Setting the EGT

[EHITCH]

Can anyone recommend a 'typical' or 'recommended' procedure for setting/operating the EGT? I believe the POH/AFM has a preferred procedure but I will not have access to it until after I need it. Aircraft is a 172P.
Thanks,
Elizabeth

 

[EdFred]

Can anyone recommend a 'typical' or 'recommended' procedure for setting/operating the EGT? I believe the POH/AFM has a preferred procedure but I will not have access to it until after I need it. Aircraft is a 172P.
Thanks,
Elizabeth

Generally peak EGT in a normally aspirated engine is just rich of where it starts to run rough when you lean it out. I just installed an engine monitor on my plane, and I get to peak EGT, just as it drops off the lean of peak side where it's not getting quite enough fuel in the fuel air mixture. Bascially, it's pull the mixture till it runs rough, and give it 3 twists rich. That's all there is to it.

 

[Jeff Oslick]

This is a fairly hot topic on a few web boards, but if there is a "general consensus" (yeah, right), for operating with a single Cessna-stock EGT gauge (EGT on one cylinder only), it would be to run 125 degrees rich of peak. Lean the engine until the highest EGT is reached, then slowly enrich the mixture to 125 degrees richer - 5 gauge increments from the peak on most gauges (should be marked what the increments are on the gauge). There is another "camp" that runs lean-of-peak, but that is much easier to do if you can monitor the EGT on each cylinder, since not all cylinders run the same EGT. Some pretty smart folks have shown that operating 50 degrees rich of peak may not be the best place to run your engine, but that is where many POHs say to run it... so all you can do is read up on all sides of the issue and make your best judgement. If you operate at less than ~65% power, there isn't as much chance of damaging parts if you aren't at the "best" mixture setting, so if you want to save gas and run lean, run it easy too.

AvWeb has some good articles on the subject.

Jeff

 

[Ed Guthrie]

Can anyone recommend a 'typical' or 'recommended' procedure for setting/operating the EGT? I believe the POH/AFM has a preferred procedure but I will not have access to it until after I need it. Aircraft is a 172P.
Thanks,
Elizabeth

Elizabeth, first, use your feet in conjunction with the EGT. Place your feet flat on the floor and feel the engine vibrations coming through the floor. Slowly pull the mixture (~10 seconds from full rich to lean). Give the engine time to respond. When you note that the floorboards have begun rumbling move the mixture very slowly back towards "rich". Note when the rumbling in the floorboards stops. Watch the EGT as you do all this. During this lean to rich pass of the mixture knob you will see the EGT needle move towards hotter temperatures, then begin subsiding. You will probably notice that the EGT needle did not reach the actual peak on the rich to lean pass of the mixture knob. IOW, you will probably see that the EGT indicated a much greater peak temperature on the lean to rich pass of the mixture knob. Make note of this peak temperature. Continue moving the mixture towards rich until the EGT needle is however many degrees rich of peak your POH suggests. Mooney 201 POH suggests leaning the mixture at 75% power and below, using either 25 degrees F rich of peak for best economy or 100 degrees F rich of peak for best power. Best power will cruise faster, but at a significantly greater fuel burn.

I tend to take two shots at finding the right mixture. One pass to find the approximate mixture knob position, the second pass to fine tune/observe the mixture region right around peak.

 

[EHITCH]

Thanks!

EH

 

[poadeleted20]

The "lean to rough and then enrich to smooth" advice is the best posted. This works just fine even without an EGT, and in over 1000 hours of flying with full engine analyzers, works just as well as the most sophisticated EGT-based techniques.

Leaning to 125 ROP is well outside the range of recommendations from Lycoming, especially if you've got only one cylinder probed. You really want it so that the cylinders are right around peak, maybe shaded to a hair above peak so all cylinders are running OK, in order to get the best combination of power, economy, and smoothness.

 

[mikea]

I tend to take two shots at finding the right mixture. One pass to find the approximate mixture knob position, the second pass to fine tune/observe the mixture region right around peak.

That's really good advice. The not-so-popular guy at JPI emphasized that you need the temperatures to stabilize so he say to set a target fuel flow and wait a few minutes before doing the Lean Find.

I make two stabs at Lean Find with finer adjustments to see where the peak on the leanest cylinder is. I set it at 50 degrees rich of peak on the leanest cylinder. You can see that all of the cylinders are running hotter and it will take a while for the temperatures to even out.

You can't see that without an engine analyzer, but you can make a second stab at it after a the engine has been running at your first stab for a few minutes.

 

[Jeff Oslick]

Leaning to 125 ROP is well outside the range of recommendations from Lycoming, especially if you've got only one cylinder probed. You really want it so that the cylinders are right around peak, maybe shaded to a hair above peak so all cylinders are running OK, in order to get the best combination of power, economy, and smoothness.

As I pointed out, some of the discussions on leaning floating around the pilot community don't fully agree with every POH. I agree that at least for first learning to lean, follow the POH. I wanted to point out to Elizabeth that there is more information on the topic out there other than what is in the POH, and I would encourage her, and all pilots, to at least become educated on what others have learned about engine leaning/management, then discuss it with an experienced CFI and (most importantly) the aircraft owner and do what they feel they should.

Jeff

 

[poadeleted20]

As I pointed out, some of the discussions on leaning floating around the pilot community don't fully agree with every POH. I agree that at least for first learning to lean, follow the POH. I wanted to point out to Elizabeth that there is more information on the topic out there other than what is in the POH, and I would encourage her, and all pilots, to at least become educated on what others have learned about engine leaning/management, then discuss it with an experienced CFI and (most importantly) the aircraft owner and do what they feel they should.

