Turbofan power vs. bleed air extraction: the mechanics

C150J

Well-Known Member
Can someone explain to me WHY thrust settings decrease when using bleed air (anti-ice, etc.). Obviously air is being taken from the HP compressor and not used for thrust. However, are the lower thrust settings dictated because the hot section will run "hotter" due to loss of airflow? Or is there another reason? I strongly believe that is has to be predicated on ITTs, as you could technically push the thrust levers beyond the carrots and get more thrust (obviously an no-no).

Thanks!
J.
 
Do you run an engine with an electronic control, or is it an engine with direct control?
 
When you're talking about thrust settings decreasing, are you talking about lower N1s, is a "request" carret moved, or is it a correction from a chart (reduce N1 by 1%)?

Or when you flip the heat on, does the engine appear to automatically reschedule.

The HP(N2) air is really used in the combustion/cooling process. All things being equal on a mechanically controlled engine, flipping on a bleed reduces the air the engine has, and with equal fuel flow, should cause EGT to rise.
 
When you use the Anti-ice bleeds (p3 air) usually that will use some of the air available to cool the engine. Therefore with less cooling air you are going to see a rise in ITT and a drop in N1.

This is from a jt15d engine.
 
Take a look at the ITT when you pop on the bleeds (whether 14th stage A/I or 10th stage bleeds). You are mentioning the CF24-3B1 which would narrow it down to a CRJ-200.
 
Take a look at the ITT when you pop on the bleeds (whether 14th stage A/I or 10th stage bleeds). You are mentioning the CF24-3B1 which would narrow it down to a CRJ-200.

Yes, that's what happens, and is why it's prudent to reduce CLB thrust before actuating A/I. I'm wondering about the actual mechanics of the ITT increase. Polar's explanation seems to hit the nail on the head. Regardless, thank you everyone!
 
Well if you think about it, you're taking compressed air from the 14th stage usually on the CF34's (-4's and up I believe are 10th stage?). This will cause less air to move onto the combustion section resulting in a smaller explosion. Smaller explosion equates to less power moving over the turbine blades, and at a slower speed than before. Add in the effect of cooling that would have been given with faster exhaust speeds, and you have a rise in ITT with a decrease in thrust provided.

Now, to attempt to keep max power, you can enriched the fuel/air mixture (stoichiometric ratio?) but this will lead to a higher ITT than before with even less airflow. The main limiting factor to this will be the material engineering of the combustion section and the turbine blades. While you could increase the fuel flow to maximize power, eventually the hot section will lose integrity and fail.

The new 787's got around this little issue. If I remember correctly, they are using tertiary compressors that are AC powered by the GENx's generators. Since not all power is used by the aircraft, they can avoid the damage done to the engine by using bleed air and use the generators instead to drive the extra compressor. More weight, but you can use a smaller engine overall which would make up the weight difference. Plus, no bleed air, you can ensure turbine cooling and have longer part life, meaning less MX.

Now, I don't fly the CF34's on the line nor do I fix or build them. So if I got something wrong please let me know.
 
Well if you think about it, you're taking compressed air from the 14th stage usually on the CF34's (-4's and up I believe are 10th stage?). This will cause less air to move onto the combustion section resulting in a smaller explosion. Smaller explosion equates to less power moving over the turbine blades, and at a slower speed than before. Add in the effect of cooling that would have been given with faster exhaust speeds, and you have a rise in ITT with a decrease in thrust provided.

There is no explosion in turbine engines, it's a constant fire. Except start up, there might be an explosion when the fuel lights off.

It has nothing to do with less air moving over the turbine blades and everything to do with less cooling air. The fuel nozzles inject fuel at high pressure (I've heard numbers from 600 to 1000 psi) into the burner can(s). It will mix with whatever air it need.

Now, to attempt to keep max power, you can enriched the fuel/air mixture (stoichiometric ratio?) but this will lead to a higher ITT than before with even less airflow. The main limiting factor to this will be the material engineering of the combustion section and the turbine blades. While you could increase the fuel flow to maximize power, eventually the hot section will lose integrity and fail.

While you're right that heat is the enemy of turbine engines (In a similar fashion, Hete is the enemy of ABX pilots), I think the stoichiometric ratio (someone just took an engines class) applies to recipricating engines.

The new 787's got around this little issue. If I remember correctly, they are using tertiary compressors that are AC powered by the GENx's generators. Since not all power is used by the aircraft, they can avoid the damage done to the engine by using bleed air and use the generators instead to drive the extra compressor. More weight, but you can use a smaller engine overall which would make up the weight difference. Plus, no bleed air, you can ensure turbine cooling and have longer part life, meaning less MX.

