EPR

[Carpass]

What does EPR mean when applied to a FADEC system? Is it to do with measuring the torque/power output of a turbofan engine? Can I also confirm that N1 is fan speed and N2 is turbine shaft speed? Many thanks to all those who posted answers for me on Air Leg. Just got Human Factors to resit now. Phoned the CAA up today and it's a 28 day period before applying for a resit.

[jetfueldrinker]

EPR = Engine Perssure Ratio. In lay mans terms, the means the difference between the pressuer of the incoming air to the pressure of the outgoing burned gasses in a jet engine, or the diffenende between the pressure of the air in the inlet section of a compressor and the compressor outlet pressure, depending on the particular application. EPR is a ratio, so naturally it has no units. EPR can directly give an accurate indication of the engines' thrust, so is used in some, but but all fan and jet engines.

As to the second part of your question, N1 relates to Low Pressure spool of a multi spool jet engine, N2 relates to the second spool(on Rolls Royce RB211 engines there are 3 spools, GE and PW tend to use just 2 spools), N3 for the third spool etc. N1 is the fan and low pressure compressor, AND the low pressure turbine, so you can deduce from this that a SPOOL consists of a compressor and a turbine section, connected by a shaft.

I hope that this is of some help, but if not, then some rotter has nicked my screen name and password, and it isn't really me posting this!

Cheers JFD

[Firkin L]

You need to be a bit careful when talking about N1 and N2. The GE CF700 (Falcon 20)has an aft fan which is referred to as N2 or Nf and N1 or Ng refers to the core engine. Just depends on the manufacturer and engine design I suppose.

[DoctorA300]

Carpass,
EPR has no particular infulence on a FADEC engine, compared to a hydromechanical controlled engine, as JFD said, it´s just a relationship between the pressure in the inlet versus the pressure in the exhaust. But if you consider the way it is measured, then the fun begins.

EPR on most comercial Jets is TOTAL pressure in the exhaust diveded by TOTAL pressure in the inlet, PT7/PT2 on a JT9 as an example, but the total pressure in the inlet changes with Airspeed, thus appearing that with higher speed you lose power compared to the same powerlever angle at a lower speed. This is especially important in the T/O roll with autothrottle engaged, at a certain speed, normally around 60 to 80 knots, the autothrottle system will no longer be alowed to adjust engine power until after liftoff, this is called "clamping"
Hope this might help you, good luck.
Brgds
Doc

[This message has been edited by DoctorA300 (edited 21 March 2001).]

[jetfueldrinker]

Franklin and Doc, thank you for your replies. I have been in aircraft maintenance for 28 years, but still recognise that I don't know it all. Glad to see that even I can still learn something new; however I have never worked on the engines that you mention, but if ever I do, then I will at least not be too complacent.

[Spoiler Mixer]

Regarding EPR, I understand Rolls Royce feel that because EPR is based on both the fan and gas generator exhaust pressure ratios, it provides the most accurate indication of engine thrust.

N1 (fan speed)is used by other maufacturers for setting power because the large fan closely approximates the fixed-pitch propellor and because of the fact that a large percentage of the total thrust is generated by the fan.

SM

[Coriolis]

Just to add to the N1/N2/Ng confusion, Allison 250 has N1 as gas generator spool, N2 for free turbine...RR Tay uses N1 for (LP) fan, N2 for HP...RR Gem has Nh for high press spool, Nl for LP spool & Nf for free turbine...GE CT58 Ng for gas generator, Nf for power turbine...etc, etc...bottom line is - don't get tangled in an argument about what N1 stands for (even on a CAA oral) 'cos different manufacturers/different engines use the same thing for different things (if you see what I mean) :)

------------------
Ground tested, no fault found

[Carpass]

Thanks a lot. Dave