Where did you learn your physics buddy?
Mechanical engineering course at university.
Dumping more fuel to reach a target temperature is less fuel efficient.
We are not trying to reach a target temperature. We're limited in how much fuel we can dump into an engine by the allowable TIT.
If the allowable TIT was higher, we could reduce the amount of cooling air we need to squeeze through the compressor, which would dramatically increase fuel efficiency.
We would simply build a much smaller engine, run it at much higher TIT, and generate the same thrust with vastly less fuel used.
Or, if we keep the engine the same size, yes we would increase fuel consumption, but we would raise the thrust so dramatically that we would actually improve fuel efficiency since the thrust would raise much more than fuel consumption.
So specific fuel consumption per thrust produced would go down.
Your turbine and your propulsive stream is driven by the expansion of air from the combustor outlet.
Yes, and, notably, not from the expansion of air the compressor compressed like you previously suggested by implying the compressor does useful work. If that was the case, then we could forgo injecting fuel at all and have our perpetuum mobile.
The total pressure change from combustor outlet to turbine is what drives your turbine and also your propulsive mass.
Not pressure, enthalpy. You are ignoring volume. Pressure can trivially be manipulated by reducing or increasing the diameter.
The Brayton cycle efficiency is indeed tied to the pressure ratio (Pr), but not because pressure is the energy source. It’s because a higher pressure ratio creates a larger spread between the compressor and turbine.
The fundamental efficiency of a jet engine (Brayton Cycle) is:
Efficiency = 1 - [ 1 / (Pr ^ ((k-1)/k)) ]
This higher ratio allows for a much higher temperature drop across the turbine relative to the temperature rise in the compressor. You aren't "driving" the engine with pressure. You are using pressure to create a state where the heat expansion does more work than the compression cost.
Why dump more fuel to reach the same stream energy when you can use less?
You can think of it the other way around. Keep the amount of fuel exactly the same, but reduce the compressor size and reduce the airflow to raise TIT and approach ideal stoichiometric ratio.
This would reduce compressor parasitic load dramatically, thus we could reduce turbine load dramatically and make the turbine smaller. The turbine would suck less mechanical energy from the gas stream, so we have more "turbojet" thrust.
Or we keep extracting the same amount of energy from the turbine and send it to a fan, which produces thrust more efficiently, hence why you see fuel economy optimized commerical engines with gigantic bypass ratios. Minimal turbojet thrust, high fan thrust.
Or you could also reduce the fuel quantity you inject, save fuel, and build a smaller engine.
Increasing TIT and reducing compressor outlet temperature is always leading to good things in terms of performance. It is up to design decisions if that is used for more thrust, more fuel economy, etc.
What you care about ultimately isn’t the temperature of the stream but the total pressure expansion.
Yes but more bang means more heat and that means more expansion. It's not like someone is waving a magic wand to expand gasses, it is made happen by injecting fuel and burning it ... no free lunch.
3. To circumvate the turbine material limit, we bleed air from the compressor and shove it directly to the turbine for cooling. The problem is that is inefficient, because to compress air you need energy, which is taken from the gasses that passes the turbine.
This is an auxiliary system not essential to the main mechanism producing thrust, and beside the actual point of contention.
No it is crucial and at the core of your misunderstanding. If we could raise the allowable TIT to the maximum combustion temperature the fuel can provide, we could reduce this "bleed air" as he called it to zero, thus reducing the compressor load dramatically. This would mean that the fuel we burn can do a lot more useful work, like producing thrust.
Higher air mass (actually more precise to say more compressed mass) = more efficient combustion for lower fuel mix.
The combustion efficiency increase (if any) is vastly smaller than the losses created by having to compress it in the first place.
Try to quantify these on an example engine, frankly any engine, and you will see the error in your ways.
High pressure is not only defined by high temp. What you care about at the end is not the temperature gain but the pressure difference from inlet to outlet.
What else but increase in temperature by fuel burn would yield to useful work?
As I said before, you care about enthalpy change, pressure alone is not a useful metric.
Mechanical compression is also driving your pressure difference from inlet to outlet, not only added heat from combustion. Another way to put it is that you can have propulsion with only compression alone (a fan) but you can’t have propulsion with only combustion alone (is a campfire propelling anything?), because it is pressure, not heat, that is driving your propulsion system (and also your turbine).
A rocket, also part of the family of reaction mass engines you see me so unknowledgeable in, do not do any compression whatsoever. Only combustion. Yet they seem to produce plenty of propulsion.
I suppose your fan gets its mechanical energy by magic?
Adding heat at a constant volume (or in a restricted flow) increases pressure and/or velocity. Again, that is how we get useful work into the system.
A fan is a way to move air, but it isn't an engine. If compression alone provided propulsion, we wouldn't need fuel tanks. We would just spin the compressor with a hand-crank.
The only reason a jet engine works is that the heat from combustion expands the air, making it do more work on the way out (turbine) than it took to squeeze it on the way in (compressor). A campfire doesn't propel anything because it isn't in a high-pressure duct. A rocket, however, is a "campfire" in a duct, and it seems to move just fine without a compressor.
You are confusing the propulsor (fan/nozzle) with the prime mover (the heat cycle). Without the heat, the pressure from the compressor is just borrowed energy that you have to pay back with interest.
