Why lean at higher altitudes in a turbo'd plane?

I'll toss this in the mix, but, Turbo's work by extracting energy from the exhaust gases. Hot gas has more energy than cold gas :) ( i know, technical right?) leaning for an egt that is close to peak maximizes the efficiency of your turbo, while at the same time reducing your fuel burn. It's a win win!
 
TUCKnTRUCK said:
I'll toss this in the mix, but, Turbo's work by extracting energy from the exhaust gases. Hot gas has more energy than cold gas :) ( i know, technical right?) leaning for an egt that is close to peak maximizes the efficiency of your turbo, while at the same time reducing your fuel burn. It's a win win!
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but that would also make the turbo run hotter
 
As you go higher a turboed airplane keeps the same manifold pressure, so in my mind the same amount of oxygen into the combustion chamber. Why is it that you have to lean the mixture out at higher altitudes?
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A proerly tuned aircraft engine provides way more fuel at full power than is neccessary to keep the engine cool. Typically this produces EGT at 1300 degrees. At least with the GTSIO 520 it is imperative that you have full rich when at full power regardless of altitude. Regardless of the turbo system altitude has NO effect on BHP up to critical altitude, only temperature and power setting does. Therefore the ratio of fuel to air stays the same as per mixture setting and power setting as you climb and descend.

As you pull power back to cruise climb say 75% BHP leaning may be accomplished as per the AFM for FF or 1450 EGT. With the GTSIO 520 this is 32.5in hg/1950 RPM this is 160 pph. This gives EGT's of around 1500. This limiting factor however is NOT EGT's, it is CHT's. One must keep CHT's below 380 degrees. There is much proven science that on going CHT's above 380 produce greatly accelerated cylinder wear. Continental redlines their engines at 450 which is pure BS as far as the science goes. Make your new redline at 380. EGT's eventually effect CHT's but there is a lag, if 1450 causes 380 to be breached than you must increase speed or mixture. For our engines I prefer to keep EGT's at 1450 in climb as well as climb at 130 KIAS which is faster than the 120 KIAS reccomended. 1450 EGT means 165 pph mixture setting. You could lean back to 130 pph and save fuel and gain more BHP but CHT's would quickly climb above 400, not good. You'd be buying cylinders like crazy.

During cruise at 60-65% BHP which is 29 in hg / 1800 rpm one can lean to peak EGT. Lean of Peak (LOP) is actually preferred and is actually much easier on the engine than rich of peak (ROP). I lean to peak untill we get balanced fuel injectors due to the vibration. Fuel is not balanced enough on the stock GTSIO 520 for LOP operation. All things considered peak EGT is still better than 50-100 degrees ROP. Yes the engineers are wrong on this one based on much evidence. At this power setting with the GTSIO 520 this produces 1590 EGT. So why can you run the higher number in cruise rather than climb? This is due to the fact that their is much less power as one has brought the power and RPM's back to what equated to 60-65% BHP versus 75% BHP and due to the fact that their is much more cooling airflow over the cylinders. As one wished to descend from FL200 one does not need to touch the mix down to shutdown.

With most turbos if you keep the EGT's below 1600 in cruise and 1450 in climb you will be fine. So the limitation on leaning is keeping CHT's below 380 and this can be done both ROP and LOP as one desires.

Mixture management in a turbo is much easier than in a normal. So therefore you lean to power setting which has nothing to do with altitude and only a little to do with OAT.
 
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