While not really a question, the following note exchange occurred over several days, regarding engine leaning, EGT, and CHT. I decided to just cut and paste it into an FAQ, in case anyone else was interested.
From: Rick
To: Mike Rellihan
Sent: Sunday, December 19, 2004 11:06 AM
Subject: Re: [musketeermail] Erratic RPM at runup
I think I have it!! Or at least a far more clear foundation for understanding it.
And, it sounds like a key element to remember in all of this is that condtions (alt, DA, temp, etc etc). will affect/change these benchmark numbers continually… today’s peak EGT is not necessarily the same as last week
I occasionally make a flight from Denver to Des Moines – cruising at 9000′ to the fuel stop then cruising at 3000′ or 5000′ descending to 900′. That’s a large Alt spread in one flight! I think I should be adjusting mixture more than I have been on this flight.
Anyway, you can see why I posted a note on MML saying, if you stop contributing there, there will be hard feelings – MINE!!!
Thanks so much, Mike.
You know this little discussion we’ve had might make a valuable treatise on the subject to be made available on BAC. I can’t be the only person who needs a primer on best leaning practices (at least I hope I’m not!!) If we ever get the newsletter up and running, it would make a great article for that, too!
Thanks again.
-Rick
—– Original Message —–
From: Mike Rellihan
To: Rick
Sent: Sunday, December 19, 2004 7:31 AM
Subject: Re: [musketeermail] Erratic RPM at runup
The purpose of identifying the EGT at peak power/peak RPM is so that you know what that benchmark number is. For takeoff and climb you want to be rich enough to stay maybe two ticks lower (cooler) than that benchmark.
You want to know peak EGT as a different benchmark. That is where you want to cruise for best economy. You also want to make sure that you do not operate continuously within the range of one to two ticks richer (cooler) of that number. You either want to be at peak EGT, or at three or more ticks richer (cooler) of that number. Which may, in fact, coincide with best power EGT (or close to it).
I know that this can sound confusing. Just keep working to understand the relationship between power, EGT, and CHT. Manufacturers publish a single number, which is an effort to simplify it for the pilot masses, but that is not a meaningful effort. The actual peak power, peak EGT number, and fuel flow number varies continuously with throttle setting, airspeed, and density altitude. What I have suggested to you is a way to establish two baseline numbers, so that you can interpret numbers wisely when they vary from those numbers. For example, you will know what your peak power/peak RPM baseline EGT is at maybe 8,000 feet density altitude. If you climb another 2,000 feet, that number will go down (cooler). You can lean a little bit more and it will go up very slightly, but you will not be able to get back to the original best power number. All that will happen is that you’ll go to peak EGT instead (best economy). The engine is producing less power due to higher altitude (thinner air), so it will produce less EGT heat at best power. But if you understand what is happening, you can find the new “peak EGT”, and go four ticks rich again for best power, or you can again lean for peak RPM/airspeed. This assumes that you are operating at full throttle, controlling max power with altitude. If you wish to operate below full throttle in cruise, either due to lower-altitude operations or for economy, there is no point in leaning for best power. You should instead operate at peak EGT for best economy.
—– Original Message —–
From: Rick
To: Mike Rellihan
Sent: Sunday, December 19, 2004 12:40 AM
Subject: Re: [musketeermail] Erratic RPM at runup
Mike,
At the risk of sounding dense on this, I need to clarify a bit more…
The Ojective:
– NOT to operate the engine in a configuration that puts it 40-50 degrees rich of peak for a prolonged period of time.
The Question:
– Is that rich of peak RPM or peak EGT?
In other words, you’ve outlined how I can establish TWO indications (marks) on my EGT gauge face: a FIRST mark that relates to peak RPM and a SECOND that relates to peak EGT. So, to meet the objective, am I’m trying to avoid 40-50 degrees rich (cooler) of the FIRST mark or the SECOND one?
I’m embarassed to keep asking but I feel I’d like to get it clear in my head. Thanks for your patience!
