What is Engine Pressure Ratio?

I kind of know what it is but I don't fully understand it. Wikipedia says,

the engine pressure ratio (EPR) is the total pressure ratio across a jet engine, measured as the ratio of the total pressure at the exit of the propelling nozzle divided by the total pressure at the entry to the compressor.

I know where the entry of the compressor and exit of the propelling nozzle but I don't get what they mean by pressure. Do they mean air pressure?

• Related: Points of measure in a turbofan. EPR is more dependent on P2/P1 than P3/P0. Source.
– mins
Oct 16, 2017 at 0:17

The wiki quote refers to total pressure at intake and outlet. Total pressure is defined as static pressure $p_s$ and dynamic pressure $p_d$ which for incompressible flow = $\frac{1}{2} \cdot \rho \cdot V^2$.

So the Engine Pressure Ratio is $$\frac {(p_s + \frac{1}{2} \cdot \rho \cdot V^2)_{exhaust}} {(p_s + \frac{1}{2} \cdot \rho \cdot V^2)_{freeStream}}$$

The turbofan exhaust gas is a mixture of the jet exhaust and the bypass air exhaust.

Yes. It's the ratio of the pressure of the exhaust gases just before exiting the diffuser to the pressure of the air entering the intake. On high bypass turbofans this measurement is taken from the fan air just before exiting the cold diffuser.

The big question is how do you physically measure the thrust of a jet engine?

Thrust can be thought of as the pressure that the engine produces to force the airplane forward. So if you measure the pressure at the back of the engine, and allow for the pressure at the front of the engine offsetting it, then you end up with something which can be used as a measure of the thrust, and thus used to manage the engine.