Mixture Control on Turbines

[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electronic controls were superseded by the hydromechanical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!