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.

 

[A Squared]

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

from the FAA's website:


In the late 1940s and early 1950s, when the gas turbine engine
and fuel control technology were being developed, a rudimentary
analog computational technology was available to implement these
controls either electronically or hydromechanically. Engine
controls based on each approach were being developed. However,
the analog electronic fuel control technology developed more
quickly, and the initial gas turbine engines were controlled with
full authority electronic controls, albeit using vacuum tubes.
Early models of engines on the U.S. Air Force B-52 bomber were
fitted with these electronic engine controls. However, the
electronic controls were superseded by the hydromechanical
controls because they demonstrated an improved reliability over
the electronic engine controls.

From the 1950s to the late 1970s, semiconductor technology
became available and advanced rapidly from transistors, through
various levels of integration, and operational amplifiers, to
solid state memories and microprocessors. Until the late 1970s,
the electronic engine controls were used only to perform
functions to protect the engine from exceeding design limits for
temperature, speed, pressure, or torque. The full authority
analog electronic engine control on the Concorde aircraft and a
few engines with limited authority controls were the exceptions
to this generalization.

 

[avbug]

From a practical perspective, the big difference between "mixture" on a reciprocating piston engine and a turbine engine is that the more fuel you dump into a turbine engine, the hotter it gets. Enrichen the mixture, add more fuel in a piston engine, and it gets cooler and cooler and eventually quits.

All turbine engines utilize a form of mechanical fuel control. This is often augmented today by various forms of electronic control. DEEC and EEC, and FADEC are various forms of electronic controls that fine tune what is mechanically commanded in the engine. In some cases, pilot in put is not direct mecanically, but is only a "request" to the fuel computer. In the event of a fuel computer failure, manual reversion takes the computer out of the fine tuning business and sets the fuel controller back to a basic setting for continued operation.

The principles behind the various engines is virtually identical. Even between a piston and a turbine engine, the concept of intake, compression, combustion, and exaust still applies. The difference between the piston engine and the turbine engine is that all the stages of the combustive process take place in one location, where in the turbine engine, they take place in separate locations throughout the engine, and are continuous. In a piston engine, the process is repeated, but only one process takes place at a time in each cylinder. A piston engine cycles, while a turbine engine operates closer to what is called a brayton cycle, a continuous flow of air and fuel. A piston engine stops and starts many times over with fuel flow, air flow, exahust, whereas the turbine does not.

From a pilot perspective, the power levers, throttles, thrust levers, whatever, are pushed up to go faster, climb faster, increase thrust, and they're pulled back to slow down. Not much difference there.

The piston engine responds almost instantly (if handled properly), whereas the turbine engine experiences a lag.

Don't get too wrapped up in electronic augmentation devices; these do not control the fuel in the engine; they only fine tune the fuel control. In the case of FADEC, which is also applied to piston engines, the concept is one of taking multiple parameters into account in one computer to limit the engine's performance, fine tune it, and protect it. The engine still uses fuel injection (turbine and piston), or carburetion (piston) for fuel control...the electronics only help, they don't do the work.

 

[Donsa320]

Chris

See, I told you, books can be written on the fuel control.<g>
Now standby, soon we'll build one for you here. <VBG>

 

[Lead Sled]

From a practical perspective, the big difference between "mixture" on a reciprocating piston engine and a turbine engine is that the more fuel you dump into a turbine engine, the hotter it gets. Enrichen the mixture, add more fuel in a piston engine, and it gets cooler and cooler and eventually quits...

...Don't get too wrapped up in electronic augmentation devices; these do not control the fuel in the engine; they only fine tune the fuel control. In the case of FADEC, which is also applied to piston engines, the concept is one of taking multiple parameters into account in one computer to limit the engine's performance, fine tune it, and protect it...

Well said Avbug. Guys, don't get too wrapped trying to pound square pegs into round holes. There is one other aspect to consider - the reason you lean the mixture on a piston engine to control the fuel/air ratio. This allows for richer fuel flows (more cooling effect) for takeoff and climb while allowing for optimized air/fuel ratios for reduced fuel consumption during cruise. At the risk of way over simplifying a very complex subject, the only way you "lean" the mixture on a jet engine is to climb. Flow flows are significanly high at low altitudes. For example, the idle fuel flow on a Lear 25 on the ground is approximately equal to the cruise fuel flow at FL410. To one degree or another, the same principle applies to every turbine - prop or jet.

