Mixture Control on Turbines

[uwochris]

Hey guys,

Is mixture control on a turbo-prop or jet a/c automatically done, or do you have to do it manually?

Also, are "condition levers" the equivalent of a mixture control- if they are, are they used in the exact same way as the red knobs in the small piston a/c? (for example, would you have to manually enrichen/lean the mixture as you climb/descend/change power/thrust settings?).

Thanks in advance,

Chris.

 

[JAFI]

Most turbine engines have a fuel control unit that automatically controls the fuel to the engine. You don't touch a "mixture" control to operate the engine.



A condition level sets a "condition" of/to the engine - flight or ground, high rpm/low rpm depending on the engine/configuration and how the manufacture wants you to operate the aircraft.



Other than that the turbine engine has a fuel cut off switch in the cockpit, either electric or cable controlled to shut down the engine by shutting off the fuel to the engine.


You will be taught how to specifically operate each aircraft at a company or a training facility when you get hired to fly a specific type of aircraft.

JAFI

 

[Gulfstream 200]

Older jets tend to have "fuel controllers" a unit on the engine (or somewhere else) that automatically regulates fuel. These still have a cable connecting the throttle to the controller..

Newer equipment may be DEEC (Digital engine control?)or FADEC (Full Authority Digital engine control?) equipped. These units electronically regulate fuel by monitoring many different parameters, from engine speed to temps to throttle position, etc...

but you really dont care, you just push and pull as necessary to get your speed -- or better yet just set your speed on the auto-throttle and let it do its job..

Keep in mind in most equipment these days there is no real manual connection between the throttles and the engines - just electrical signals.

The above is very basics...but to answer your first question NO, you dont regulate the mixtures. -- flying is now very easy.

 

[Headwind]

NO. Turbine aircraft have fuel computers. The condition levers do not control mixture and you as a pilot have no way to control mixture. They introduce fuel and ignition in the start process. They shut off fuel in the shut down process. Back is off and forward is on. The engine needs some compressed air ( usually 12% n1) before introducing fuel and ignition in the start up.
On some aircraft like the PT6-42 you have two positions for on - one is high speed idle, the other is low speed idle.
HEADWIND

 

[FracCapt]

Newer equipment may be DEEC (Digital engine control?)

Close....Digital Electronic Engine Control.. It seems to me that using the terms digital AND electronic is kind of redundant....but, what do I know, I'm just a pilot..

but you really dont care, you just push and pull as necessary to get your speed -- or better yet just set your speed on the auto-throttle and let it do its job..

For some reason, whenever I set the speed for the autothrottles, it doesn't hold very well. Usually he's distracted doing something else, like getting ATIS...

 

[Diesel]

The King air with Walter engines I fly has what they call "logic"

You turn the fuel "on" by moving the fuel controller lever out of cutoff to on.

You then hit the start switch.

At a speed of n2 depending on pressure, temp and some other stuff the engine fires up. It usually lights off around 3percent. Then it cuts the fuel off and lights it off, cuts it off, turns it on, and then stabalizes.

Temp limits are watched by the system and will perform and auto shut down
A neat engine that has a ton of power.

 

[Donsa320]

Chris

My.my, you sure got some fancy answers. <g> But perhaps the simple one is: Turbine engines run at a fuel/air mixture which is VERY lean in the first place. There is about 17 times MORE air going through the jet engine than is needed to burn all of the fuel. Compare that to a piston engine where we try to get the ratio of fuel to air to about exactly enough of each to burn all the fuel. That ratio is about 17 to 1. The jet engine is 17 TIMES leaner than that!
Now, since the jet engine is so very lean, all we do to change power output is control the fuel flow. It is very much like a blow torch. The "Fuel Control", be it mechanical or digital or whatever, trys to keep the flame perfect through a wide range of air flows as the engine rpm's and throttle movements change. You can write a book on how the fuel control does that, but it does. I hope that helps.

 

[Vortilon]

First generation fighter jets in the 50s had manual mixture controls....that had to be a handful as you climbed though the flight levels!

