And Keith, for the high bypass turbofan engines, more RPM always doesn't mean
more TSFC.
You are quite correct, but I do not believe that I have ever said that increasing RPM alsays increases TSFC. If I did it was an error on my part.
At idle RPM most of the energy from the fuel is being used just to keep the engine running. Very little thrust is produced, so the TSFC is very high.
As RPM increases the fuel flow is increasing, but a greater proportion of the energy in the fuel is being used to produce thrust. So the TSFC gradually decreases as RPM increase. But this benefit continues only up to a certain RPM.
At very high RPM internal inefficiencies increase, thereby reducing the proportion of the fuel energy that is used to produce thrust.
The overall effect is that over the RPM range from idle to max RPM the TSFC starts high, gradually decreases until the optimum RPM is reached , then TSFC gradually increases again as RPM continues to increase above the optimum value.
But all of this is only part of the story, because other factors such as airspeed, air temperature and altitude all affect SFC. As I said in an earlier post, basic texts make a number of simplfying assumptions. In most cases these simplifying assumptions are not explicitly stated in the texts. So we should not be at all surprised when we find that real-life aircraft do not comply with the (over simplfied) conventional wisdom.