Question on EPR w.r.t. age of an engine

[ct976]

If a brand new engineis producing 60600lbs of thrust and indicating 1.8EPR and ten years down the road, itis still indicating 1.8EPR, will it still produce 60600lbs of thrust?

[ChristiaanJ]

ct976,
Thanks for your question.
Coming from the aerodynamics/avionics side of the fence, and having seen the EPR/N1 controversy, I finally went and looked up the exact definition of EPR...

My ha'pence worth would be that, for a given engine and configuration and under given ambient conditions, EPR translates directly to thrust.
Never mind the state of the machinery between the intake and the nozzle: same EPR, same thrust.
What would not be the same, due to wear and tear, would be things like fuel flow, N1, EGT, etc. to obtain the same EPR.

I hope some of the engine experts here will give a full answer.

[mono]

Yes, but the EGT for a given EPR (or even N1 for that matter) may well be higher as the engine may no longer be as efficient.

You may even get to the point where the engine wont achieve a target EPR/N1 as it exceeds its EGT limit.

A comp wash or similar procedure may restore EGT margin but eventually overhaul will be required. That said the "on-wing" times for some engines these days are staggering compared to early generation engines.

[onoff_mylife]

if the egt margin i increasing that is the probable sign of engine degrading....that i think is a god parameter forjudging engine performace

[ChristiaanJ]

mono and onoff_mylife,

I think the original question was: with an older engine, does the same EPR still represent the same thrust?

I also think both of you are right, in saying that an increase in EGT and N1 needed to achieve the same EPR are indications of decreasing engine performance.

But we're still waiting for an answer to the original question, too.

[FE Hoppy]

same epr same thrust.

[Bolty McBolt]

I agree. Same EPR same thrust.
EPR stands for Engine Pressure Ratio. i.e. The difference betwenn the pressure at the intake versus the pressure at the exhaust (basic terms)

As many have stated an older engine will have higher EGT or N1 due to loss of efficency ot the older/worn engine.

EGT margin is used as an indicator for the health of an engine. eg for a given air temp at set specific thrust (N1 or EPR) the EGT needs to be below a certain figure matched to the temp of the day. If it close or trending towards the upper limit. Quite often a compressor wash will provide a good shift in lowering the EGT.

Hope this helps :ok:

[Thridle Op Des]

....so can anyone tell me why a 340-500 at low weights and cruising say 4000 below optimum can show an EPR of less than 1 (stabilised thrust, RR Trent 500's)?

[stiffwing]

r u sure about an EPR in cruise of less than 1? Sounds improbable to me, unless they ain't turning!
At idle descent, EPR will be around .99 to 1.01.

[mustafagander]

RR use INTEGRATED EPR. I would expect the answer to lie in the factoring of the EPR of the hot stream Vs cold stream.

[Thridle Op Des]

That was the explaination I got - not that I particlarly understand the concept. All I can think of is "what information is this parameter really giving me?" I suspect very little, NI seems to give me more of sensible indication about the engine thrust though the thrust is being FADEC governed about EPR.

TOD

[lomapaseo]

All I can think of is "what information is this parameter really giving me?" I suspect very little, NI seems to give me more of sensible indication about the engine thrust though the thrust is being FADEC governed about EPR.

We have been through this same discusion for hundreds of posts on this board before.
The answers above are all correct in a general sense. That is, either EPR or N1 are both quite adeqate for determing thrust over the life of the engine.
If you want to pick apart the details, then they are both less than perfect when the engine begins to break down, either in the fan, compressors or turbine. I seem to recall that somebody on this board provided and excellent chart of the individual thrust contributors stage by stage in the engine. Add them all together and subtract the negatives and you get total thrust.
Take any one of them and imagine a deterioating effect on that one pressure and the thrust will be degraded some amount. Please understand that the N! as well as most EPR readings only provide the major part of the measurement, your real mileage may vary.
All in all it's relatively unimportant, since the aircraft can still be flown reliably using the measurement system of thrust designed for it.

