EPR vs N1 Engine Indication

[regor]

Can anyone confirm that modern EPR indications are more characteristic of engine thrust than N1?

I have been told that an accurate EPR indication can prolong the life expectancy of an engine by 5-10%, compared to a purely N1 indicated engine.

If this is true, why is N1 still used as a primary thrust indicator on a lot of engines?

R.

:confused:

[mutt]

EPR indication can prolong the life expectancy of an engine by 5-10%

I guess that you were told this by a Rolls Royce engineer :):)


Mutt

[m&v]

rolls /Pratt use EPR .CFM uses N1...As dictated bt the constructor.:confused:

[DoctorA300]

Not true. EPR engines do not have a longer lifespan. The hole EPR/N1 argument is accademmic, it is just different ways of indicating thrust. Epr is the relationship between Ram velocity (Pt2) and core Exhaust velocity (Pt7), in short, how much does the engine accellerate the air. But since almost all engines today are highbypass engines, and threfor 70%-80% of the thrust comes from the fan air, using EPR is just as much an assumption as using N1.
Brgds
Doc

[DoctorA300]

ASFKAP,
It might just be my teflon memory (nothing sticks) setting in, but isn't the V2500 an N1 engine.
Doc

[mutt]

We operate EFIS equipped MD90s with V2500's, engine power settings are displayed as EPR.


Mutt

[Keith.Williams.]

Regor,

I cannot claim to have more than the vaguest idea of which method is used in this or that engine, so I won't enter the debate above.

But if we can get back to the original question, we can try to answer it by looking at what the two methods consist of and what their use (implicitly) assumes.

N1 is a measure of the RPM of the low pressure spool, which on a turbofan engine includes the fan. In modern turbofans, the fan produces the majority of the thrust at low airspeeds, so it is reasonable to expect that N1 will give a reasonable indication of thrust.

But as an individual engine gets older the efficiency of its various components degrades, The fan and compressor become dirty eroded and in some cases corroded. These processes reduce the efficiency of the fan and compressor, thereby reducing the thrust produced at any given N1. This means that the accuracy of N1 as an indication of thrust gradually decreases as the engine gets older. Using the same N1 throughout the life of an engine, will cause a gradually reduction in the amount of thrust being generated. So N1 is accurate when an engine is new, but becomes less accurate as it gets older.

EPR in a non-bypass engine is the ratio of the low pressure turbine outlet pressure to the low pressure compressor inlet pressure. The low pressure turbine is the last in the series and the low pressure compressor is the first in the series, so EPR is a measure of the pressure increase produced by the engine. This increased pressure is used to accelerate the air through the propelling nozzle in order to produce thrust, so EPR is a pretty good measure of thrust.

Although the gradual reduction in component efficiency referred to above, also affects engines using EPR gauges, this does not have any significant effect on the thrust generated at any given EPR. So the accuracy of EPR as a measure of thrust remains almost constant throughout the life of an engine.

The gradual reduction in component efficiency does however mean that ever-greater RPMs and turbine temperatures are required to generate a given EPR. So if we were to use the same EPR setting throughout the life of an engine, we would eventually exceed the RPM or turbine temperature limits. In reality of course we monitor these gradual changes and replace components before specifed degradation levels are exceeded.

As stated in previous posts, the use of EPR measuring systems in high by-pass engines is a bit more complicated because it has to take accoiunt of the effects of the separate hot and cold gas streams.

At the end of the day, both N1 and EPR can be used as indicators of thrust, provided the other relevant parameters such as turbine temperature are also monitored. If excessive temperatures at any given N1 or excessive RPMs or temperatures at any given EPR are ignored, it really doesn't matter what type of thrust indication system is used. Engine life will be dramatically reduced!

[Overseasaviator]

I was lead to believe that those engines which have various bleeds that open or close at certain conditions or power settings use EPR, ie Pratts or Rollers, whereas those that do not use N1 ie GE. I do not of course refer to bleeds for anti icing purposes.

[DoctorA300]

Overseasaviator,
All GE engines have VBV's (Varible Bleed Valves).

Keith,
As about 75% of the thrust is generated by the fan alone, engine deterioration will have the same effect on thrust on both EPR and N1 engines. The deterioration of the engine is monitored and given a value called EGT margin, as you propably are aware of.
Doc

[411A]

Then of course (for those old enough), seem to remember the RR Conway had no EPR gauge at all........:eek: ;)

[dogcharlietree]

Regor, to answer your question, YES, there can be a significant difference in the indications.
I suggest ppuners re-aquaint themselves with the accident report on Air Florida Boeing 737-222, N62AF, January 13, 1982. Immediately after this accident, a lot of airlines changed their procedures, shifting priority from using EPR to N1.
Admittedly that was then, things may have changed thesedays.

[Capt Claret]

I suspect the mighty ALF-502 ;) belongs to the family you describe as having "bleeds that open or close at certain conditions or power settings", it though is very definately an N1 engine.

