EGT Start limits Turboprop vs Jet

[scramjet]

Just converting to the B737 from the Dash 8.

Does anyone know why it is that the PW123 powered Dash 100 has a Take off EGT limit of 785, a start EGT limit of 816-900 for 20 secs and 950 for 5 secs meanwhile the CFM56 powered B737 has it the other way around with a start EGT limit of 725 and a take off EGT limit of 950.

Thanks

[gaunty]

Think about when the gas path may or may not be stabilised and the consequences of it?

[tinpis]

Not a lot you can do about it is there?

[Checkerboard]

Scramjet- Cant really comment on the 737 engine but from a turbo prop point of view...........
The lower temp limit on take off compared to engine start has been explained to me by an engineer as a result of thermal core lock. If an engine fails on take off at a high temp the chance of a restart can be affected by the cooling of different components at different rates. Therefore they have to have a lower temp limit on take off to prevent thermal core lock. Make sense?? I dont profess to be an expert on this and maybe somebody else may know more, but thats just what I was told once.

[Kanga767]

I'll have a bash at explaining it.

There are two factors to take into account when discussing gas turbine deterioration in the hot section. One is combustion temperature and the other is centrifugal force on rotating components.

In toy gas turbines (turboprops, or at least all the turboprops I know), the start temp is higher than the rated take off temp because of the relatively small turbine blades and distance from centre of rotation (moment) mean that there isn't much centrifugal force on the blades at start-up, thus you can heat them up more without causing hot section stress (blade creep). There are other issues that work against this such as not as much cooling air during start because most of it is being used for combustion, however, as stated before, the dominant factor is the lack of centrifugal force.

Now, get into some real engines and the blades are bigger, heavier and further out from the centre of rotation, so now the limiting factor during start is the lack of cooling air, so the start temps are now lower than the rated take off temp.

You probably need to ignore the actual temperature values and just observe that one is higher than the other on the same engine. The PW123 measures 'EGT' in between the LP and power turbine but the CFM56-7 measures it at the inlet to the 2nd stage LP turbine. Interestingly, neither of these are actually 'EGT', more like 'ITT'. (familer with TPE 331-8/12??, they are true EGT)

The point being that the gas flow has done different amounts of work in the two engines you are trying to compare (the PW123 has driven two compressors by that stage, where the CFM has only driven the one compressor and a bit) and thus the temperatures will be different in an absolute sense.

To test the thoery, does anyone know if the temps of big turboprops like Allisons in Hercs/P3s or the PW150 DHC8-400 resemble fans??

K

[Brian Abraham]

Some figures though they dont seem to always give start figures on the FAA site.
Allison 501-D13 or 501-D13A (Lockheed Electra)
Maximum permissible temperatures:
Turbine Inlet Gas Temperature, Takeoff: 971°C
Max. Cont: 932°C
Maximum Transient (not to exceed 2 sec.) 1116°C
Rolls-Royce/SNECMA Olympus Mk 610-14-28 Concorde
EGT**
Start and relight 550C
***Contingency 883C
Takeoff (with reheat) 806C
Takeoff (without reheat) 806C
Reheated climb 755C
Maximum continuous 736C
**E.G.T. limitations vary with total temperature as presented in the approved Concorde Airplane Flight manual; the values shown herein are the highest authorized.
***Eligible only with unlimited-type fuels.
Allison 250-C20 (Bell 206 helicopter)
Take Off 810C
Max Continuous 738
Max Starting 927C for 1 second
Indicated turbine exhaust (Boeing 777)
gas temperature (T49) GE90-110B1/ -113B/ -115B
Takeoff (5 minute see NOTE 16) 1090oC***
Maximum Continuous 1050oC
Ground starts (manual or auto) 750oC
Inflight starts (manual or auto) 825oC
***30 seconds maximum transient 1095oC
SR-71 P & W JT11D-20
Max Start 565C (permitted five excursions to 649C before removal required)
In flight report EGT's experienced and the time involved any time EGT in or above the emergency opering zone is experienced (EGT above 830C below 40C compressor inlet temp; EGT above 805C above 40C compressor inlet temp), as special inspections are required.
Shutdown affected engine if,

above emergency EGT limit (845C above 40C compressor inlet temp; 865C below 40C compressor inlet temp) and below 900C for 2 minutes

between 900C and 950C for 15 seconds

over 950C for 3 seconds

When on the ground, or at low Mach numbers, engine speed varies with throttle mevement when the throttle is between idle and slightly below the Military stop. At higher settings, up to maximum afterburner, the main fuel control schedules engine speed as a function of compressor inlet temp (max limit 427C) and modulates the variable area exhaust nozzle to maintain approx constant RPM. Throttle movement in the afterburner range only changes the afterburner fuel flow, nozzle position and thrust. At high Mach number and constant inlet conditions, engine speed is essentially constant for all throttle positions down to and including idle. Once the compressor inlet temp increases to 40C the EGT remains constant in the 755C to 805C band when manually trimmed and 785C to 805C band when in auto trim.