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My understanding of a typical jet engine is that when running, the turbine extracts energy from the hot exhaust gases. This drives the turbine, which is coupled to the compressor at the front of the engine. As a result, when the turbine accelerates, the compressor accelerates. This increases the mass flow rate of air into the engine, which in turn increases the mass flow rate out of the engine, producing more thrust. It also means more energy is extracted by the turbine, which drives the compressor even faster.

It seems as though it is a cycle that should continue to increase without limit. Therefore, my question is what stops the engine spin speed from increasing to infinity (ignoring material limits)? Why does the spin speed of the engine stabilise if the compressor and the turbine are continually driving each other at increasing rates?

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  • $\begingroup$ You control the fuel delivery to prevent that. Also, sonic conditions will limit air inflow. $\endgroup$ – Phil Sweet Mar 3 '19 at 14:55
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This an oversimplification, but it should get the idea across

How does the turbine extract energy from the air? The turbine stator (nozzle) turns the air from axial to tangential (converting internal energy to kinetic energy), and then the turbine rotor removes the tangential velocity from the air and converts it into rotational energy of the rotor. So, the amount of energy extracted by the rotor depends on the relative tangential velocity between the air and the turbine. Put another way, as the turbine speeds up, it extracts less work from the air.

The exact opposite is true for a compressor. The compressor rotor add tangential velocity to the air, and then the compressor stator turns the air from tangential back to axial. So as the compressor speeds up, it is doing more work on the air.

So start at a low speed, and then dump a bunch of fuel in the combustion. Now the turbine produces a lot of torque, and the compressor is not consuming much. The excess torque accelerates the rotor. Now at the high speed, the compressor is consuming more torque, but the turbine is producing less. There's still some excess torque, but not as much. The rotor keeps accelerating. As it goes the compressor is consuming more and more and more torque, and the turbine is producing less and less and less, until some point it balances out. The turbine is producing exactly as much as the compressor is consuming. No excess torque. Therefore it stops accelerating.

Now what this simplified description has assumed is that the amount of fuel burned (i.e. amount of heat added) is constant with speed. That's not really true. But again this should get you the idea.

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  • $\begingroup$ Thank you very much for that answer. I had no idea about the relationship between the work done by the compressor and that extracted by the turbine and how they balance each other out. $\endgroup$ – AkThao Mar 7 '19 at 20:36
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Simply put, fuel.

Fuel is the source of energy by burning and expanding the flow of air in combustion chamber, and the flow of fuel is controlled by the pilot.

Pilot or auto pilot, have a chart and an algorithm for fuel demand and push fuel levers to adjust the fuel being injected into combustion chamber.

there are other applications for jet engines such as marine or power generation for electrical plants, on those the fuel control could be semi or fully automatic.

Here is Wikipedia's diagram of a jet engine, turbo fan jet

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