EPR - What is it ???

[Constelation]

I´d like to know what is EPR. Not just what it means exactly each letter but what it involves in terms of flying.
I don´t understand why some engines are controled by N1 and others by EPR.
Can anyone explain me how does it work ???

[Rainboe]

Engine Pressure Ratio. It is the Pressure behind the big fan divided by the pressure in front. It gives numbers something like 1.05 up to 1.78 on a B747 RB211 engine. There is no practical difference between measuring by this way or using N1, N2, N3 or whatever. Rolls Royce seems to prefer EPR and GE and Pratt N1. It makes no difference to the pilot.

[TR4A]

Older P&W engines like the JT8D uses EPR. EPR can give erroneous readings when ice forms on the EPR pickup tubes when anti-ice is not being used. This will give you a false EPR setting with a lower N1. This means you think you have normal power but you have a reduced power setting.

Take a look at this article:

http://www.avweb.com/news/safety/182403-1.html

[PT6ER]

Try this link...all will be explained!


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

Hope it helps!

TTFN

6ER

[Alty]

P&W still uses EPR (a "core" EPR - i.e. LP turbine exit to fan face) as the primary thrust parameter in all of their large commercial gas turbine engines, and I would assume all of their military derivatives as well. The PW4000-112" engines on the 777 are definitely EPR.

Rolls Royce, on the RB211-524 and -535, used an "integrated EPR" that is a combination of the core PR and the fan PR. I recall that the Trent 800 went to a "core" EPR like P&W.

GE has used N1 for the primary thrust parameter for all of their commercial engines (not sure about earlier military variants). For commercial transports, the first use of N1 as a thrust setting parameter was on the 747 and the DC-10.

Rolls Royce changed course on its primary thrust parameter, but I would be floored if P&W or GE would change their course in the future. The engine companies still adhere to engine design philosophies that date back several generations - you can see some of the same successes and mistakes in new designs that they have had repeatedly over the years.

[PT6ER]

Ah Alty,

do I hear the old "3 shaft" vs "2 shaft" discussion bubbling under the surface ;)

[411A]

<<For commercial transports, the first use of N1 as a thrust setting parameter was on the 747 and the DC-10.>>

Could be so, but I seem to remember the 'ole Conway powered 707's that I flew did not have EPR gauges, so N1 had to be used.

Thru the glass, darkly...:confused: :ugh:

[PT6ER]

Most of the small iron such as JT15s, PW300 series, PW500 series and TFE731s use N1.

The JT12 used EPR I believe but in my experience EPR was always the domain of the larger engines - anyone else find this to be so??

[Alty]

I'm surprised about the Conways. I have a seen a lot of 1970-ish work that was done to substantiate N1 as a primary thrust parameter for FAR 25 cert.

As for ancient design philosophies, I was thinking more about the high compressor design. One manufacturer always attempts the same OPR with fewer stages. Then again, that same manufacturer has more success at high turbines than another brand. Each brand has their typical set of problems. That makes the newer "joint" brand engines quite interesting - each company can work to their strengths, though the bar has hopefully been raised from the V2500.

[moggiee]

411a - the Conway-equipped VC10 uses both N1 and P7 (measured in inches of mercury).

Primary setting parameter is P7 but there is also a requirement for a minimum N1 of 96% for take-off.

[wondering]

From: GE-Aircraft Gas Turbine Guide

EPR

When used in conjuction with the basic turbojet and the aft fan turbofan engine, the EPR indicator is considered the primary indicator for equivalent thrust measurement.

The EPR instrument provides a readout of the ratio of the turbine discharge total pressure to the compressor inlet total pressure. On the basic turbojet the ratio is Pt5 /Pt2 where on the aft fan turbofan engine, where the turbine discharge total pressure is measured aft of the fan, it represents Pt7 /Pt2 .

The complexity of the advanced design high bypass turbofan engine has somewhat obscured the original concept of EPR as a primary thrust setting.

The EPR indicator, as applied to a front fan high bypass engine as the CF6, measures the low pressure turbine inlet pressure (Pt5.4) and the engine inlet total pressure (Pt2 ). The ratio does, however, provide a reliable secondary parameter for setting thrust.


By the way, why does GE provide a Gas Turbine Engine Guide for free but not RR? Shouldn´t RR make this information freely available to everyone for the sake of safety on the net instead of selling it in a book? :confused:

[Alty]

Never trust GE for a real-world EPR definition.

The more standard definition (ala P&W) for a dual-spool turbofan uses the LP turbine EXIT total pressure, not the inlet.

I think the first CF6-powered airplanes (N1 as primary) did have the "GE EPR" as a secondary gauge. That time has long since passed.