Why does increasing bleed air load increase ITT?

[ravisurdhar]

This doesn't seem to make sense to me. Why would turning the defog on (for example) increase the ITT? You're extracting more air prior to combustion. I would assume the EDM would react with a corresponding drop in fuel flow (to keep the fuel/air mixture roughly stoichiometric), resulting in less energy released during combustion and a lower ITT.

This is specifically for a PT6A, where the bleed air for ECS is tapped off the P3 port, but I assume the theory holds for most engines.

 

[BEEF SUPREME]

True for most engines or at least the ones I have operated. Think about it this way: 60 percent of the air going through a PT6 keeps it running cool. Take away that air and you have a less efficient, hotter engine.

 

[ravisurdhar]

You seem to be implying that the majority of the air going into the engine goes through it without combusting; ie, the fuel/air mixture is far below stoichiometric. Is that true? That seems odd to me. The cutting-edge research in jet engine design is primarily focused on making the combustion products as hot as possible, so as to extract as much energy as possible out of the fuel/air mixture. Obviously, one simple way to increase the temperature is to keep your fuel/air mixture approximately stoichiometric. This might make sense to me on a turbofan, where the majority of the air actually does not get combusted, and the bypass air does indeed cool the core, but on a turboprop this doesn't apply.

 

[BEEF SUPREME]

TBM initial was where I learned the 60 percent cooling for the PT6. The instructor gave the example of adding too much power for take off with the bleed air on for pressurization. Since there isn't enough air to run the bleeds and cool the engine a lot of guys had fried their PT6 on the ground. I don't know why the TBM was so problematic. I never ran into this problem with the KA or PC12, likely the pressurization system.

Also on the Citation X the takeoff power technique is to add about 30 percent to the engines let the temps drop and then cob in the rest.

 

[Fly_Unity]

Increasing your altitude will also increase your temps because there is less air going into the engine. Also if you notice that if you jam up the power, introducing more fuel into the combustion chamber, ITT will go up, but as the fan speed starts to spool up (it lags somewhat), increased air will THEN cool down ITT.

One way to look at it.
What is separating the flame from the burner can? Air! Its like an air cushion (Another reason we dont introduce fuel during startup until that air cushion is sufficient to provide protection from the hot flame). The thinner the cushion of air, the hotter the ITT probe is.

Turning on the Bleed Air valve wont really make the flame hotter, but provide less air pressure (and therefore less of an air cushion), and the ITT probes are now closer to the flame, making it read hotter.

 

[Patrick]

Don't forget that turbine engines use the same air that gets sucked in through the compressor to also cool the turbine. Whenever you pull bleed air for whatever you're using it for (pressurization, anti-ice, heating/ cooling, etc), you're taking air away that could have been used for either combustion or for cooling. This is why, in most turbines, as you increase bleed needs, you will see a rise in EGT/ ITT and a drop in power.

The limiting factor is in the metallurgy, as yes, in an ideal world, you would run as hot as possible to maximize efficiency, but if you want the thing to last, well, you have to stay within the limits of the materials.

 

[ravisurdhar]

Turning on the Bleed Air valve wont really make the flame hotter, but provide less air pressure (and therefore less of an air cushion), and the ITT probes are now closer to the flame, making it read hotter.

OK, I could buy that.

Don't forget that turbine engines use the same air that gets sucked in through the compressor to also cool the turbine. Whenever you pull bleed air for whatever you're using it for (pressurization, anti-ice, heating/ cooling, etc), you're taking air away that could have been used for either combustion or for cooling. This is why, in most turbines, as you increase bleed needs, you will see a rise in EGT/ ITT and a drop in power.

Where is this cool air actually going? In every schematic of the engine I've seen (including in the Dash 1), air comes in the inlet, goes through the compressor, and then either goes through the left and right P3 ports to the ECS and OBOGS systems respectively, or on into the combustor, turbines, and out the exhaust stacks. Nowhere have I seen air going into the compressor, out the P3 port, then around the engine casing or something like that. The only bleed air cooling system I've heard of is on the F110, where bleed air is routed to vents in the leading edge of the turbine blades to keep them cool.

(Side note: what does the P in P3 stand for?)

