Re: What is the maximum pressure in psi at the inlet of a Jet Engine?

Greetings:

Your question is complicated by what is used for the definition of inlet pressure.
Typically there are seven regions where pressure measurements are taken when designing or
testing a jet engine. These pressure regions are called stations P1 through P7. In operation
many, but not all, jet engines maintain a fairly constant pressure ratio between the
various regions for optimum operation. The turbine inlet pressure is generally not
a working parameter.

Engine working parameters for a core engine would be:
P1 ( compressor inlet ), P3 ( compressor discharge -- inlet to combustion chamber(s) ),
P7 ( exhaust, immediately aft of the last turbine stage ).

Around the core engine, fans can be added and inlets which deliver ram air to the P1
region. Inlet design is usually part of the design of the airframe, especially in high performance
aircraft. Interesting numbers for the impact of adding a turbofan to a core engine are
presented on the United Technologies (Pratt Whitney) technology pages on the web at :

http://www.utc.com/discover/index.htm

To answer your question I will discuss the core engine parameters for Pratt &
Whitney's JT8D series engines which are, according to the UTC web site:
Quote
are the most popular modern commercial engines ever made. More than
14,000 of them have been built, amassing more than half a billion hours of reliable
service since 1964. The JT8D is the workhorse powerplant for the airline industry.
More than 350 operators use the JT8D to power more than 4500 aircraft - nearly a
third of the world's commercial fleet. By setting the industry standard for low
maintenance cost, the JT8D helps make Boeing 727's, 737-200's, and McDonnell
Douglas DC-9's and MD-80's the most economic aircraft to own and operate. The
eight models that make up the JT8D family cover a thrust range from 14,000 to
17,000 pounds. The newer JT8D-200 series offers 18,500 to 21,700 pounds of
thrust, and is the exclusive power for the popular MD-80 series aircraft. The
JT8D-200 builds on the family's excellent reliability and low maintenance costs
while meeting noise and emissions regulations.
End Quote.

As for your question aircraft mechanics tell me that :
In an engine that utilizes EPR ( engine pressure ratio ) as a performance
measure, the P7 will be about twice what the P1 would be...at max power. EPR would be
around 2.0, although it could be 1.8 or 2.1 depending on atmospheric conditions. We
have measured P7 directly as after trimming an engine with our test equipment and on a
JT8 "small block" ( pre-217's ) it is about 57 inches of mercury at high power settings. So
the inlet pressure ( vacuum? ) would be roughly half of that at high power.

To convert inches of mercury to pounds per square inch (psi) you multiple by 0.4912.
Therefore P7 is about 28 psi, which is about 2 times the standard atmospheric pressure
at sea level at zero degrees C (14.7 psi). Thus using the ratios, P1 ranges from
13.3 psi to 15.6 psi.
Note that 13.3 psi is below standard air pressure and at sea level the engine would be
sucking in air. I have seen a DC-8 parked on a ramp near sea level, suck up a vortex of
water from a pan of water on the ground, while running up the engines. However, at
flying speed, ram air would increase P1 to well above the ambient air pressure and the
engine would have to reduce the inlet air pressure to reach optimum P1. Even one psi difference
over the very large diameters of today’s engines causes very high velocities of
flowing air mass. See the UTC web site for typical mass flow numbers. Also the UTC
site indicates that pressures within the engine are very great compared to P1 or P7.

Best regards, Your Mad Scientist
Adrian Popa