Will jet engines always require fossil fuel to operate, or could battery technology evolve to where electric jets are feasible? Though, is a jet engine that doesn’t burn fuel even possible?
More than possible, they exist - https://www.cnbc.com/2019/06/18/all-electric-jet-firm-eviation-announces-us-airline-as-first-customer.html
But note that the Eviation Alice (interesting choice of name) in your link is not a jet - it uses three pusher props.
The article refers to the plane as a “jet”, but I don’t think it actually is. It pretty clearly has propellers.
Also, my understanding is that one of the defining characteristic of a jet engine is that at least some of the thrust is produced by expelling exhaust gasses from burning jet fuel (although with modern high-bypass turbofans less of the thrust is produced that way than older jets). So while electric planes might be possible, I don’t think they could properly be called “jets”.
But from the looks of it, that isn’t a jet – it’s a prop plane with three electric engines powering three props (one at the rear) with a maximum speed of 240 knots, which is nowhere near the speed of a jet.’’
I’m sure we have engineering experts who can chime in, but as a WAG what I’m thinking of here is one of the very early WW II German jet prototypes. It always struck me that the efficiency of a jet engine is hampered by the fact that the expanding hot gases in the combustion chamber have to fight to some extent with the back pressure from the compression turbines, which force all the expansion out the rear nozzle. I wondered how efficient a jet engine would be if the compressors had some other source of power. Turns out, that’s how one of the early German jet prototypes was built (can’t find a cite right now) – the fan and compressors were powered by a separate piston engine. The interesting thing about that arrangement was that the majority of the engine’s thrust was actually due to the front-end fan and compressors.
Which leads to the possibility of a hybrid electric-jet where an electric motor drives the compressors, and perhaps ultimately to an all-electric version where the electric motor is the only source of power for the ducted fan, and compressors may nor may not be helpful. Modern efficient high-bypass turbofan jet engines get the majority of their thrust from the fan anyway, not from the exhaust gases. Such an arrangement would look just like a jet, could be capable of jet speeds, and be all-electric.
Electric ducted-fan engines, which are “sort of” jets, are fairly common in radio-controlled models.
The big challenge for scaling this up to airliners is the relative energy density of petroleum-based jet fuel vs batteries. The current story with state-of-the-art batteries is that jet fuel has roughly a 20 to 1 advantage.
I’m pretty sure you could build a jet engine, just like the kerosene ones, except with a big resistance heater in place of the burner cans. The problem, as stated by Xema, is that the electric power source would be prohibitively heavy.
So… hydrogen fuel cells, or bypass the electric part and just burn hydrogen directly? Of course, carrying around a lot of hydrogen in an aircraft was one of the pioneering accomplishments of the Hindenburg. :eek:
What do you think of this?
Whenever you compress any gas (including air), part of the energy goes in increasing pressure and part of it goes into increasing its temperature. (Very generalized)The energy that goes into increasing the temperature is mostly unrecoverable; only the Pressure part of the Energy is recoverable.
So a fan that moves air will always be more efficient (since there is very little compression and consequently no temperature rise) than using a compressor and a nozzle to generate thrust.
We’ve got at least 3 definitions of ‘jet’ 
- A high-speed high-cost executive passenger airplane
- A turbine engine with a shrouded propeller (turbo-fan)
- A turbine engine with a rocket-exhaust (turbo-jet)
… and then you’ve got ramjets (which probably won’t be replaced by electric) and turbo-props (which might be replaced by electrics). Turbo-props are the same kind of thing as turbo-jets and turbo-fans, but are designed differently and you can see a propeller.
For the purpose of the original question, I think that definition (1) is the most interesting. At present, there is no suggestion that electric airplanes can replace (2), mass tourist transport, or (3) manned fighter aircraft.
“jet” is just a stream of fluid. an electrically powered ducted fan which directs a stream of air out a nozzle to produce thrust is as much a jet as one burning hydrocarbons to do it.
the core stumbling block is that electrically powered transport aircraft would have to be completely re-thought, you’re not just going to take an existing airliner and swap the turbofans for electric ducted fans. just to give a sense of the scale we’re talking about, the core of the Boeing 777’s turbine engines is also used for power generation (GE LM9000) where it produces about 90,000 horsepower. that’s a tall order for an electric motor and battery setup to match. especially for something that has to get off of the ground.
The same turbine produces about 101,000 horsepower, if you spray water to cool down the air after compression (as stated above).
I believe if you have input-heat-output you generate thrust, and an electric heat pump could generate the heat (or microwaves as mentioned above). I think an electric jet engine is possible, but i don’t think it would be practical for transportation even if you ignore energy storage.
Electric planes are possible and even have special applications (long duration solar planes, perhaps very short routes) but those use high efficiency motors and propellers.
Brian
If you can use water also you might be able to compress the air electrically which will heat it up, spray liquid water that would flash into steam then come out the back.
electric jet engines are theoretically possible and a nuclear powered one was built and tested in the 50’s. Instead of using batteries to drive a heat exchanger they used the heat generated directly from a nuclear reactor. You can read about it here X-39 derivative of J-47.
The existing technology to power aircraft electrically are limited to much slower speeds than commercial jets and don’t have the equivalent range. If you add more speed to the equation then range goes down further.
Microwave air plasma thruster. Of course, the question is, how is the electricity generated.
Very interesting question, I have wondered about this myself for a long time. I’m sure it is possible but I don’t know remotely enough about thermodynamics to guess what the numbers would be.
Here’s my concrete question:
I want to fly a supersonic Concorde with electric Olympus engines. That is, I want to build an Olympus engine that drives the compressor with electric motors, and that has the same performance as the normal jet fuel engines, not counting the afterburners. (A Concorde can reach supersonic speeds without using its afterburners, in addition obviously to maintaining supersonic cruise without afterburners.)
What power do I need to drive the compressors?
Some info at the aforementioned Wiki link. In particular, we have an air mass flow of 186 kg/s (I guess during cruise) and a fuel mass flow of 1.2kg/s (that might be Takeoff not cruise, not clear). But anyway. The electric motor has to make up for the energy content of the fuel, in addition to creating the pressure at the required mass flow. And note that the conventional Olympus benefits from a relatively large ramjet effect, which I think the electric Olympus would entirely share.
What kind of ballpark power are we talking about? And would this actually work, given that such a small yet powerful motor and power source would exist?
I might note that a fuel flow of 1.2 kg/s equates to a thermal power of 54MW.
I think the compressor mechanical power would be less. Maybe half? I don’t think it would be the same? Definitely not more?
I tried to understand the formula on the Wiki page for the De Laval nozzle (which the Concorde uses).
If the jet fuel raises the T by a factor of 10, that increases the ve by a factor of 3
To achieve the same increase in ve only by raising the p, even a factor of 100,000 is not enough! Starting out with pe/p = 1/10.
(Ignoring the isentropic rise in temperature, or the compressor inefficiency, I don’t think they are that significant.)
Could it actually be true, that you CAN’T make a supersonic jet engine just by driving the compressor with an electric motor? That instead, you should just replace the conventional combustor with an electric resistance heater??