Hi Everyone,
First of all thanks for taking the time to reply to my original post. As is often the case with PPRUNE I do not really feel that I have a clear concise answer to take away from the discussion. However I believe compromise one of the universal tenets of science and indeed aviation and I certainly appreciate that there is no single factor that controls the efficiency of an engine nor the need for/benefit of a compressor section.
HOWEVER... I was really looking for a simple thermodynamic explanation that would provide the basic model on which I could add any number of exceptions and limitations. In this sense I think that slippery pete's explanation helped a lot. I have been trying to find an answer independent of this forum and have enjoyed mixed success.
From what I can summarise, and I'm happy to be corrected:
1/ Why do turbine engines require a compressor section?
There are obvious and practical reasons for having a compressor section such as were mentioned in the early replies such as the need to avoid a "bonfire" etc. but fundamentally the reason seems to be to increase the overall efficiency of the engine.
2/ Why does increasing the compression ratio (piston) or pressure ratio (turbine) improve efficiency?
I think that the general thermodynamic reasoning is that, compression when viewed as an ideal process is reversible. Meaning that if I expend a certain amount of work compressing air inside a cylinder then the same amount of work can be expelled by the piston in returning to it's original state. As it is compressed air (whether inside a piston or compressor) will increase in temperature.
The efficiency of any system is:
what you get out / what you put in
Carnot's Theorem tells us that in reality even with an idealised engine (no friction losses etc) efficiency is determined by the difference between the temperature at which heat enters the engine and the ambient temperature of the surrounding environment. Since we cannot really hope to change the ambient temperature of the environment the best we can do is improve the temperature at which the heat enters the engine. In aircraft this is done by increasing the temperature at which the fuel air mixture is ignited.
One must also consider of course the effect of practical real world factors such as material temperature limitations, volumetric efficiency effects, flame propagation, points of maximum pressure and so on so forth.
Any thoughts,
J