I’ve often wondered about the feasibility of a hybrid design analogous to diesel-electric locomotives. Instead of multiple turbofan engines, have a single electric generator, essentially a giant APU, which burns jet fuel. Then connect that to electric ducted fans.
Of course, for redundancy you’d probably want two generators in parallel so one can fail without you falling into the ocean.
The Wiki page lists many hybrid electric (including series hybrid electric) aircraft projects.
One of them is Zunum Aero
I don’t see chemical batteries as ever being economically viable for airplanes, at least with current technology. They’re just too heavy.
I asked a related question eight years ago:
Nice! I had many of the same thoughts. Except I now think that the volume expansion is not relevant, only the mass flow is. And obviously the air mass flow dwarfs the fuel mass flow in a jet engine. (I gave some numbers, just from Wiki specifications.)
The specific heat of the gases (CO2 and H2O versus N2 and O2) might be significant, however.
I would also be interested in hearing from an expert!
There’s obviously truth to this as evidenced by the evolution to larger and larger diameter high-bypass turbofans instead of the older turbojets; they’re more efficient and quieter, too. But I’m not sure that this adequately explains the physics behind it.
Whether the energy that goes into increasing temperature is recoverable or not seems to me to depend in large measure on how much of it is wasted by, for instance, being radiated into the atmosphere. But ISTM that in a jet engine, the compression is almost entirely isentropic; the air behind the last compressor stage typically reaches temperatures of as much as 250°C. The temperature+pressure is then greatly increased by burning jet fuel, but surely that initial high temperature is a contributor to increased velocity through the turbines and increased exhaust velocity. Again, just looking at it from a simple intuitive standpoint, the airflow is so rapid that not much of the compression heat can be lost, but contributes to increased pressure (Charles’ Law and all that).
Perhaps the issue is that the high post-compression temperature produces back pressure against the compressor blades, whereas once in the combustion chamber the gases are much more confined to being directed rearwards toward the power turbines and exhaust nozzle?
I can’t quote the physics I’m afraid, but the general rule is that accelerating a large mass of air a little bit is more efficient than accelerating a small mass of air a lot, so a high bypass engine is more efficient than a pure jet. AIUI this is true for a small prop rotating at high speed vs a large prop rotating at slower speed, so not necessarily related to the jet engine itself.
Yes. It has to do with the fact that when an aircraft engine is producing thrust by acting on air, force is the product of the mass and the acceleration of air molecules, whereas the energy required is proportional to the product of mass and velocity squared. If you can act on more mass per unit time and thus keep the velocity change (relatively) low, you reduce the insidious effect of the squared term on the energy expended.
Yes - it’s a feature of producing thrust, and in no way specific to any type of engine.
I have some experience with radio controlled electric jets (known as EDFs, Electric Ducted Fans) and they’re fast and fun but consume battery power at a prodigious rate. Not really suitable for scaling up, at this point.
Batteries appear quite viable for hybrid small short range passenger planes such as Zunum and others are designing based on existing battery technology. For very small commercial planes battery only will work. The various efforts must prove themselves, but they don’t assume any breakthrough in battery technology.
Battery powered 737/A-320 and above size airliners are universally agreed to be beyond known battery technology, at useful ranges. Even studies of hybrids at that size show only moderate reductions in fuel consumption.
That’s if electrical motor driven ducted fans count as ‘electric jet engines’. If ‘electric jet engine’ meant a gas turbine with electric resistance heaters powered by batteries as the heat source, that’s physically possible but a dead end. A battery can’t compete w/ jet fuel to simply produce heat. When the jet fuel is producing heat that a gas turbine has to turn into useful work at far less than 100% efficiency, but a battery powered electric motor is producing work directly, then even current batteries can compete at small enough scale and short enough range.
I’m not sure that works out, since now you’re adding conversion (mechanical-electric-mechanical) losses into the chain. Plus in a weight constrained vehicle that has to get off of the ground, you have the mass of two “engines” somewhere in the plane (which would be almost as heavy as the turbofans they replaced) plus the mass of the motors and electrical cabling capable of transmitting that much power.
at any rate, locomotives are diesel-electric largely because doing it that way is a lot easier than designing a mechanical transmission for the application.
Thanks to both for the explanations. Makes sense.
As a side note, but somewhat related to the question of electric power requirements, one of the salient features of any fuel-burning turbine engine in general is the ability to dump in large amounts of fuel (compared to a piston engine) and create large amounts of power, albeit not necessarily efficiently. One of the features of a nuclear power plant I visited years ago was an impressive row of ground-based jet engines for emergency backup power if the nuclear reactor(s) had to be shut down. They could have used locomotive-type giant diesel engines, but obviously concluded that this was the superior solution, despite the fact that any thrust produced was a wasted side effect. (Digression to a digression: after the Lac-Mégantic rail disaster in Quebec which among many other things interrupted the town’s power supply, they actually did use diesel locomotives to provide temporary emergency power to the town.)
