EPR vs N1

[N2]

Why is it that some engine manufacturers use EPR and yet others use N1 as the primary thrust setting parameter? Certain EPR set engines will revert to N1 operation (N1 mode) anyway if the EPR system fails, so why bother? I have a tendency to lean toward N1 simply because there is less to go wrong with N1 over that of EPR indication. Your thoughts on this or on the basic functionality of these systems would be appreciated.

[quid]

With the advent of high bypass engines, EPR became a less important measure of thrust. Total thrust is fan thrust and core added together.

EPR measures core thrust, and in the old days that's all you cared about. Now N1 (the big prop) produces the lion's share of the thrust, and that's not measured by the EPR syatem.

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[cfmblast]

Totally agree with quid!!
It's all about straight jet and fan engines.
The EPR gauge has become redundant instrument for engines with a high bypass ratio.

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cfm blast

[N2]

OK,
Why then does RR and Prat still insist on using EPR as the primary thrust setting parameter, even on their latest engines?

quid,
An intergrated EPR system actually puts the EPR probe aft of the fan into the fan stream. This gives a higher EPR readout now reflecting fan power. There still must be some benifit here over N1 for all the extra agro EPR systems can present.

[Ronin]

You could measure thrust in bananas..... (as long as you know that 1.68 bananas is max thrust for the day) :)

[Drambuie]

G.E. to the best of my knowledge never had EPR..it was always N1. Pratts were nothing but EPR, and considered N1 rpm not a true measure of the ratio of reactive thrust, ie. what went in, what you did with it, and what went out. They then thought that was the best measure of "available thrust". Pt2 vis Pt7. Just way the Rollers use EPR, I don't really know, but as you say, when the EPR fails go to N1. I use EPR for the reverse reasons...Indication of icing, temp inversion, A/T malfunction, and if you have the ol' PMS on the -300, a good check.
And after all that...I use N1 for all the good reasons. T/O, CLB, CRZ opt, and last but not least Turbulence penetration. And if you were ever on the 727, then its "spin up" on final.
I just love the N1.

[HotDog]

The RB211 series uses IEPR (integrated engine pressure ratio) which closely relates to gross thrust and maintains it's accuracy despite changes in engine efficiency due to deterioration. The indication is obtained by taking pressure signals from both cold stream (PF, behid the fan) and the hot stream exhaust outlet total pressure (P8) and comparing them with intake pressure (P1). Probably a more accurate measure of engine thrust but if you are using FFRATS, N1 is a more stable source of input.

[gas path]

Hot Dog and Drambuie you got it just about right there..
The RB211 uses IEPR which is based on the on the ratio of the combined low p/x turbine exhaust (P5) and the low p/x fan exhaust (PF) over the inlet pressure (P2).The combined P5 and PF are refered to as pressure integrated (PINT).It is as you say a more accurate measure of thrust.
P and W also uses EPR but it is just a measure of exhaust p/x (P7) over ambient p/x (P2) ambient because its probe is on the side of the strut!
The RR trent epr system is similar to the Pratt. in as much it is now just low p/x turbine exhaust (P50) over inlet p/x (P20).
Accurate measure of thrust is carried out in the test cell and various forms of ballast are used to match individual engine characteristics,this is due to manufacturing tolerences.
Also for interest, on the Pratt, the turbine exhaust sleeve and plug are a matched pair and (by mix and match) you could trade off thrust for EGT margin.
GE as far as I know always went for straight N1 as the measure of thrust.
At the end of the day I don't suppose it really matters as long as you get airbourne!!! :)

[ironbutt57]

which series rb-211's we talking about?

[gas path]

Sorry Ironbutt..
That was for the RB211-524g,h,and h/t engines
for the 22b and b4 (Tristar) and the b2/c2/d4 (B747) variants although the tappings are the same the station no's were different being PF (fan p/x) and P8 (exhaust p/x) over P1 (inlet p/x).
The changes in station no's were to bring them in line with other engine manufacturers. (read American!!! :) )

I see your a 757 driver no wonder you were slightly http://www.pprune.org/ubb/NonCGI/confused.gif :)

[This message has been edited by gas path (edited 14 January 2000).]

[N2]

Gentlemen,
Thank you for your input. I agree with many of the points raised here. This, however, keeps me coming back to my original question. Why have it? Particularly as "THE" main engine indication when N1 speed is provided anyway. In an age when aircraft design is directed towards reducing crew work load, aircraft weight and complexity, EPR seems unnecessary. It is a system that is prone to error (leaks, blockages, probe icing, xmitter faults...) and has been on more than one occasion a contributing accident factor. If Rolls or Pratt are listening in, why this extra instrument to deal with? (too much to hope for I suppose).

[Drambuie]

I will keep it simple rather than going into physics. Not too good at that either!
We have two lots of engine performance, primary indications.
1. Power
2. Thust
It was always my understanding that EPR and Fuel Flow gave you the power indications, and N1 (on the Pratt anyway) was the Thrust.
Interesting to note that the Military certify and use N1 for their performance envelopes.
That is, all performance must be achieved and is based on "MIL" power. Which in our world is approximately 90%N1. Everything else is a bonus. As the engine deteriorates, they tweek it (MIL-screw) to get the guaranteed performance MAX, which is approx 90%N1. Admittedly their N1 'top' limitations are lower than some of the commercial versions of the Pratt. The ol J-79 / JT3 was a good example.
Might not answer your question, but I hope it doesn't raise further ones.

[This message has been edited by Drambuie (edited 16 January 2000).]

[ironbutt57]

just curious, as the info (minimal at best) available to us 75'ers, indicates the EPR is based on intake vs. output of the fan stage...i am a bit skeptical of this..anybody enlighten me? rb 211-535e4

[gas path]

To be honest Ironbutt, I'm not sure about the 535c and e4 on the 75 but my guess it's the same as the earlier 524 on the 74 ie. pf+p8/p1.
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anybody else ??