 

[mikecweb]

Pressure

 

[SteveC]

OK, I could buy that.


Where is this cool air actually going? In every schematic of the engine I've seen (including in the Dash 1), air comes in the inlet, goes through the compressor, and then either goes through the left and right P3 ports to the ECS and OBOGS systems respectively, or on into the combustor, turbines, and out the exhaust stacks. Nowhere have I seen air going into the compressor, out the P3 port, then around the engine casing or something like that. The only bleed air cooling system I've heard of is on the F110, where bleed air is routed to vents in the leading edge of the turbine blades to keep them cool.

(Side note: what does the P in P3 stand for?)

Patrick and flyunity are saying the same thing. "Cooling" air = blanket of air between flame and ITT probe (and walls of combustion chamber.

 

[Autothrust Blue]

(Side note: what does the P in P3 stand for?)

P3 = the pressure of air at, for lack of a better term, station 3 in the engine. Big Pratt turboprops have P2.5 and P3 bleed air. P2.5 is between the LP and HP compressors, and P3 is at the HP compressor. (Disclaimer: only tangentially familiar with the PT6 herself; this is PW118A/B-speak, but I'd imagine they use the same nomenclature. Disclaimer on top of that: It's late.)

 

[z987k]

PT6 has the P2.5 as well. At least the -27,28 and 36. I'd bet all of them do... I'd think they have to have it... with such a stupid design and all.

 

[jeflies]

I miss flying turbine


Sent from my iPhone

 

[Polar742]

When you are talking about bleed air, the only reference is from the "core" or gas generator part of the engine. Commonly this seems to be denoted as "the core" "N2" or "Ng".

The bleed air extraction can come from 1, 2, or 3 places along the compressor section, often referred to as "LP" - Low pressure, "IP" - Intermediate Pressure or "HP" - high pressure. Engines usually have a source of bleed air for their own internal use ( pushed through the turbine stages to keep them cooler). Also there is a means for systems required extraction, (anti-ice, packs, hydraulic reservoir head pressure).

As pointed out by the other posters, most air going through the core is used for cooling and flame control. As you pointed out, there is advanced research into flame patterns and burning all the fuel versus pushing it unburned out the back (think of old school jets with thick black smoke trails). However, the more air we take from the engine, the less we have to control flame propagation and cooling the turbine stages. If we take too much air, the flame will actually touch the burner can and the resulting effect will be much the same as taking a cutting torch to that metal.

When you accelerate a turbine engine, you first see an EGT increase as there is more fuel added, then as the rpm increases, you get more airflow and the egt temp decreases as there is more cooling air once the engine stabilizes at a set rpm. You can also see the effect of HP and LP valves opening and closing. The best way is on an engine that automatically switches, in the ground. It's a hot day outside, so you want as much air as possible through the packs. You can push up the power on one engine, watch the EGT rise, then settle back. However, if you keep pushing the power up, you'll see the EGT suddenly cool as the HP valves close and the LP valves open. Same reason on a cross-bleed start, most engines require a specific N2 to start the opposite engine. It is close to the limit of the HP valve. If you push the power too far, you'll have the HP valve closed then the LP valve opens and you may have more power on the engine (80% N2 vs 70% N2) but less air to actually start the engine.

 

[ravisurdhar]

Thanks Polar742, that was pretty helpful. So it sounds like what you're saying is that yes, a significant amount of air that goes into the engine does not undergo combustion, but just stays between the flame and the burner can, and exits out the exhaust as just regular, non-combusted (but really hot) air. So this cooling air actually isn't bled off, but if you do increase the bleed load somehow (like tuning on your environmental systems), there's less non-combusted air to insulate the burner can from the flame, hence the ITT rise.

Man, I wish we had a cutaway engine in class I could look at. Maybe I'll go down to the maintenance shop tomorrow and talk with some folks there. I really want to get up close and see it in person; all we ever do is talk about it using CAD models that look like they were made in 1985.

Just for reference we call it N1 on the T-6, not N2. And there's only one engine.

