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Computer drawing of gas turbine engine showing the pressure variation
 through the engine. Engine Pressure Ratio (EPR) = product of pressure
 ratio of all engine components

Most modern passenger and military aircraft are powered by gas turbine engines, which are also called jet engines. The first and most simple type of gas turbine is the turbojet.

On this slide we show how the flow pressure varies through a typical turbojet engine. The pressure is color-coded with blue indicating the lowest pressure and white the highest pressure. Air is brought into the turbojet through the inlet at the left of the computer drawing. At the rear of the inlet, the air enters the compressor. The compressor acts like many rows of airfoils, with each row producing a small increase in pressure. At the exit of the compressor, the air is at a much higher pressure than free stream. In the burner a small amount of fuel is combined with the air and ignited at near constant pressure. Leaving the burner, the hot exhaust is passed through the turbine. Energy is extracted from the flow by the turbine to turn the compressor, which is linked to the turbine by a central shaft. Some pressure is lost in the hot exhaust during this process, but the pressure entering the nozzle is still greater than free stream. The nozzle then converts the high pressure and temperature into high velocity. Because the exit velocity is greater than the free stream velocity, thrust is created as described by the thrust equation.

The engine pressure ratio (EPR) is defined to be the total pressure ratio across the engine. Using our station numbering system, EPR is the ratio of nozzle total pressure pt8 to compressor face total pressure pt2. EPR can be easily measured on an operating engine and displayed to the pilot on a cockpit dial. That is why the ratio is not defined in terms of free stream conditions. Total pressure losses in the inlet are not contained in the EPR. But if we know the EPR, the inlet losses, and the corresponding engine temperature ratio, ETR, we can easily determine the thrust of an engine using the nozzle performance information and the thrust equation. The EPR is simply the product of the pressure ratio across all of the engine components.

EPR = pt8 / pt2 = (pt3 / pt2) * (pt4 / pt3) * (pt5 / pt4) * (pt8 / pt5)

EPR = compressor pressure ratio * burner pressure ratio * turbine pressure ratio * nozzle pressure ratio

For a given engine design, we can determine the pressure ratio of each component as given on each of the component thermodynamic slides.

Here is an animated version of the graphic:

Computer animation of basic turbojet pressure variation

You can investigate the variation of pressure through an engine by using the EngineSim interactive Java applet. You can vary the performance of any of the engine parts and investigate the effects on thrust and fuel flow. EngineSim can also plot the variation of pressure through the engine.


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Editor: Nancy Hall
NASA Official: Nancy Hall
Last Updated: May 13 2021

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