# Why do turbofan engines have low pressure compressors?

In my understanding turbofan engines usually have 2 stages of compressors:

• one low pressure compressor
• followed by a high pressure compressor

But it looks like the high pressure compressor cancels the job done by the low pressure compressor, doesn't it?

What's the point of decreasing pressure in the first compressor if you then increase it in the second compressor? Why not using only the high pressure compressor?

• where did you get the idea that a compressor would decrease the pressure? – Federico Sep 6 '15 at 21:52
• It's not unique to turbofans. All things that compress have parts that are slightly lower pressure before the part that's slightly higher pressure. That is because going from atmospheric pressure to high pressure requires the pressure to change. And since there cannot be an infinite pressure difference, the pressure must increase by a bit first then by a lot. – slebetman Sep 7 '15 at 15:10

The low pressure compressor does not reduce pressure. It increases it, but operates at a lower pressure than the high pressure compressor. Some engines also have an intermediate pressure (IP) stage.

https://en.wikipedia.org/wiki/Gas_turbine_engine_compressors

https://www.grc.nasa.gov/www/k-12/airplane/compress.html

Each stage increases the pressure as the air is forced back into the flame tubes.

• by less it is not given that the delta_p is lower, it simply works at lower absolute values of pressure (it comes firt, after all), hence the name – Federico Sep 6 '15 at 21:54
• @Federico Yes, bad choice of words. I'll edit. Thanks. – Simon Sep 7 '15 at 6:32

Early jet engines had only one set of compressor disks which were sitting on the same shaft and spinning at the same speed. More modern designs use two or even three concentric spools, each spinning at its own speed. All spools have several compressor discs at the forward and one or a few turbine discs at the rear end, and allowing them to run at different speeds allows each compressor-turbine combination to run more efficiently. Note that all compressor components increase pressure downstream.

Gas turbines must be able to run efficiently not only at full thrust, but also at intermediate thrust settings. Especially in cases where the airplane is not fully loaded, airlines prefer to reduce thrust somewhat, because this stresses the engine less. Enabling different speeds allows to adapt the engine better to the operating conditions. Adding a spool adds complexity and introduces new failure modes, but the difference in speeds between the components makes obvious how much benefit they bring. In case of the GE-90, the low pressure spool runs at 2261.5 RPM when N1 is 100%, and the high pressure spool runs more than four times faster at 9332 RPM when N2 is 100%.

Rolls-Royce's success in the engine market is largely due to its ability to tailor its Trent engine to the many variants of aircraft which are sold by Boeing and Airbus. This tailoring is done by adapting the low speed components while leaving the core identical between models. By having three components to tinker with while its competitors General Electric and Pratt & Whitney have only two, Rolls-Royce has an advantage in the business of offering tailor-made engines.

Another reason for having two independent rotating assemblies can be found on vertical take-off jets: By spinning the low- and the high-pressure components in different directions, their gyro forces can be greatly reduced. While this is not relevant to regular airplanes, hovering a vertical take-off jet is much simpler when its engine does not add a pitch motion to every yawing motion and vice versa.

• Three-spool may be today's best technical compromise between efficiency and complexity. I imagine the best effective engine would have a different rotating speed for each disk, compressor and turbine, but is far too complex to build. – mins Sep 7 '15 at 10:50