[FE Hoppy]

NI seems to give me more of sensible indication about the engine thrust

N1 is just RPM of the fan. alone it gives no indication of thrust at all.

[ChristiaanJ]

lomapaseo,We have been through this same discusion for hundreds of posts on this board before. IMHO, the original question here is, to what extent EPR is an objective representation of the actual engine thrust (under given conditions, and on a given aircraft), and to what extent is it independent of the ironmongery inserted between the p2 and p8 sensors.
If that exact subject has been discussed before, could you point us to it?

FE Hoppy,N1 is just RPM of the fan. alone it gives no indication of thrust at all.Couldn't agree more.

[411A]

N1 is just RPM of the fan. alone it gives no indication of thrust at all.

Ah so, with Rollers, absolutely correct.
Quite frankly, I fail to see just why some folks want to nit-pick their AFM...what you see (and read) is what you get.

Don't agree?
Proceed to the penalty box.:ugh::ugh:

[Thridle Op Des]

ChristiaanJ, I think lomapaseo's comment was directed to me for not trawling through 'hundreds' of previous posts which explained why I was seeing an EPR of less than 1 in the cruise (I'm still to find one which related directly to this feature, I know it it to do with IEPR, but this is surely one situation where this particular dimension is simply not working in a practical sense)

I would be interested to hear why Airbus prefer to relate NI to thrust rather than EPR in all their Unreliable Speed Indication procedures.

For those more tolerant of different queries I came across this link which I though was a very interesting demonstration if the various T/P/N parameters and how they interact, though no doubt this has been posted 'hundreds' of times before.

http://www.grc.nasa.gov/WWW/K-12/airplane/ngnsim.html

[Short_Circuit]

I have been taught that the EPR will definitely reduce with wear / age at a given fan speed in all engine types as will compression ratio of a piston eng for the same basic reason. When they are tested in at test cell, the boffins fudge the indication using, trimmer resisters or CDX units or programme plugs (EPR & EGT) etc.To make it appear to produce the correct EPR. There are numerous ways to fool you jet jockeys into thinking the thrust is spot on, as indicated on the dial. Even new engines are trimmed just to make all look good and even. EGT can be fudged down with resistors to prevent limiting due to inherent indicating anomalies in individual engines. EGT will appear to drift up as the temp probes age and break. N.. figures are basic RPM displayed in %.

For post #8 & #9

EPR of just <1.0 can be normal during DES at IDLE in big inlet hi bypass fans because, EPR is the ratio of the ram air press in the nose cowl to what is pumping out the back, (fan & exhaust on some) displayed as a number 1 : 1.XX (the 1 is just not displayed to simplify the display). If the engine is at idle and not using all the air forced into the nose cowl due to ram, the incoming air will compress and spill out and not pass through the engine Therefore the ratio of inlet press to outlet press is less than 1:1 ie < 1.0 EPR.
Short CCT

[ChristiaanJ]

ShortCCT,I have been taught that the EPR will defiantly reduce with wear / age in all engine types as will compression ratio of a piston eng for the same basic reason.I suppose you mean definitely :)

What you are talking about is max attainable EPR as the engine ages.
The question here is : as long as I can run an engine at a given EPR (say 1.6) do I get the same thrust at EPR=1.6 when the engine is new, as when it's clapped out?
Theory seems to say yes. Other parameters (N1, EGT, sfc, etc.) will all have changed due to wear and tear, but "same EPR same thrust" should hold.

The rest of your first paragraph is irrelevant... we're not talking about fudging, but what happens under identical circumstances.

Your second paragraph is not quite on the same topic, but thanks for that.
I understand it as reminding us, that indicated EPR depends on a lot of things, such as sensor location, engine configuration, calibration, combination of several sensor inputs to obtain an optimum value, etc., and hence EPR=1.0 does not necessarily translate to thrust=0.

[Short_Circuit]

CJ
Short answer… yes….
But all other parameters will trend up to produce the same thrust EPR) due to wear. 1.80 indicated is 1.80 indicated new or 10 years old.

Definitely too much grape juice.:ouch: