Nothing like being put in one’s place by an e= xpert. I accept all comments with humble appreciation.

Bob

From: Lancair Mailing List [mailto:lml@lancaironline.net] On Behalf Of Walter Atk= inson
Sent: Thursday, June 27, 2013 9:05 AM
To: lml@lancaironline.net
Subject: [LML] Re: GAMI test

Bob:

Based on hard data, there are some significant misstatement= s in your post. I will comment in line.

**To run a TSIO 550 lean of pea= k, it must be operated below 75% power.**

Many of these engines are ope= rated above 75% power as a matter of routine while controlling CHTs. This usually requires operating it LOP. As Confucius say, &quo= t;Do not tell man something impossible when he already do it!" <g&g= t;

**but it’= s important that you know--and operate the engine at lower power settings--= if you want to run LOP. **

There is absolutely NO reason= not to operate the engine LOP at high power settings. Literally, tho= usands of pilots are doing so quite successfully.

**When you run = rich of peak, some of the fuel is used directly for cooling and passes through the engine without burning. As a matter of course, it cools by eva= poration, and leaves nasty lead and other deposits behind…on plugs, v= alves, etc. **

The heat of vaporization of a= ny extra fuel in a ROP mixture is not even a blip on the radar screen in BTUs available for cooling. The reason a richer mixture runs cool= er than, say 50dF ROP, is that the richer mixture burns slower placing the = thetaPP further from TDC. This results in lower internal cylinder pre= ssures and fewer BTUs transferred across the thermal boundary layer. Extra fuel does NOT cool a cylinder. A= full rich mixture relying on heat of vaporization for cooling would not be= 1 degree cooler!

**So when you r= un LOP and eliminate this source of cooling, you must offset it by either reducing power (total heat produced) or increasing cooling (better bafflin= g). **

When LOP, the leaner mixture = slows the burn even mores than a richer mixture and cooling is even better. It is possible, therefore, to operate at higher power settin= gs LOP with the same CHTs as ROP, or at the same power setting with cooler = CHTs.

**So as you lea= n the mixture toward max cylinder EGT, you’re reducing the amount of fuel available for cooling and the CHT’s go up.**<= /span>

No. See above.

**With most Con= tinental engines, this doesn’t happen at the same fuel flow for all cylinders, that is, one cylinder reaches peak EGT at a given fuel flow (ar= ound 16-17 GPH if you’re running about 65% power) as you lean, while = the other cylinders are still rich of peak EGT. You need to note the fuel f= low when the first cylinder reaches peak EGT. As you continue to lean, the EGT (and CHT to a lesser extent) of the = first to peak starts going down, while the remaining cylinder EGTs (and CHT= s) continue to go up. Eventually, all cylinders reach peak EGT and start de= creasing. Note the fuel flow when the last cylinder reaches peak EGT and just starts to decrease. The differ= ence in fuel flow between first and last cylinder EGT to peak is the “= ;GAMI lean test.” Ideally, the fuel flow between first and last shoul= d be less than 1 GPH; use of GAMIs can tune this to less than half a gallon per hour, BUT NOTE THAT THIS IS OPTIMIZED = FOR ONLY ONE POWER/RPM setting, so you should test/set up the engine at the= normally-expected cruise power setting.**

**Here’s = the rub: You need to be sure all cylinders are at least 30 degrees below th= eir peak EGT when operating the engine in cruise power, and this is critically= important when LOP. Ideally, all cylinders are all operating at about the = same amount below their individual peak EGTs, but the actual temperatures a= re not significant…it’s the peak value and delta below--for each cylinder--that’s significant. If the= fuel flow is not well balanced among the cylinders, the first to peak EGT = will get so lean by the time that you get the last to 30 degrees below peak= EGT that the first to peak starts mis-firing. My cylinders start mis-firing at about 120 degrees LOP as a reference point. Mine are well enough balanc= ed (no GAMIs) that I can operate the richest cylinder at 60 degrees LOP and= the leanest cylinder (first to peak when leaning) will remain below 100 de= grees LOP. That equates to about a 0.5 GPH fuel flow difference FOR MY ENGINE…but this varies a lot f= rom engine to engine, and with the power/RPM setting. **=

How far LOP one should set th= e mixture is dependent upon the power being produced, not some magic number that always works. For example, at 65% power, 20dF LOP is ade= quate. I operate routinely at 87% power and set the mixture at 70-80d= F LOP.