As far as the Garret TFEs go, DEECs aren't FADECs and DEEC equipted engines don't have the same levels of automation or protection as FADEC engine do. What DEECs do really well is record any "exceedences" in their memory. They do have a certain advantage when it comes to setting takeoff power, but they provide few, if any additional protections beyond those provided by EECs. Granted, they have "Takeoff", "Climb", and "Cruise" positions on the quadrant, but putting the power lever in the "Climb" position isn't a guarantee that the engine is at the highest permitable climb power setting, same thing when you retard the power levers further back into the "Cruise" - you're not necessarily going to be at an appropriate for conditions max cruise power setting either. Disagree with that statement? Next time you're climbing out or at cruise, check the DEEC generated N1 verses the power charts. You'll typically find that the engines can be pushed up a bit. Personally, I think that DEECs are being pushed on us TFE731 operators as a way to keep MSP costs down - lower temperatures = less heat related engine wear = less MSP claims; but hey, that's just me.

'Sled

 

[starchkr]

Lead sled...
Our TFE's with DEEC's do not have a different setting for TO, climb or cruise. We have one setting, "to the stops", and we monitor that we do not exceed temp limits (on the -2C engines at least). Yes they do provide a good source of recording info on how the engines are operating, and they need to be downloaded every 40 cycles...unless something has happened in the engine beyond paramaters then they flash and let us know to download sooner. The DEEC's monitor temps and altitude and a few other paramaters to best "put" fuel into the engine and it will do it all on its own as we climb, there should be no "messing" with the levers once set, and the engines should spool up to actually +.75% above book numbers(engine wise) to make sure we can make the published numbers(performance wise). As far a slower temps go, our DEEC-2B engines can hold the same temps as the ones with EEC's. Now our -2C engines(always with DEEC's) can actually hold higher temps, so i don't know about the less heat related engine wear with MSP's.

As far as the other long posts with talk about mixture and fuel mixture/ air ratio...umm yeah, they were too long for me to read so i will just say, yes.

 

[Lead Sled]

Lead sled...
Our TFE's with DEEC's do not have a different setting for TO, climb or cruise. We have one setting, "to the stops", and we monitor that we do not exceed temp limits (on the -2C engines at least). Yes they do provide a good source of recording info on how the engines are operating, and they need to be downloaded every 40 cycles...unless something has happened in the engine beyond paramaters then they flash and let us know to download sooner. The DEEC's monitor temps and altitude and a few other paramaters to best "put" fuel into the engine and it will do it all on its own as we climb, there should be no "messing" with the levers once set, and the engines should spool up to actually +.75% above book numbers(engine wise) to make sure we can make the published numbers(performance wise). As far a slower temps go, our DEEC-2B engines can hold the same temps as the ones with EEC's. Now our -2C engines(always with DEEC's) can actually hold higher temps, so i don't know about the less heat related engine wear with MSP's.

As far as the other long posts with talk about mixture and fuel mixture/ air ratio...umm yeah, they were too long for me to read so i will just say, yes.

I'm most familiar DEECs on the -40's, but I assume that the retrofitted -3s will be similiar to what you have. The -40's have the markings on the quadrant.

I know it very common practice fly 'em "at the stops", but take another look at the AFM before you tell me it's OK to fly them that way. Temps aren't the only limitations on TFE731 engines. There are some other recommendations as well. Oh well, cruise and power management is a whole nuther thread.

As far as the temp thing goes, I'd bet a nickle that the differences are due to the ITT harness. I flew a Lear 35 once that had perfectly matched ITTs going in and 10 to 15 degree split after the "new" style harness was installed on one of the engines. We raised a stink - 15 degrees is a lot of power to lose at altitude. Garrett finally admitted that the new harness helps control temperatures - you can read that as it help them to keep their MSP costs down.

'Sled

 

[TonyC]

from the FAA's website:


In the late 1940s and early 1950s, when the gas turbine engine
and fuel control technology were being developed, a rudimentary
analog computational technology was available to implement these
controls either electronically or hydromechanically. Engine
controls based on each approach were being developed. However,
the analog electronic fuel control technology developed more
quickly, and the initial gas turbine engines were controlled with
full authority electronic controls, albeit using vacuum tubes.
Early models of engines on the U.S. Air Force B-52 bomber were
fitted with these electronic engine controls. However, the
electronic controls were superseded by the hydromechanical
controls because they demonstrated an improved reliability over
the electronic engine controls.

I am, as always, impressed. Not the least bit surprised, still impressed.

Thanks A Squared!