Fuel injection - pressure of fuel. 3

[Nabla1]

For fuel injection systems to work, the fuel must be under pressure, so that when the injection valve is released, fuel is pushed through it. How is the fuel from the tank kept under pressure?

 

[j79guy]

thruthefence: Well, this thread has fell off the tracks a ways back, so shall we carry on? Fewer stages of compressor on one hand "should" equate to lower overall simple cycle efficieny, due to lower overall compression ratio, however, the combined axial/centrifugal flow compressors of the C18/C20 family of engines were not particularily good in terms of overall compressor efficieny. If they were freshly overhauled, and the fellow who sprayed/machined the abradeable coating in the case halves was aggressive in the machining, and run them real tight, the end losses of the axial stages could be kept to a minimum. However after a few crash shutdowns, and the coatings crazing, the efficiency quickly fell off. They are just too darn small to be efficient with an axial flow, without prohibitively expensive materials and machining techniqiues. Look at a single 250 axial flow compressor stage in your hand, they're not particularily sophisticated. (Plus a low Reynolds number for each axial flow stage.)
To answer your questions, I would say Yes, Yes, and Kind of.
The original Allison 250 design was conservatively laid down quite a few years ago now, and certainly better modeling techniques have come into play in the design of the later models such as the C40. Better machining? perhaps.
The newer engines do have a higher compression ratio across the centrifugal stage, which claims back some of the "lost" efficieny by not having the axial flow stages. (Lost efficieny via lower overall compression ratio)
The single biggest increase in overall simple cycle efficieny of the newer 250 engines, comes via an increase in firing temperature. The C40 units fire considerably hotter than the old C20 units, leading to higher thermal efficiency due to basic gas turbine thermodynamics.
Yes, I too like the 250 units, and have one installed in a marine propulsion drive application. It is informative to make changes to these engines, in a non life-critical application, where you are not strapped by the FAA or DOT.
I cannot afford a C40 unit, but the older units are relatively inexpensive and available to beat on.

j79guy