So I guess the bottom line here to producing electric-powered engines that have the performance (speed and comparable range) of jet aircraft is that it’s possible in theory but requires so much energy input that we have no such source of high energy density electric power (unless fuel cells can be refined for use as such). Otherwise it would require the kind of batteries that Elon Musk can only dream of.
I am not sure we are on the same page. Some reading on gas turbines will be good.
Radiation heat loss in industrial settings doesn’t matter below about 1000C. It has nothing to do with heat loss but the 2nd law of thermodynamics: The compressor is doing work - some part of that energy will be wasted and some part will be recoverable you do work again. You can not ever recover all of it.
There is nothing special about gas turbine compressors. Most centrifugal compressors can be modeled as isentropic compression. They are more polytropic but isentropic is a good approximation:
This is a limitation not a feature. If the same compressor is operated in an industrial setting, it will have inter coolers and aftercoolers to cool down. Centrifugal compressor efficiency is a direct function of gas density, if the gas is hot (lower density) then the efficiency is lower.
No. The compressor discharge pressure is the highest pressure. Pressure doesn’t increase by burning jet fuel; temperature does. If the pressure increased, you will have reverse flow in the compressor.
The pressure remains the same, however the number of gas molecules at this pressure increases when combustion happens.
As a general statement, this is false. Example, a reciprocating-piston compressor increases both the temperature and pressure of the working fluid; if no heat transfer takes place, the compression is reversible, i.e. you can expand the fluid back to its original state, recovering the compression work entirely.
Post-compression water injection keeps temperatures down so you can put more fuel in without roasting the turbine downstream. Post-compression water injection isn’t for improving efficiency; if anything, lowering the specific heat ration for a Brayton cycle should reduce efficiency.
[quote=“Machine_Elf, post:34, topic:853450”]
As a general statement, this is false. Example, a reciprocating-piston compressor increases both the temperature and pressure of the working fluid; if no heat transfer takes place, the compression is reversible, i.e. you can expand the fluid back to its original state, recovering the compression work entirely.
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Not going to argue on entropy.
Already covered in post #13
Are you saying I’m wrong? If so, can you clarify why?
Also, just two posts up, you said:
so I’m not clear on what you mean by “not going to argue on entropy.”
Post #13 is what I was responding to. I understood your claim (re: increased power output with water injection) to be made in support of your earlier claim that lowering temperatures improves efficiency (especially with the parenthetic “as stated above” remark). Did I misunderstand the purpose of your statement in post #13?
To clarify, my concern was to achieve supersonic exhaust velocities with only electricity as the input.
So is it correct to state that it is in fact physically impossible to achieve supersonic exhaust from a mechanical propeller?
There exist supersonic propellers but those aircraft don’t even come close to achieving Mach 1 in straight level flight with them.
There exist supersonic wind tunnels but AAUI, just like supersonic aircraft they achieve supersonic speed with a convergent-divergent nozzle, not a propeller.
Is it physically impossible to propel an aircraft to supersonic speeds with a propeller or merely a materials and engineering problem?
The other question was, is “gas turbine with electric resistance heaters” the ONLY way to achieve supersonic speed with electricity? I initially thought you could achieve it with compression alone. Which is obviously physically true but is it really practically impossible?
Machine Elf - allow me to elaborate:
Say you compress a gas (it doesn’t matter with a reciprocating or centrifugal compressor). And the gas stores energy in pressure form (higher pressure) AND thermal form (higher temperature). The energy stored in pressure form is highly recoverable as work however the energy stored in thermal form is recoverable to a maximum of Carnot engine efficiency. You cannot beat that by using a piston method or any other method.
So for a gas at 250C (compressor outlet) and an ambient tempt of 20C, (regardless of pressure) The very maximum work you can get out of the thermal energy is about 44%. So you cannot recover the “compression work completely” as stated by you.
Please note that 44% is thermodynamic best - in reality the number is far far less at low temps.
Carnot heat-engine efficiency is not applicable to the simple compression/expansion processes that we’re discussing here. If adiabatic compression of a given mass of gas from V1 to V2 requires W amount of mechanical work, then adiabatic expansion back to state V1 will return W amount of mechanical work.
If the gas is at V2 and you then input Q amount of heat and expand the gas back to V1, then you are operating a heat engine and Carnot tells you what percentage of that heat Q can be converted to mechanical work (and yes, it will be less than 100%).
Machine Elf - I respect you dude and I know you are a dedicated old timer with superior intellect. The Corona virus has left me crabby and may have effected you too.
As much as I want to believe you, you can’t beat thermodynamics.
Just think - if you could get back all the work, humankind’s search for battery technology will be over. All you would then need is a compressor to compress air from solar power during sunlight hours, store the gas well insulated, and expand through an expander during the night.