**BTW, the TIT’s ARE CRITICAL, and you’ll want to watch these while y= ou’re fiddling with the EGTs. Continental says the max TIT is 1750 an= d the max continuous is 1650. I am more conservative, and have a “yel= low warning” set at 1550 and a “red alarm” set at 1650. As you lean the engine, TIT’s will increase with EGTs, and will cont= inue to increase for a short time after the first EGT peaks are reached, an= d then start decreasing. I normally see about 1500 on my TITs in cruise fli= ght; I lean the mixture more if they go above 1550 in cruise flight.**

I know of no scientific data = to support your recommendations of keeping TITs under 1550. Doing so may keep you 100dF ROP or more, but it is doing nothing for the life of th= e turbo. The 1650 TIT limit is for max rotor speed at max temperature= . The turbo may be safely operate at 1649 all day long without concer= n. A s a matter of fact, that number is conservative unless one is at critical altitude.

**Continental says the max CHT is 460 degrees. I am pretty convinced you’ll cook y= our cylinders if you operate them anywhere close to that in normal cruise f= light. I have a “yellow warning” set at 400 and a “red al= arm” at 420. I only see these temps during high power climbs between about 12,000 and 16,000, when the engine is still producing lots o= f power but the air is starting to thin/provide fewer molecules to carry aw= ay the heat. In any case, when my CHT’s get to 420, I increase the fu= el flow (low boost if not already on) , richen the mixture control (climbing ROP anyway) and/or increase airspee= d. As a last resort, I’d pull the power back, but have never had to d= o this…yet.**

Agreed!

**As to CHTs duri= ng cruise flight. Get the fuel flow to the cylinders balanced first…e= ffectively you are making each cylinder produce approximately the same amount of powe= r (yields a smoother running engine)--and heat. Remember that this will be = optimized only for a small range of power/RPM settings. Then go to work on = balancing the CHTs. This is (at least theoretically) much easier than balancing the fuel flows and EGT pea= ks because you can do many things to change the cooling air flow across the= cylinders--and that’s the only variable IF you’ve balanced the= fuel flow/cylinder power output. Working CHT’s first just won’t yield good results…you’ll wind up doing= it over and over…**

That's excellent advice.

Back to precautio= ns you asked about:

1) <= span class=3D"apple-converted-space"> If you’re running LOP, be sure you have the engine producing less than = 75% of its rated power; below 65% if you’re conservative.

= ; &n= bsp; WHY???

2) <= span class=3D"apple-converted-space"> Be sure that every cylinder is LOP and that the hottest EGT is at least 30 de= grees below the cylinder’s max EGT (without consideration of the actu= al value, nor the values of any other cylinder)

= ; &n= bsp; That depends on the power be= ing produced.

3) <= span class=3D"apple-converted-space"> Keep the TITs below 1650 degrees; ideally below 1550.

= ; &n= bsp; No scientific reason for thi= s recommendation concerning 1550.

4) <= span class=3D"apple-converted-space"> Keep the CHT’s below 420; ideally below 400 (If you do the above things, = they will likely be in the 300-340 degree range)

= ; &n= bsp; Excellent advice.

I hope my response has imp= roved your appreciation of the issues. If I can expound on any of the= se, feel free to ask. You would benefit significantly by taking the APS class as all of these things will be shown with data to = support what I have said above.

Best regards,

Walter Atkinson

Advanced Pilot Seminars