Just like our space program, using yesterday's technology to power us into tomorrow. The oldest turbine engines used a mechanically driven compressor to provide compressed air for auxiliary systems. While it's true the engine doesn't suffer a loss of internal air, the engine still must produce enough power to drive the mechanical compressors. Throw in an extra set of oil seals on the gear box to leak, an extra set of gear teeth to inject shrapnel into the accessory gearbox, more moving mechanincal parts to maintain, I'll call it a wash in the reliability department.

Obviously, I'm not a GE engineer, but we'll see how it goes.

Now, I don't fly the CF34's on the line nor do I fix or build them. So if I got something wrong please let me know.
 
Ok, first off, this is my reply, but I'd like a second voice on top of polars, just because I know books vs line vs mx vs mfg are all different in some sense. Anyone here an engineer and build these things?


There is no explosion in turbine engines, it's a constant fire. Except start up, there might be an explosion when the fuel lights off.

Call it a continual explosion if you will. You're right that there's not a spark to speak of, but Jet A plus oxygen, plus plasma equates to ignition. A flame increases temperature as some volume is converted to a plasma state and neighboring air increases in temperature. In an explosion you're dealing with a rapid rise in temperature and mainly pressure. That pressure must drive the turbine blades and is henceforth the really important part. If we could do it without the heat... great!

It has nothing to do with less air moving over the turbine blades and everything to do with less cooling air. The fuel nozzles inject fuel at high pressure (I've heard numbers from 600 to 1000 psi) into the burner can(s). It will mix with whatever air it need.

We're still dealing with less air though right? The inlet is being rammed with a certain amount of air that is then being compressed by the stages. At the end of that, some air (even on the low amount let's say 5%, and I say low since the stage will be low pressure compared to the compressor section, so high pressure to low pressure, a lot of volume will be lost. The air would rather go through the SOV valve than go into the combustion section, which is high pressure... hence compressor stalls when the inlet doesn't ram enough air in...) will go to the anti-ice and not the combustor. Less air equates to a smaller boom unless fuel takes a great hold, which it's not due to just less air for cooling, but also a greater percentage of fuel, leading to the higher ITT.


While you're right that heat is the enemy of turbine engines (In a similar fashion, Hete is the enemy of ABX pilots), I think the stoichiometric ratio (someone just took an engines class) applies to recipricating engines.

Actually took the engine class 4 years ago, and advanced aerodynamics 2 years ago I think... it's been a while. I just still teach it. Thanks though!

Just like our space program, using yesterday's technology to power us into tomorrow. The oldest turbine engines used a mechanically driven compressor to provide compressed air for auxiliary systems. While it's true the engine doesn't suffer a loss of internal air, the engine still must produce enough power to drive the mechanical compressors. Throw in an extra set of oil seals on the gear box to leak, an extra set of gear teeth to inject shrapnel into the accessory gearbox, more moving mechanincal parts to maintain, I'll call it a wash in the reliability department.

Obviously, I'm not a GE engineer, but we'll see how it goes.

Hmmm, but if you already need the generator on there, simply increasing it's weight by say 5% for a 15% increase in power (I don't know if that's the amount, but it normally works along those lines. Kindof the same way as a bigger inlet increases the power disproportionally to the area of inlet increased), wouldn't that be better than robbing the precious thrust? The chance for failure should be theoretically less on the engine. Sure, now you have another system to fail, but talking about just the engine, it should be better for that. Oh, and the stoichiometric ratio is NOT just for piston engines. It applies to all engines, it's simply that you don't take it into account like you do on a Cessna with a mixture control. The stoichiometric ratio has more to do with chemistry that it does with pilot calculated performance.

I appreciate your reply polar, but I would like some clarification to make a composite of not just the pilot handbook, but the physics and mechanics of this.
 
Can someone explain to me WHY thrust settings decrease when using bleed air (anti-ice, etc.). Obviously air is being taken from the HP compressor and not used for thrust. However, are the lower thrust settings dictated because the hot section will run "hotter" due to loss of airflow? Or is there another reason? I strongly believe that is has to be predicated on ITTs, as you could technically push the thrust levers beyond the carrots and get more thrust (obviously an no-no).

Thanks!
J.

Another fun trick to watch the ITTs on the ground. The engine idling, then as power is added to begin taxiing, and the HP valves close (the reducing the demand for bleed air), as the engine decelerates before the HP valves open up, then as the HP valves open.

Even electronic controlled engines don't regulate, and it really brings out the theory.
 
Actually took the engine class 4 years ago, and advanced aerodynamics 2 years ago I think... it's been a while. I just still teach it. Thanks though!

I think with an attitude like that, I'd be dropping that class fast! Polar 1, Proud 0.
 
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