-Rick
—– Original Message —–
From: Mike Rellihan
To: Rick
Sent: Saturday, December 18, 2004 9:17 PM
Subject: Re: [musketeermail] Erratic RPM at runup
Rick, my sincere apologies; I was just seeing the “Rick” due to the nickname display in the email From field. I just scrolled down and saw the Koch in the first note. Just glossed over it the first time. Of course I know of your big contributions to BAC.
Peak RPM and peak EGT most likely will not coincide. The best way to validate the numbers is during low-AGL flight, if you can. I know you are pretty high there already. If there is an open area where you can cruise at maybe 2,000 AGL at Wide Open Throttle, without dodging hard stuff that would hurt you, that will give you the best results. Operate the mixture until you get the peak RPM (which will also be peak IAS). Make note of where the EGT needle is. You can even use a grease pencil on the gauge face. Then start leaning until you find peak EGT (it would normally be three to four “ticks” higher (hotter), and you should lose airspeed and RPM. The engine will start to get rough right around peak EGT, on the carbureted engine; possibly even before then, if you have a single-cylinder probe. Make note of where peak EGT occurs, as well as the onset of roughness if they don’t coincide. If roughness permits, your best economy point will be at peak EGT/slight roughness. The mark you first made will be peak cruise power at full throttle. One or two ticks rich of that peak power point (which will cause lower EGT) will be the best takeoff and climb EGT setting. You can make semi-permanent gauge face markings using thin pinstripe tape from the auto stores. I do this to mark all my critical airspeeds on the Airspeed Indicator.
The complicating factor is that there is a wide temp spread between cylinders on a carbureted engine. Using full throttle in cruise, while using altitude to control power output, usually helps to improve mixture distribution. Any density altitude over 8,000 feet will give less than 75% at full throttle, and even 7,000 feet DA won’t hurt a thing on your engine.
This is far easier with an all-cylinder EGT-CHT instrument, and easier still with fuel injection. I’m not deliberately trying to make it confusing.
—– Original Message —–
From: Rick
To: Mike Rellihan
Sent: Saturday, December 18, 2004 8:33 PM
Subject: Re: [musketeermail] Erratic RPM at runup
Mike
Thanks again for the valuable info. I JUST found this email. I had been disappointed for several days thinking you chose not to respond for some reason. This additonal info fills in some gaps for me. Thanks!!!
One follow up question: I understand my EGT is a relative indication, but for me to avoid the 40-50 ROP zone, how would I determine that exactly using the only relevant indicators I have – RPM and EGT? Do I find peak RPM, note the EGT indication at that point and be aware of 1-1/2 to 2 ticks rich, or cooler, that that point? (oddly phrased, but hopefully you’ll get what I’m asking)
And finally, yes I’m a BAC member. I’m the club’s art director, actually. Cloyd asked me to serve another year to which I heartliy agreed.
Thanks again, Mike.
-Rick Koch
BTW, I flew today under similar conditions to that previous flight that prompted the original question – with no problems. I leaned a bit more aggresively than last time.
—– Original Message —–
From: Mike Rellihan
To: Rick
Sent: Wednesday, December 15, 2004 6:42 AM
Subject: Re: [musketeermail] Erratic RPM at runup
I didn’t say you were “too rich at 50 ROP”. I said that 40-50 ROP generates the fastest burn and the sharpest pressure rise. It is the hardest on the engine and will create the highest CHT. You don’t want prolonged operation there. You have to pass through that point during the leaning process. Passing through won’t hurt anything. Leaner is better for economy, while richer (100 ROP) is better for power, and much richer (125-150 ROP) is better for internal cylinder cooling during takeoff and high power climb.
The EGT gauge you have is only useful for relativity (going up or down, and how much). The absolute number you wind up with doesn’t matter. It will change with OAT, altitude, power setting, etc. There is no such thing as too high an EGT on a non-supercharged, non-turbocharged plane. Most of the original EGT gauges don’t even have actual temperature markings on them for that very reason. Your markings probably are 25-degree points.
