About Engines

luvjascha

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What are twin spool type, Supercharged, Turbocharged engines?
I understand that this topic is too vast to cover in the forum, but nonetheless, could someone explain the three in a nutshell?

Thanks
 

avbug

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Twin spool is a term applied to turbine engines, in which one part of the engine drives another. In a typical twin spool engine, two or more turbine stages are provided. One drives the compressor section, or in other words, serves itself. The other drives a fan, which produces much of the engines thrust. Both "spools are the core of the engine...both are part of the engine producing thrust, but second and third spools drive fans, which are really just big multibladed propellers contained within a housing.

A supercharged engine uses an impeller to boost the induction air pressure in a piston engine. It drives the impeller by gearing off the crankshaft. Part of the engine power is used to boost it's own power. The higher the pressure in the induction (intake air) for a piston engine, the more power it can produce. More air available to burn more fuel means greater potential to produce more power. Large radial engines benifit from supercharging.

A turbocharged engine uses exhaust gasses to do the same thing. Instead of using gearing from the engine itself to drive the device that compresses the air, exhaust gasses are used to accomplish this.

A third system was used on the R-3350, which involved the Power Recovery Turbine. This used exhaust gasses to drive a fluid coupling that returned power back to the crankshaft directly. The engine also employed supercharging, which gave it the benifit of both supercharging and turbocharging. The difference in this case was that the turbocharging benifit didn't boost air intake or induction pressure, but ran straight to the crankshaft, in effect eliminating the drag and penalty of the supercharger.
 

mzaharis

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As usual, a picture is worth a thousand words. Here are some cross sections of some commercial jet engines (GE90 and PW4000).

http://adg.stanford.edu/aa241/propulsion/largefan.html

Both are twin-spool engines. In the GE90 series engine cross-section, which is numbered, you'll notice that the fan (1), compressors (2 for low pressure and 3 for high pressure), combustion chamber (4), high pressure turbine (5), and low pressure turbine(6).

The Fan(1), low pressure compressor (2) and low pressure turbine (6) all rotate on the same shaft, at the same speed. The fan and low pressure compressor are driven by the low pressure turbine.

The high pressure compressor (3) is driven by the high pressure turbine (5), on a second shaft, or spool (hence, twin spool). This allows the fan and low pressure compressor to be spun at its optimum speed, while the high pressure compressor can spin at its optimum speed (which is much higher, as it's trying to squeeze those molecules ever and ever closer). So having the "low pressure system" (fan, low pressure compressor, low pressure turbine) and the "high pressure system" (high pressure compressor and high pressure turbine) spin separately, they can optimize the speeds so both perform at their most efficient.

Rolls-Royce often brings it a step further, creating 3-spool engines. The first spool is the fan, driven by the low pressure turbine; the second stage is the Low pressure compressor, driven by the "Intermediate pressure turbine", and the 3rd stage is the high pressure compressor, driven by the high pressure turbine. It allows even more optimization of the rotational speeds of each component, but introduces more mechanical complexity.
 
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Regul8r

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mzaharis said:
Rolls-Royce often brings it a step further, creating 3-spool engines. The first spool is the fan, driven by the low pressure turbine; the second stage is the Low pressure compressor, driven by the "Intermediate pressure turbine", and the 3rd stage is the high pressure compressor, driven by the high pressure turbine. It allows even more optimization of the rotational speeds of each component, but introduces more mechanical complexity.
You can always bet on British Engineers to find a way from A to B with three stops in between...
 

mzaharis

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I pulled up the old articles from aviationnow.com.

Some interesting points:

1. Pulling the power off of the intermediate shaft makes the Rolls engine less suseptible to compressor stalls (again, a claim that Rolls-Royce themselves make, but they back it up with a bunch of cool looking graphs ;-) ).

2. The aircraft requires 5 times as much electrical power as a normal airliner (!).

3. The only thing that bleed air is used for is engine cowling deicing - even wing deicing is electric.

4. The real advantage of bleedless engines is that bleed air from an engine is hotter and more compressed than optimal, mostly due to the fact that you can't take the bleed air off on the stages where there are variable stators, and you need to wait until a higher stage than optimal. Electric air compressors can be set to compress the air no more than necessary, and you needn't waste energy compressing air more than necessary for cabin compression.

5. They're using the generators as starter motors - this is really a limiting factor - to get the low speed torque required to start the engines, you must run much more current through the generator/starter motors than those motors are normally designed to handle.

6. Controlling all this power flowing through the aircraft is quite a trick - the controllers require liquid cooling.
 
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