[gas path]

Ironbutt.. you drive 75 with 535e4 that epr system is ratio of p/x signals from fan exhaust (PF)and engine air inlet (P1) only. The 535c engine uses IEPR (PF+P8)/P1.
(sorry I just had to look it up).
Back to the original thread though, as far as I can make out N1 or EPR for thrust measurement is a historical thing, but EPR is a more accurate measurement of gross thrust.
The control laws are more accurate and power is more tightly defined.
For interest with for instance the RB211 a given EPR with a tired engine N1 and N2 will be up and N3 will be down.
cheers.

[ironbutt57]

thanks for the info gaspath, hard to find with the manuals supplied...cheers

[N2]

Thank you all for your input, good info.

[dununder_upover]

% N1 is a measure only of the rotational speed of the the fan, the actual Thrust developed will vary with the efficiency of the fan. This is in turn affected by pressure / Temperature / Density. Put simply the fan that is bent by a bird strike may still be spinning at 100% rpm but will not devlope the same thrust as a perfect unit.

EPR is a measure of the real thrust produced. ie. Input vs output pressure expressed as a ratio.
As both input & output pressures are equally effected by ambient pressure / temperature & density variations thus the indication is not affected.

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[Stamatis]

Speaking of N1%, can someone please enlighten me as to what is the base against which this percentage is measured from? In other words, what does the 100% N1 signify?

[VnV2178B]

Interesting technical points guys, but I have a sneaky feeling that EPR is used for control to keep the fuel burn down and hence the profits up.
The impression I was given on Rollers was EPR for fuel efficiency as the various bits wore out and only revert to N1/root theta if something packed-up completely (or GYH if things got really bad !).
VnV...

[Gnd_Chk_Satis]

Having seen several EPR over-read situations -doesn't take much take much damage to the fan/core rake to cause this. Famous one was a Tech Log write up, "EPR reading correct but all other parameters low". IMHO EPR indication only has a place if the fuel control uses EPR as the primary control parameter...otherwise N1 is more reliable...but not necessarily as accurate.

Go for N1 indication- control on a modern engine is using a 'model'with inputs of speeds,EGT,ff etc etc and no one controlling parameter anyway....and besides it's super easy to change a speed probe (Rollers exempt as per usual!?!) and difficult to fix a damaged EPR rake.

Ground Checked Satis- Please report futher.

[togaroo]

In the critical phase of take off wouldnt an early indication of engine problem be useful, therefore monitoring EPR would show a problem earlier than a reduction in fan speed N1 and allow direct action to be taken earlier?

As was mentioned before as long as parameters are set and agreed to before t/o roll then any deviation from the norm is an indication that something is wrong.

Technically I'm not sure if this is why EPR is still used but it would make sense to monitor the core function which at the end of the day is driving the big fan around to get you airborne.

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Its life Jim, but not as we know it!!

[bizjet pilot]

EPR is rather elegant but its a ratio, not a value. As for it being a "truer" measurement of thrust, only half convinced. Seems to me that fuel flow is the best for answering the question: how much thrust are the engines putting out. Fuel flow in lb or kg per hour will give you virtually the same indicated air speed at any indicated altitude, weight and configuration being constant. more reliable than either thrust or EPR. But problem there is fuel flow is never operationally limiting, whereas all kinds of temp or power limits expressed as N1 or EPR.

[N2]

Togaroo,

"In the critical phase of take off wouldnt an early indication of engine problem be useful, therefore monitoring EPR would show a problem earlier than a reduction in fan speed N1 and allow direct action to be taken earlier?"

The best indication (although not the only one) of a potential engine problem would be a sharp rise / spike in engine vibration with a close eye then kept on the other primary engine parameters (noteably EGT).

The fan is the big driver, the one doing the lions share of the work and the reliability of its indication is (IMHO) better than that of an EPR system. On todays N1 system's there are no moving contact parts, with the probe end itself burried inside the engine where its nice and warm and free from ice. As well I'm with Gnd_Chk_Satis, its an easier system to maintain (Roller aside). As an example, the T700 on the A330 with the common nozzle assy has the nozzle EPR trimmed with the engine. If a problem is encountered with the nozzle, you cannot just replace it, it also has to have the correct EPR trim value as well. Makes maintenance all that much more stimulating.....

GE has got the right handle here.

[Streamline]

I kind of like this question EPR versus N1 ?

What is EPR actually ?

It’s a pressure ratio, that’s all, nothing else.

It doesn’t measure thrust at all.

This pressure ratio will give an accurate image of the thrust if the physic dimensions of the engine are known in advance, which of course they are.

But suppose your big fan’s efficiency has worn a bit, or there is a bit of icing on the fan blades, or an acoustic panel ….whatever ( for example: freezing fog, we all know that just taxiing in freezing fog gets them iced up a bit, or dents in the fan etc… .

If under those conditions you set your target EPR you might find that you N1 is really revving it up to deliver your target EPR.

If now you turn around the reasoning, and you use N1 as the target value you may not have the required EPR.

On the other end if your EPR is not reading accurately you will see a bit of a strange N1.

So really it’s a bit like taking the heart beat and the blood pressure of your engine at the same time.

I am not a doctor but if blood pressure is low and heart beat high I guess something is really wrong.
I know it’s not 100% physics here, but then again, physics is only a mathematical concept to grasp reality with numbers as accurately as possible and that’s what EPR and N1 are actually doing for us.

They put one of them in the loop for the FADEC and you have a look at the other one. If the first fails you can swap the controlling one with the push of a button.


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Smooth Trimmer

[This message has been edited by Streamline (edited 28 September 2000).]