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Recently, I read manual about engine, PW JT8D-217A/C, and there is a chart making me so confused.

Manual show the reference table tells you what number, EPR, is normal in specific temperature and atmospheric pressure.

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(Click to enlarge)

We all know EPR can be related to engine thrust.

I thought higher Atmospheric Pressure meaning higher air mass density, so EPR should be higher.

At lower temperature making higher efficiency (Higher EPR) is quite easy to understand.

But according to the manual, lower Atmospheric Pressure can produce higher thrust when temperature is constant.

Also when temperature is higher than 102.2F, the EPR doesn't change anymore in different atmospheric pressure.

Why is that?

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    $\begingroup$ Compression must be reduced at high atmospheric pressure to avoid exceeding the absolute pressure limits for the inside of the engine. A lower pressure ratio results in less thrust. $\endgroup$
    – Peter Kämpf
    Commented Apr 21, 2018 at 19:42

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You can also notice that for one atmospheric pressure, the EPR remains constant until it starts to get really hot.

That's a flat rated engine (or an engine that is not easily temperature limited).

The three limits for a jet engine are: pressure limit, temperature limit, and N1 RPM limit.

How can a lower atmospheric pressure produce higher thrust when temperature is constant?

Cold weather—high EPR is as you said is straight forward.

Normally a decrease in pressure would decrease the EPR. However, engine manufacturers are willing to push the pressure limit when the ambient pressure decreases (altitude rises).

This allows more thrust for a takeoff from a high airport, which otherwise would significantly limit the engine/airplane. This increased tolerance comes at the cost of more frequent checks for the engine, i.e., more money.

We are frequently asked why this is the case. The engine manufacturers, recognizing that good takeoff performance may be critically important on the more challenging routes, allow some increase in the pressure-limited takeoff thrust setting at altitudes above sea level.

Jet Transport Performance Methods - Chapter 15

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  • $\begingroup$ Nice answer. So it's like manufacturer preset system. Engine will push the RPM to produce higher EPR when situation is allowable (low temperature). At higher temperature (like 50C) , engine already reach the limit, so even in low Atmospheric Pressure, engine still can't produce higher EPR. Is my expression correct? Thank you very much. $\endgroup$
    – ReinFore
    Commented Sep 28, 2017 at 9:56
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    $\begingroup$ @ReinFore. Yes, that's correct, as I understand it. To explain another way, it's not the lower pressure causing EPR to increase, it's the lower ambient temperature that occurs as altitude increases, that allows the engine control to increase EPR without exceeding the limits ymb1 mentions. The engine control is trying to achieve a thrust level as altitude changes that keeps aircraft performance relatively constant, so the pilot is familiar with its behaviour. $\endgroup$
    – Penguin
    Commented Sep 28, 2017 at 10:48
  • $\begingroup$ @ReinFore - I clarified the answer. $\endgroup$
    – user14897
    Commented Sep 28, 2017 at 11:22
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    $\begingroup$ @Penguin Thanks. Seems engineers put lots of effort on it in that old days. Such a outdated engine still is quite intelligent. Not as straight forward as I thought. $\endgroup$
    – ReinFore
    Commented Sep 28, 2017 at 11:54

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