 

[A Squared]

Close....Digital Electronic Engine Control.. It seems to me that using the terms digital AND electronic is kind of redundant....but, what do I know, I'm just a pilot..

No, it's not redundant, electronic and digital are not synonymous. It would be entirely possible for a turbine engine to have Analog electronic engine control. I can't name specific models, but I would be surprised if there weren't engines with analog electronic controls. Certainly there are plenty of other machines which have analog electronic controls.

 

[Donsa320]

Vortilon

Ahh, the words "mixture control" and "jet engine" really don't go together.
I think that what you are referring to in the early jets was the crude fuel controls.
In those days, pre 1950, it was very possible to destroy a jet engine by too rapid an advance of throttle. One had to carefully monitor the engine while opening the throttle and watch for rpm stagnation, overtemperature, compressor stall and flame-out. The fuel controls of the day could not do a good job of adding fuel exactly correctly as required to respond to the throttle movement. Now-a-days one can move the throttles rapidly with impunity thanks to great improvements in fuel controls and the compressor acceleration bleed relief valve engineering.

But mixture contol...no such thing in jets.

-D.C.

 

[Mel Sharples]

No, it's not redundant, electronic and digital are not synonymous. It would be entirely possible for a turbine engine to have Analog electronic engine control. I can't name specific models, but I would be surprised if there weren't engines with analog electronic controls. Certainly there are plenty of other machines which have analog electronic controls.

The mighty Saab-o-matic had an ECU (electronic control unit) and then upgraded to a DECU (digital electronic control unit) on the B-models. So you are indeed correct that you can have electronic without digital, and they do exist.





.

 

[Vortilon]

Donsa320,

That's what I was referring to....didn't know this place was a checkride.

 

[Lead Sled]

The mighty Saab-o-matic had an ECU (electronic control unit) and then upgraded to a DECU (digital electronic control unit) on the B-models. So you are indeed correct that you can have electronic without digital, and they do exist.

Ditto for the Garretts, er... Allied Signal, er... Honeywell, or whatever they're calling themselves now. The engines originally came with EECs (electronic), which evolved into DEECs (digital), now we're looking at FADEC. Honeywell is in the process of getting the EEC engines out in the filed upgraded to DEECs via various MSP incentives, etc.

'Sled

 

[starchkr]

The TFE731-2-2b has EEC's normally, but you can have the upgraded DEEC's installed.

The TFE731-2-2c has the DEEC's already with it (along with higher temp limits...but that was not the topic above.)

With the EEC's we must physically set power using the N1 speed as to not exceed book limits for temp and altitude.

With the 2b DEEC's we still need to set power using N1 so as not to exceed book values.

However, with the 2c DEEC's we push the levers to the stops and let them ride, just verifying that we are getting close to book values.

But, for the original question of the post...what are mixture controls???? We have two levers, one for each engine, that's it, pretty simple. Anymore than that and i get confused as to which one to use. You need to keep it simple for feeble minds such as my own.

 

[A Squared]

We have two levers, one for each engine, that's it, pretty simple. Anymore than that and i get confused as to which one to use. You need to keep it simple for feeble minds such as my own.

lessee....., for each engine we have a control for: throttle, prop, mixture, carb heat, cowl flap, oil cooler door........that's why I'm glad I'm not a flight engineer anymore, I wasn't smart enough to keep track of all that.

 

[ATR-DRIVR]

Ahhh... to be back on the Connie, 6 or 7B....those were the days...eh Mar?

 

[mar]

Not the sentimental type.

Yeah, well, maybe I'm still starry eyed at the idea of flying something that was manufactured after my birth....but I kinda like having a job where I don't have to wash my hands *before* I use the toilet.

Asquared, you're slipping my friend. You forgot water injection, mags and oil transfer!

Or maybe those were just implied with the (...).

Actually I do kinda miss the smoke a little.