Run-up RPM (and RPM drop) should be what the POH specifies. That number was developed for the specific airframe, prop, and engine combination, to load the engine enough to validate ignition, etc., but without causing high CHT and oil temps in ground ops, and without unnecessarily overstressing the prop blades. EGT readout is meaningless during a run-up. The exception is if you have an all-cylinder readout and it shows one cylinder reading exceptionally low (like half of normal), but you would probably feel that as vibration too.
When leaning for peak power, peak RPM is the best indicator with a fixed-pitch prop. That will usually occur at 100 ROP, or close to it. You then want to go a bit richer than that for takeoff and a full-power climb, to protect CHTs. If you are at or above 8,000 feet density altitude, especially in winter temps, you no longer have to worry about running extra rich to protect CHT, you can save the extra fuel. Just avoid running in that 40-50 ROP range.
Plug fouling can crop up almost any time; especially as the plugs age. The absence of past problems doesn’t mean it will never happen. You can minimize the chances by good leaning practices, as you seem to be doing most of the time. It is always possible that you had (or have) a carburetor problem, but it is unlikely. If the float temporarily sticks open due to debris in the needle seat (as they used to do on cars fairly regularly), you will go very rich until the problem clears. Those are transient conditions but they can happen.
The CHT gauge question would take a great deal of additional writing. I’m hoping you are a BAC member, so I can ask you to log on and read all the info I have already put up there. I have no current info on prices. The best deals are usually at Sun-N-Fun and Oshkosh.
—– Original Message —–
From: Rick
To: Mike Rellihan
Sent: Wednesday, December 15, 2004 12:07 AM
Subject: Re: [musketeermail] Erratic RPM at runup
Mike,
(Yes, I meant 50 deg RICH of peak)
You’ve offered some great info but as I sit here pondering all of this, I find myself getting more and more confused. I’m either overthinking this, or it’s a semantics issue. You say I’m too rich at 50 ROP. But then you say I should be 100-125 ROP, How can I get to 100-125 with out enrichening even MORE? As I enrichen the mixture (lever forward), I watch the EGT decline (get cooler – needle moves to the left). Conversely, if I lean the mixture, the EGT rises (gets hotter, or so I thought, needle moving to the right). I guess I need a lesson in proper, or at least more thorough EGT/RPM interpretation and interaction.
My EGT displays non-numeric tick marks, that I have assumed are in 25 deg increments. There is a… not sure what you would call it… a conspicuously larger mark (~3/4 of the way up the scale) that I never can seem to reach when going for peak EGT. Not really sure if I even need to. I have better luck watching the RPM for peak while leaning, then noting the EGT “reading” and trying to further adjust from there (the 50 deg ROP that I’ve been doing).
What do you look for, Mike, in terms of RPM and EGT readouts during a typical runup? And what do you do, specifically, to ensure the proper interactaction of the two with each other, during your mixture adjustment procedure?
I don’t have a CHT gauge. What would you recommend there? And how much? Install costs?
In any event, I will try a leaner runup mixture, to be sure. I still think it’s interesting that I’ve never had any difficulties until last weekend (actually I don’t know that I’d classify that as a difficulty – rather just different). But I’m anxious to hear about your specific mixture practices.
Thanks so much!! Your help is greatly appreciated.
-Rick
—– Original Message —–
From: Mike Rellihan
To: Rick
Sent: Tuesday, December 14, 2004 5:39 AM
Subject: Re: [musketeermail] Erratic RPM at runup
10-4. Sounds like you are doing almost everything quite well. I just wouldn’t use full rich at all at that altitude, except perhaps for finding out where the setting is (half-inch out, etc.). You definitely don’t want to be full rich during the run-up, as the engine load is very low; the load is just enough to create enough cylinder pressure for a valid ignition test. You’ll run across people who deliberately use even lower power for their run-ups, saying that it is easier on the engine. They are taking a risk that they don’t fully understand, as many ignition problems will not show up under low engine loads.
