?

 

[tonycondon]

eh doing anything to a radial will blow a jug, like starting it, running it, shutting it down.
Radials don't leak oil, they mark their territory

 

[Steve]

Hehe, yep, "Big Red" (painted fire engine red with gold lightning stripes down the side), the BE-18 that commands the one of the rear corners in the main hangar, does just that!

eh doing anything to a radial will blow a jug, like starting it, running it, shutting it down.
Radials don't leak oil, they mark their territory