 

[TonyC]

No, it's not redundant, electronic and digital are not synonymous. It would be entirely possible for a turbine engine to have Analog electronic engine control. I can't name specific models, but I would be surprised if there weren't engines with analog electronic controls. Certainly there are plenty of other machines which have analog electronic controls.

That's exactly what I thought initially. To be sure, not all things electronic are digital; analog electronics are everywhere around us.

But once you've stipulated digital, does the word electronic (in this case) add anything meaningful? I suppose there could be a digital mechanical linkage. Any control that has only 2 positions (on/off) or any number of specific positions (off/lo/hi, Park/Reverse/Neutral/Drive/Overdrive) could technically be considered a digital control and yet be served by non-electronic (mechanical, pneumatic, hyraulic, etc.) means. So, I guess the answer is yes, the word "electronic" does add some degree of specificity. However, I believe that, for the most part, and to most people, adding "electronic" to "digital fuel control" leans towards the redundant.

And then the thought occurred to me, I really don't know that much about fuel control units, and the above discussion is based solely on knowledge of the meanings of those few words. What if there really ARE no "analog electronic fuel controls"? What if all of the fuel control units in the world that are analog are non-electronic? Although I know quite a bit about automobile mechanics, I don't consider myself to know anything close to everything about them. That said, I know of Electronic Fuel Controls, and as far as I know, they're all digital. The only analog devices I'm familiar with are called carburetors. (In fact, I still own a few of those ancient devices.)


So, anyway... now that I've availed myself of the privilege of engaging in a conversation about a technical subject with A Squared, the question on my mind is this:

Is there such a thing as an Analog Electronic Fuel Control?

 

[enigma]

Donza gave a very good answer, but I'd like to add some clarification. A turbine engine does not run any leaner than a recip.

To understand a turbine engine vs a recip, one must understand that the flame burns continously in a turbine vs. a the "start and stop" cycle of the recip.

In a turbine engine, a lot of air is blown into the combustion chambers (or burner cans), into which fuel is being sprayed. The flame takes the shape (approximately) of the fuel spray pattern. I'm sure that you've probably observed a candle flame. The candle flame is somewhat like the flame inside a turbine engine. In both, the flame is surrounded by air, but only the portion of air necessary to combine with the fuel gets burned, and the flame only exists in an area of fuel supply. Therefore, to say that a turbine burns lean is not exactly correct. But, if you understand that distinction, Donza is pretty much right on.

Now, on to my explanation. The turbine engine sprays fuel into an area that is being continuously supplied with pressurized air by the compressor section of the engine. During the start sequence, ignitors (aka spark plugs) provide a spark to start the fire. Once the fire is lit, it doesn't go out. Just as the candle burns until the wax is gone. The fuel only needs around 25% of the available air for combustion and the rest of the air actually insulates the metal from the flame and helps shape the flame. The combustion chamber is open, meaning that the fuel and O2 are being continously resupplied.

The recip engine, with it's closed combustion chamber, must reignite the mix every power stroke (which requires that the mix must be within a narrow window) , and it must also have a fairly precise mix in order to ensure complete combustion.

A turbine engine doesn't need a mixture control, it only needs a fuel injection system to supply the proper fuel for each combination of compressor pressure and power required.

I just deleted a bunch because I don't want to confuse the issue. It's been a long day. Tomorrow might demand a re-write. Especially if anything here can be applied to a GV

enigma

 

[Donsa320]

OK all you reip guys, you want to see busy, it would be a Convair 240/340/440 copilot in icing conditions. With the augmentor vanes closed, every little change in power would send the head temps and carb air temps wild. And on the go-around why is that Christly fire bell ringing with no engine fire lights?....oh yeah, augmentor overheat. <g>

Enigma, yes thanks for noting that the chemistry of fuel combustion is the same...I did not want to get into burner can dynamics, just that there is plenty of extra air moving through the engine to support complete combustion, the rest is for the jet reaction. I would like to add that in a flame-out the engine must spool down some so that the fuel air spray can spread out enough to actually touch the igniters and some compressor bleed valves usually need to be open. It will not be an immediate relight if it is a true complete flameout.