You said “50 degrees lean of peak for takeoff”. I’m assuming that you actually meant rich of peak. But 50 rich is not enough for takeoff. You do not normally get full power unless you are 75 to 100 degrees rich of peak. Without a CHT, you should probably try for 125-150 ROP, to make sure that the leanest cylinder is being protected from excessive CHT; though at that altitude this would only be a risk during low airspeeds and full throttle. 40 degrees rich is also the point where CHT will go the highest, especially in takeoff and climb when airspeed is lowest; and it is the point where internal cylinder loads are the highest. The ten degree difference between 40 versus 50 degrees isn’t enough of a spread. The altitude doesn’t matter for how rich of peak you are; all altitude does is change where peak will be. The relative spread remains the same for any altitude. While many people do it, the instrumented tests done by the GAMI people have proven that 40-50 ROP is the worst place to run an engine. If you want CHT protection at low airspeeds and full throttle, you want to use 125-150 ROP at sea level, and maybe 100-125 degrees at higher field elevations. If you want max power (at any altitude), once you are at cruise airspeed, you need to be close to 100 ROP. If you want best economy on a carbureted engine, you want to run on the edge of lean roughness, which is usually going to be close to peak EGT. On my fuel injected engine I can usually run 25-50 degrees lean of peak, but the power loss is significant when set that lean.
It is an Old Wives’ Tale that peak EGT is a bad thing, on a normally-aspirated engine. If you had a CHT gauge you could prove it to yourself. Your CHT is lower at peak EGT than it is at 50 ROP. CHT is the life-limiting number on our engines. You have to understand the cylinder combustion process to understand “why” this is the case. At peak EGT, some of the cylinder heat is being released into the exhaust rather than held inside the cylinder, due to a slower rate of burn. At 40 ROP, the burn occurs at peak speed, the pressure spike is the highest, and the most heat is transferred to the cylinder head and piston. That’s why the EGT looks cooler. It is also an OWT that a lean mixture burns faster and hotter. It actually burns slower and cooler. That’s why in a car engine, as the mixture gets leaner, the spark timing is automatically advanced to compensate for the slower burn rate. This particular OWT got its start from the fact that if you go from full rich to maybe 50 rich, the burn IS faster and hotter. But that’s just going from very rich to slightly rich; it isn’t really going to a lean mixture.
Truly lean-of-peak mixtures give the best economy, lowest power, cleanest burn, coolest temperatures, and require the best ignition. The closest you can usually get to that on a carbureted engine is peak EGT, due to cylinder mixture imbalance. One interesting aside: on longer trips, you may find it worthwhile to go up to 8,000 feet or higher, where you can probably run wide open throttle in cruise. That will often allow you to lean a bit further without excessive roughness. You can do some experimentation yourself. Each carbureted engine/airframe combination tends to have a throttle position that gives the best mixture distribution due to the airflow patterns in the induction system. On most engines that is wide open throttle, but not always.
—– Original Message —–
From: Rick
To: Mike Rellihan
Sent: Monday, December 13, 2004 10:22 PM
Subject: Re: [musketeermail] Erratic RPM at runup
Hi Mike,
Thanks for the info. To clarify, I do lean it out as much as possible for ground ops, taxi, etc. After start, I set the throttle at 1000 RPM and lean until max RPM which is ~1200. I taxi to the runup area ( a good little ways at my airport) and begin my runup. I push mixture forward (close to if not full) as I increase throttle to 2200 RPM. I do mag tests, carb heat THEN lean mixture to max RPM and then add a little more rich ( half inch or so) before throttling back to 1200. If tower gives me a quick release I go, otherwise if I suspect I’ll be sitting a while, I’ll reset mixture for ground ops again and then reset a final time with a quick throttle up to 2200 and adjust mixture for max RPM. Perhaps I should adjust RPM FIRST for max RPM, the do mag and carb heat tests.