Good thoughts. For Lycoming engines like the O-320-D2J in her C-172P, see the Lycoming Key Reprints available at http://www.lycoming.textron.com/main.jsp?bodyPage=/support/publications/keyReprints/index.html, primarily in the Operations and General sections. I especially recommend the ones on "Leaning Lycoming Engines" and "EGT and Fuel Management." If you look there, you will find that when leaning with an EGT, they say that for best power you want 100F ROP, and for best economy, you want peak EGT. Note that with a fixed pitch prop, you don't need an EGT to lean for best power -- just lean to peak RPM (best economy is slightly less than best RPM). 125F ROP is too rich for either, and will promote lead fouling as well as wasting gas.

I also recommend review of Lycoming Service Letters 185B, 192B, and 197A (available unofficially at http://www.prime-mover.org/Aviation/Manuf.html or officially from Lycoming -- only 185B is officially available on line -- you have to call them to fax a copy of the other two) for more information on leaning in all phases of flight and ground operations.

 

[gismo]

I'd like to add a couple thoughts. One is that whenever you are setting the mixture you must move the mixture much slower than most pilots tend to do. One tick on most analog EGT gauges is usually 25 C and it should take several seconds to move that far. Typically it's best to move the mixture a little bit and wait 10-15 seconds for things to settle down before moving it again. Of course you should use much of this time to scan the sky for traffic. Leaning shouldn't be five minutes of head down time.

The other is to caution you WRT the fact that peak EGT will occur at a different mixture setting for each cylinder on most engines and if you only have a single probe, chances are that cylinder isn't the leanest (the one that peaks first). In fact the leanest cylinder often varies with throttle position and RPM.

In any case I agree that setting the mixture by feel / roughness on a carbureted engine works fine and that with an engine like the one in your 172 it's difficult to abuse the engine by leaning during cruise. OTOH if you don't lean fairly agressively, you won't make anywhere near the POH specified range so pad your fuel accordingly.

 

[grattonja]

I run the skyhawks I rent 50 degrees ROP, which is two ticks on the EGT, assuming the usual quality cessna engine gauges are working correctly that day, and the EGT works. But, that is a 172SP, which is an IO360 lycoming, instead of your O320. Don't know if that makes a difference.

Jim G

 

[poadeleted20]

I run the skyhawks I rent 50 degrees ROP, which is two ticks on the EGT, assuming the usual quality cessna engine gauges are working correctly that day, and the EGT works. But, that is a 172SP, which is an IO360 lycoming, instead of your O320. Don't know if that makes a difference.

Fuel injection makes a huge difference in the equalization of the fuel/air charges going into each cylinder. Carbureted engines tend have substantial differences between cylinders in when peak EGT occurs, but FI engines tend to have all cylinders peak much closer together.

 

[EHITCH]

Further questions, as always -- I was taught to "roll" the mixture out (not to pull it) until the engine starts to run rough, then make two complete revolutions in the opposite direction. This was on a 172P without an EGT gauge. That plane is now gone and its replacement HAS an EGT, so I am assuming it is proper to continue "rolling" the mixture out; however I will be next reading the Lycoming info Ron posted.
Thanks,
E

 

[Ed Guthrie]

Further questions, as always -- I was taught to "roll" the mixture out (not to pull it) until the engine starts to run rough, then make two complete revolutions in the opposite direction. This was on a 172P without an EGT gauge. That plane is now gone and its replacement HAS an EGT, so I am assuming it is proper to continue "rolling" the mixture out; however I will be next reading the Lycoming info Ron posted.
Thanks,
E

Vernier? "Rolling it out" meaning screw out the Vernier versus release the lock and pull?

I would agree, although I might release and pull until somewhat close to peak EGT, then go to the "roll it out" technique from there.

 

[EdFred]

Further questions, as always -- I was taught to "roll" the mixture out (not to pull it) until the engine starts to run rough, then make two complete revolutions in the opposite direction. This was on a 172P without an EGT gauge. That plane is now gone and its replacement HAS an EGT, so I am assuming it is proper to continue "rolling" the mixture out; however I will be next reading the Lycoming info Ron posted.
Thanks,
E

I did training in a 172P model as well. I was told to pull it out slowly and then 3 revolutions in. No real difference in either way it's done.

 

[poadeleted20]

No set number of turns of the vernier or clicks of the "standard" mixture is right. Lean to rough, enrich to smooth -- period.

 

[Ed Guthrie]

No set number of turns of the vernier or clicks of the "standard" mixture is right. Lean to rough, enrich to smooth -- period.

Good catch. I focused on twist versus pull as the adjustment technique and failed to notice the "2 turns rich" part later in the post. Twist to fine tune and find peak. Use vibration or EGT to know when you've hit peak or moved rich of peak.

 

[Ghery]

No set number of turns of the vernier or clicks of the "standard" mixture is right. Lean to rough, enrich to smooth -- period.

The great thing about this approach (which I use) is that I can set the mixture without interrupting my traffic scan. No danger of having my head down looking at a guage when I should be watching for other airplanes.

 

[poadeleted3]

The great thing about this approach (which I use) is that I can set the mixture without interrupting my traffic scan. No danger of having my head down looking at a guage when I should be watching for other airplanes.

I'll drop my head for a couple seconds after using this approach to make sure the EGT or CHT (depending on what plane I'm flying) is in the green. I agree, it beats spending all that time heads down staring at a guage!