Massive state of the art thing it is too! :O

[Keith.Williams.]

DOCTOR A300,

You are of course correct in saying that if the EPR system does not include the by-pass flow in its calculations, the resulting accuracy will deteriorate as the engine gets older, in much the same manner as N1 (as an indication of thrust) does. But more modern engines also measure the fan outlet pressures. In effect they take separate measurments of the the pressure rise in the hot stream and that in the by-pass stream. They then integrate the results to take account of the different mass flows. This sytem should (in theory at least) provide a much more accurate indication of thrust than can be provided by N1 alone.

The real problem with EPR systems is that blockage of the low pressure tapping at the compressor inlet can cause the system to over indicate the EPR. If the autothrottle or pilot are using EPR to set take-off thrust, this over-indication can have disasterous consequences, with insufficient thrust being avaialble to complete the take-off.

It is rather more difficult to imagine a situation where an N1 measuring system would give such an over-indication, but even the most inconceivable situations tend to occur eventually!

If however the problem is one of a sudden loss of fan/compressor efficiency, due to FOD for example, then an EPR system will be far more effective than an N1 system as an indication of thrust.


PITCAIRN,
I think that you are being a bit unfair in comparing a duplicated N1 system with a non-duplicated EPR system. What would be the effect if both the primary and secondary N1 systems went down?
A duplicated EPR system would case no more problems than an equivalent N1 system.

[lomapaseo]

It is rather more difficult to imagine a situation where an N1 measuring system would give such an over-indication, but even the most inconceivable situations tend to occur eventually!

If however the problem is one of a sudden loss of fan/compressor efficiency, due to FOD for example, then an EPR system will be far more effective than an N1 system as an indication of thrust.



I think think the latter statement cancels the word "inconceivable'

Takeoff/early climb thrust becomes most critical in a multiple engine bird ingestion. In that case the N1 values will not equate directly to book thrust values by a significant amount.

This disadvantage is not unique to N1 engines since as stated earlier EPR managed engines can revert to N1 when things get dicey.

In either case the pilot should still have the capability to advanced the throttles to maintain his desired pitch setting before the EGT burns out the turbines because there is no real EGT margin left after the engine is allowed to deteriorate for several months/years on-wing..

[DoctorA300]

Keith,
you said : If however the problem is one of a sudden loss of fan/compressor efficiency, due to FOD for example, then an EPR system will be far more effective than an N1 system as an indication of thrust.
I have to disagree a little bit with you one this. On a PW JT9D-59A powered A300 a few years ago, I wittnessed a birdstrike which remodelled the fan stage quiet drasitically, however the core airflow was not disturbed and therefor the EPR remainded the same, the a/c yawed very dramatically befored the crew recovered.
It is true that if you measure the fan outlet pressure, then the EPR indication is as close to an actual measurement of thrust as you can get on wing, but I only know of a couple of engines that measures that (CFM56-5 and 7) and on those it is only used for engine health monitoring.
Brgds
Doc

[lomapaseo]

Doc

I believe that the yaw was more due to the transient effects of an engine surge than a permanent large loss of fan flow capacity.

In the events where a single birds messes up the fan, as long as there is not a large rub-out of the fan rub-strip at the fan blade tips and the airfoil of most blades are still present to pump air, the engine will recover at a slightly higher N1.

In the events where the remants of a fan blade failure take out the fan tips rub strip over a large arc than the engine will either run down (older engines) or on new models, recover at a lower power by opening up bleeds.In the later case the actual thrust produced is unreliable at the gages and even may result in loss of N1 signal as well as EPR, although the EEC will synthesize a fake N1 in order to maintain what it thinks is being commanded by the throttle setting.

[DoctorA300]

Lomaspaso,
Ehhh, no. ALL the fan blades were destorted from the midban snubber and outwards, but the inner 1/2 remained undisturbed, well nearly. There was no surge, and the yaw tendancy remained until power was reduced. The EPR only dropped for 1 to 2 seconds and then recovered, the Autothrottle was engaged and corrected accordingly. The JT9D unlike the CF6-50 does not have a CDP cutback tube mounted outside the fan casing, and all blades remained attached anyway, the N1 however decreased 9%.
I know all this because I was the poor unluck Engineer that ended up with a precasionary engine replacement down route.
Brgds
Doc

[lomapaseo]

Doc

Would that be the EI-CLS in 97?

[DoctorA300]

Lomapaso,
No, it was OY-CNL in '96. I didn't know that Transair had P&W powered Busses.
Brgds
Doc

[spanner-do]

What happens when it goes wrong!

In the company I work for we recently had the situation where on a 757 with RB211 engines a damaged EPR sensor rake caused the engine to produce excess thrust.

The commanded EPR and actual (as measured ) EPR values disagreed which left a command arc showing on EICAS and apparently caused the Autothrottle to further command additional thrust.

This situation came about due to the loss of a single pitot type probe head on the EPR sensor rake.

If the engine was being controlled on commanded N1 rather than EPR then the problem would not have arisen.