I try to be careful with leaning and not run too rich. The plane came from a sea level environment and since March of this year, with the leaning procedure I’ve described along with my 6700″ field, I’ve never had a bit of trouble and the pipe even stays pretty clean as a result. Yesterday was the first I’ve experience this jittery RPM indication (along with a sooty pipe). I’m hoping it was an anomaly.
I will try your suggestions for finding mixture setting for best power and takeoff. I do have an EGT and usually try to set it 50 deg lean of peak for takeoff but then I push the mixture a half inch or so more rich “for good measure”. I’ll not do that next time.
Will keep you posted.
-Rick
—– Original Message —–
From: Mike Rellihan
To: Rick Koch
Sent: Monday, December 13, 2004 7:51 PM
Subject: Re: [musketeermail] Erratic RPM at runup
The spark plugs get more sensitive to an over-rich mixture as they age. At your field altitude, and especially your density altitude, full rich is much too rich. Ground taxi should be done with the engine as lean as you can get it without it sagging and stalling. The run-up should be leaned to best power. This will actually be a bit richer than the taxi setting, but nowhere near full rich. The run-up power setting is probably no more than 40% power or so, so you can lean it all you want as long as it keeps running; but to get a good run-up test, you just want to lean to best power (peak RPM increase during the run-up).
For takeoff, when the actual density altitude is most likely 6,000 feet or so where you are (at an ambient of 37 to 38 degrees), full rich is too rich. If you have an EGT gauge, lean to about 150 degrees rich of peak for the takeoff. If you don’t have one, set the mixture halfway between full rich and the run-up mixture setting point. You can also do your own test, to find the mixture setting point for best power and takeoff. Just lock the brakes, aim at the wind, and go to full throttle. Then briskly lean the engine to peak static RPM. That is the best power point. If you don’t have an EGT gauge, just go a half-inch richer on the mixture for takeoff. When you don’t have a turbocharged or supercharged engine, it is very difficult to hurt the engine through leaning at density altitudes of 5,000 feet and higher (and for that matter, even at sea level).
If you don’t have EGT but do have CHT, just stay rich enough to keep the CHT to maybe 380 degrees or below. There is no limiting EGT on a non-supercharged/turbocharged engine. The CHT limit for normal ops is around 400 degrees, with most builders and manufacturers considering 450 degrees the redline for CHT. While the ideal instrumentation is an EGT-CHT analyzer for all four cylinders, if you have to choose a single inexpensive instrument, for our planes the best choice is a CHT gauge.
Please send me some feedback on how this works out for you. It is possible that it will self-clean the plugs. Otherwise you may need to pull them and clean them, and start fresh with the leaning regimen.
—– Original Message —–
From: Rick Koch
To: musketeermail@yahoogroups.com ; Bob Steward
Sent: Monday, December 13, 2004 11:15 AM
Subject: [musketeermail] Erratic RPM at runup
Something I haven’t experience before…
Hello everyone,
Yesterday during runup (2200), mixture full rich, my RPM was erratic and not
“locked” onto the 2200 tick. Mag tests dropped 100, then 150 back to 100,
down just 50… Adjusting mixture a tad leaner seemed to make RPM a bit
more stable. I was curious what might cause this. Was it too rich? Plugs?
I usually runup full rich, test mags and carb heat, then quickly set mixture
for takeoff and back throttle off to fast idle (1200) until take off.
My run up RPM has always been rock steady until yesterday.. Possible causes?
78 Sundowner
O360 A4K
OAT was ~ mid thirties
(I don’t recall dewpoint but wrote it down from ATIS at the time – can
retrieve it later)
field elevation 5670′
wind 12kts
I flew in the pattern for an hour or so with no problems and after landing,
a finger in the exhaust pipe showed a ‘normal’ amount of carbon- possibly a
little blacker than normal.
Thanks,
-Rick Koch