 

[z987k]

Thanks Polar742, that was pretty helpful. So it sounds like what you're saying is that yes, a significant amount of air that goes into the engine does not undergo combustion, but just stays between the flame and the burner can, and exits out the exhaust as just regular, non-combusted (but really hot) air. So this cooling air actually isn't bled off, but if you do increase the bleed load somehow (like tuning on your environmental systems), there's less non-combusted air to insulate the burner can from the flame, hence the ITT rise.

Man, I wish we had a cutaway engine in class I could look at. Maybe I'll go down to the maintenance shop tomorrow and talk with some folks there. I really want to get up close and see it in person; all we ever do is talk about it using CAD models that look like they were made in 1985.

Just for reference we call it N1 on the T-6, not N2. And there's only one engine.

N1 is the speed of the compressor stage. N2, the turbine stage. There's no way you can call N2 N1. On the pt6 in your t6, N2 is more or less your prop rpm * the reduction ratio.

 

[ravisurdhar]

N1 is the speed of the compressor stage. N2, the turbine stage. There's no way you can call N2 N1. On the pt6 in your t6, N2 is more or less your prop rpm * the reduction ratio.

Right, sorry, of course you're correct. N2 is exactly the prop RPM x the reduction ratio, as they're physically connected. We have gauges for N1 and Np.

 

[z987k]

Right, sorry, of course you're correct. N2 is exactly the prop RPM x the reduction ratio, as they're physically connected. We have gauges for N1 and Np.

Of course if you had a engine not designed by BACKWARDS Canadian's up front, N1 = N2 and we don't have Np gauges because that'd be pointless. N1 gauges tell you N1,2,p.

 

[Autothrust Blue]

Of course if you had a engine not designed by BACKWARDS Canadian's up front, N1 = N2 and we don't have Np gauges because that'd be pointless. N1 gauges tell you N1,2,p.

Not all Canuckistani engines flow backwards, just saying. We call them Nh, Nl and Np (separate shaft) on the "correctly" flowing Canuck engine.

I still don't like the screaming poop-house that is any fixed-shaft turboprop engine, no matter how pure it is from an engineering point of view.

Thanks Polar742, that was pretty helpful. So it sounds like what you're saying is that yes, a significant amount of air that goes into the engine does not undergo combustion, but just stays between the flame and the burner can, and exits out the exhaust as just regular, non-combusted (but really hot) air. So this cooling air actually isn't bled off, but if you do increase the bleed load somehow (like tuning on your environmental systems), there's less non-combusted air to insulate the burner can from the flame, hence the ITT rise.

Man, I wish we had a cutaway engine in class I could look at. Maybe I'll go down to the maintenance shop tomorrow and talk with some folks there. I really want to get up close and see it in person; all we ever do is talk about it using CAD models that look like they were made in 1985.

Just for reference we call it N1 on the T-6, not N2. And there's only one engine.

I read a good P&W pamphlet on the PT6 once - here (PDF target) it is. A lot of it is ownership stuff, but there are a few good operational tips in there too, and a bit of mechanical information specific to the power plant above and beyond what you find in an AFM/POH.

 

[shdw]

You seem to be implying that the majority of the air going into the engine goes through it without combusting

Yes, more like 75 percent is cooling and 25 percent is for combustion. The 60 percent Beef referred to is the amount of the energy, post combustion stage, that is lost to drive the compressor blades.

 

[ackeight]

TBM initial was where I learned the 60 percent cooling for the PT6. The instructor gave the example of adding too much power for take off with the bleed air on for pressurization. Since there isn't enough air to run the bleeds and cool the engine a lot of guys had fried their PT6 on the ground. I don't know why the TBM was so problematic. I never ran into this problem with the KA or PC12, likely the pressurization system.

Also on the Citation X the takeoff power technique is to add about 30 percent to the engines let the temps drop and then cob in the rest.

To answer your question it is because the 700 model uses p3 to pressurize. The 850 p2.5. They figured out the cabin size was small enough to use the lower pressure air and thus allowed the engine to be cooled better in a hot takeoff with the bleeds on. A bleeds off takeoff is not a tbm published procedure but is a commonly practiced one in the real world. Once you get good airspeed reduce the power a tad and flip the bleeds on. I think it's partly due to the smaller air inlet opening compared to other pt6 installed planes.