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Computer drawing of gas turbine engine showing the temperature variation
 through the engine. Engine Temperature Ratio (ETR) = product of temperature
 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 temperature varies through a typical turbojet engine. The temperature is color-coded, with blue indicating the lowest temperature and white the highest temperature. 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. The increase in pressure is accompanied by an increase in temperature since the compressor is doing work on the flow. In the burner a small amount of fuel is combined with the air and ignited at near constant pressure. The temperature of the flow reaches a maximum in the burner. 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. The temperature decreases across the turbine during this process, but the temperature 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 temperature ratio (ETR) is defined to be the total temperature ratio across the engine. Using our station numbering system, ETR is the ratio of nozzle total temperature Tt8 to compressor face total temperature Tt2. ETR can be easily measured on an operating engine and displayed to the pilot on a cockpit dial. If we know ETR, and the corresponding engine pressure ratio, EPR, we can easily determine the thrust of an engine using the nozzle performance information and the thrust equation. The ETR is simply the product of the temperature ratio across all of the engine components.

ETR = Tt8 / Tt2 = (Tt3 / Tt2) * (Tt4 / Tt3) * (Tt5 / Tt4) * (Tt8 / Tt5)

ETR = compressor temperature ratio * burner temperature ratio * turbine temperature ratio * nozzle temperature ratio

For a given engine design, we can determine the temperature 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 temperature variation

You can investigate the variation of temperature 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 temperature through the engine.

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Editor: Nancy Hall
NASA Official: Nancy Hall
Last Updated: May 05 2015

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