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

Cessnaflyer

Cessnaflyer

Wooooooooooooooooooooooooooooooo
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?
 
To get the fuel burn way down. In the turbo'd airplane that I fly the most, it is not uncommon to burn ~32GPH with everything full forward. Once leveled off, and up to speed, I can get the burn down to 15-23GPH depending on power setting, altitude, and if we're on the rich or lean side of peak. Adjusting the mixture on the way down also keeps TITs constant as you back power off. With this big 6, I suspect it would get WAY too hot climbing out without wasting all the fuel that full rich wastes.

Not a very technical "why", but more of a practical "how" we do it.
 
Yeah that is what we do in a C182T. What I guess I am asking is, why is the stoichiometry different when you still have air getting packed in there?
 
Yeah, I think I understand the "spirit" of your question. I really don't know, but I was thinking that the stoichiometry doesn't change with altitude due to the reason you suggest. My guess is that we have to lean because full rich is setup to cool the engine and is highly inefficient from a fuel efficiency standpoint.

I'll be interested to see what others have to say about it.
 
Cessnaflyer said:
Yeah that is what we do in a C182T. What I guess I am asking is, why is the stoichiometry different when you still have air getting packed in there?
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It's not, but you don't have to run an engine at 14.7:1 all the time. You can go leaner to save fuel. Or in the case of best power, it's always produced more rich than 14.7:1. So on climb out, if you could hook up a wideband O2, (just guessing) I bet you would see something along the lines of 12:1 at full power. Lean it back to 15-16:1 in cruise to save tons of fuel.

Ever notice how best power is made so many degrees rich of peak. That's because it's made rich of the stoichiometric ratio.

Also, if you are pulling the power back below 30" or so, then you are below sea level pressure at that point and would need to still lean for that.
 
born2aviate said:
In that case...I don't see how you would ("could") save more fuel flying at 36k than at 5k.
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Up to the critical altitude of the turbos, burn will depend on power setting and leaning technique from my observations. I know when I level at 16k I'm burning the same GPH that I was at 2000'. The great thing about getting up to O2 altitudes is the increased TAS and taking advantage of winds, pulling back the power, leaning it out, and saving the fuel with altitude that way... If there is nothing preventing a climb to up there.
 
Like mentioned before.
Full rich is overly rich for take-off power. Cruise and Climb power is typically less than T/O power, which also means the mixture has to be adjusted to less.
 
If the waste-gate is closed (above FL200 say), you have to lean out (decrease manifold pressure).
 
There are basicly three types of turbo instalations in airplanes.

1st, the kind that produce a LOT of boost and can maintain sea level HP all the way up to the aircraft's service celing (Cirrius, Lancair). With these, you lean to the same GPH regardless if you are at sea level or at FL230.

2nd, Turbonormalized engines that keep sea level HP up to a given altitude (say 14000 MSL) after which they lose power just like a normal engine. Once you have reached the trubos max altitude, then you have to lean it out.

3rd, fixed wastegate turbos produce more power in thiner air, but that power drops off as you climb just like a non turbo engine.
 
Cessnaflyer said:
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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As others are saying, you have to keep most big turbo pistons way rich for cooling low and slow. Up in the colder air at cruise speeds, you can run much leaner.
 
Cessnaflyer said:
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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because burning 40 GPH sucks when you can burn 33-35 GPH for the same speeds. over the course of a year that is lot of extra fuel.
 
These are the columns you are looking for.

drunkenbeagle said:
As others are saying, you have to keep most big turbo pistons way rich for cooling low and slow. Up in the colder air at cruise speeds, you can run much leaner.
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Lean (of peak) is cooler too. Not looking to start the "OMFG U RUN LEAN OF PEAK OVERSQUARE CALLING FLIGHT STANDARDS" conversation here, just pointing out that leaner can be cooler.

As you pull the mixture control back, CHT increases, BHP increases, and EGT increases. Brake horsepower specific fuel consumption decreases (lower is better). Power (BHP) peaks first, followed by CHT, then EGT. At 50F rich of peak EGT, a common cruise mixture setting, CHT is actually highest (lower is better). At peak EGT, BHP and CHT are lower than at 50F rich of peak. On the lean side of peak EGT, BHP and CHT are lower than at 50F rich of peak.

To be precise and nitpicky: very high power settings require a richer than normal mixture to retard the combustion event and prevent detonation. Retarding the combustion events results in a lower total internal cylinder pressure, which means that temperatures will be lower. The cooling is a byproduct of the former.
 
Autothrust Blue said:
These are the columns you are looking for.



Lean (of peak) is cooler too. Not looking to start the "OMFG U RUN LEAN OF PEAK OVERSQUARE CALLING FLIGHT STANDARDS" conversation here, just pointing out that leaner can be cooler.

As you pull the mixture control back, CHT increases, BHP increases, and EGT increases. Brake horsepower specific fuel consumption decreases (lower is better). Power (BHP) peaks first, followed by CHT, then EGT. At 50F rich of peak EGT, a common cruise mixture setting, CHT is actually highest (lower is better). At peak EGT, BHP and CHT are lower than at 50F rich of peak. On the lean side of peak EGT, BHP and CHT are lower than at 50F rich of peak.

To be precise and nitpicky: very high power settings require a richer than normal mixture to retard the combustion event and prevent detonation. Retarding the combustion events results in a lower total internal cylinder pressure, which means that temperatures will be lower. The cooling is a byproduct of the former.
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Agreed. Most manufactures recommend not running above 75% power in cruise anyway, so it is usually fuel consumption we are aiming for.

I, personally, go by what the book says and do that ;). Someone took the time and money to waste a bunch of gas in test flights, so I go with what worked for them.

For modern turbo pistons, that means watching the TIT and running LOP.
 
Autothrust Blue said:
Lean (of peak) is cooler too. Not looking to start the "OMFG U RUN LEAN OF PEAK OVERSQUARE CALLING FLIGHT STANDARDS" conversation here, just pointing out that leaner can be cooler.
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I got my aircraft back from annual last week. I checked the engine's logbook entry (as I always like to do after annuals) for the compression test results.

Anyways, I always cruise at LOP, and after one year of doing so, every single cylinder's compression was higher than last year's compression. (75/80, 73/80, 76/80, 74/80 vs. 74/80, 71/80, 70/80, 72/80).

So suck on that LOP haters. That's right, suck on it. :bandit:;)
 
By the way, I'm not inferring that my engine compressions were higher directly as a result of always running LOP; however it does illustrate that running LOP is not detrimental if done correctly.
 
Apophis said:
By the way, I'm not inferring that my engine compressions were higher directly as a result of always running LOP; however it does illustrate that running LOP is not detrimental if done correctly.
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The folks that build the airplanes wouldn't suggest it otherwise ;)
 
Apophis said:
They do, you'll just have to go to China to find them now. (*cough* Teledyne/Continental *cough*) :cool:
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The state of our nation's general aviation industry is humiliating. :(

We're so risk-averse, we've forgotten that sometimes you just have to be bold, take chances, and try something new.
 
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