4 ms·

Yes, the 15x factor (14x in your quote) is what is often cited in discussions, given all else equal. What complicates is that fossil fuels can drive propellers

by quanto 5y ago

Yes, the 15x factor (14x in your quote) is what is often cited in discussions, given all else equal. What complicates is that fossil fuels can drive propellers, turbofans, and jets, where as electric motors can only drive propellers -- each technology with its own efficiency envelop.

Robotbeat 5y ago
That’s not true. You can drive a turbofan or jet with electric motors as well. For high bypass turbofans, a prop fan is functionally identical. For jets, the heat increase can be produced by multistage compression (although there’s little reason to do that versus a single stage turbofan/prop fan except for maybe supersonic, and even then single-stage makes a lot of sense).
- guntars 5y ago
  Also there’s nothing that says a jet engine needs to use jet fuel. You could use electric heaters to heat the air, because ultimately it is a heat engine and it doesn’t care where the heat comes from. Surely, if you took an existing jet engine and put some electric heaters in it would be very inefficient, but if you played to the strengths of electric heat, you could potentially make something interesting. For example, there are no potentially corrosive combustion products and in fact you don't even need to have oxygen flowing through it - it might as well be CO2 (hello martian electric jets). The second benefit is that you can deliver electric heat into a closed system continuously without cooling it from fluid flow. It's more beneficial for furnaces and things, but maybe there's a design out there that can take advantage of it. Probably more relevant for some sort of closed cycle piston aircraft. You could reach higher internal temperatures as long as the materials can support it.
  - Robotbeat 5y ago
    The issue is that jets are super inefficient for thrust. That’s why all modern passenger jets use the jet core just as a power source pretty much and rely on the bypass fan for almost all of the thrust. There’s no good reason to replicate the jet core when you can just use an electric motor to drive the bypass fan directly.
- nbadg 5y ago
  Technically that would be a different thermodynamic cycle. For a true jet engine, you need compression, heating, and then expansion. Using the heat of compression doesn't count; it's thermodynamically equivalent to a propeller (just less efficient because not all of the work is being converted directly into kinetic energy, but rather a bunch is converted into heat). One could however make an electric jet engine. You "just" need to replace the fuel-based heating element with an electric-based one. Then the front compressor would be electric, and you'd have no use for the turbine, since its sole purpose in conventional jet engines is to extract energy from the combustion to power the compressor. I have this recurring daydream of an electrodynamic jet engine. The inlet ionizes the airflow, the compressor is electromagnetic, the heating element inductive. Just one fancy-shaped tube with a bunch of complicated electronics around it. That would be really interesting from a whole lot of perspectives -- for example, noise reduction. It also would open up a lot more flexibility in terms of power density (which is more or less determined by the amount of heat you can dump into the engine, which is more or less limited by the melting point of the turbine blades). Something else that's interesting about that idea is that it doesn't depend on oxygen in the air, which means you could use it on Mars and Venus. I've never looked into the numbers for Venus, but as I remember, the local speed of sound is a significant challenge for rotorcraft on Mars, so having a rotorless, purely-electric jet engine there could open up a lot of flexibility in terms of aircraft performance.
  - idiotsecant 5y ago
    That would be interesting! The only way I know to ionize air en masse is a strong electric arc. These planes would sound like science fair Tesla towers!
  - guntars 5y ago
    That's funny, I just wrote about the exact same idea! I literally had not heard about it until now, but clearly many people have had the thought. Do you know of anyone actually attempting to build one, toy scale or otherwise?
    - bityard 5y ago
      Someone get Colin Furze in here
akira2501 5y ago
People seem to be forgetting that the fuel is also a heat sink, and is often used to dump heat from the air conditioning systems. Planes can and do take off in +100 degF weather, I'm not entirely sure how you're going to manage all that heat on an electrical plane with no readily available sink like a wing full of fuel.

by quanto 5y ago

Robotbeat 5y ago

That’s not true. You can drive a turbofan or jet with electric motors as well. For high bypass turbofans, a prop fan is functionally identical. For jets, the heat increase can be produced by multistage compression (although there’s little reason to do that versus a single stage turbofan/prop fan except for maybe supersonic, and even then single-stage makes a lot of sense).

guntars 5y ago
Also there’s nothing that says a jet engine needs to use jet fuel. You could use electric heaters to heat the air, because ultimately it is a heat engine and it doesn’t care where the heat comes from. Surely, if you took an existing jet engine and put some electric heaters in it would be very inefficient, but if you played to the strengths of electric heat, you could potentially make something interesting. For example, there are no potentially corrosive combustion products and in fact you don't even need to have oxygen flowing through it - it might as well be CO2 (hello martian electric jets). The second benefit is that you can deliver electric heat into a closed system continuously without cooling it from fluid flow. It's more beneficial for furnaces and things, but maybe there's a design out there that can take advantage of it. Probably more relevant for some sort of closed cycle piston aircraft. You could reach higher internal temperatures as long as the materials can support it.
- Robotbeat 5y ago
  The issue is that jets are super inefficient for thrust. That’s why all modern passenger jets use the jet core just as a power source pretty much and rely on the bypass fan for almost all of the thrust. There’s no good reason to replicate the jet core when you can just use an electric motor to drive the bypass fan directly.
nbadg 5y ago
Technically that would be a different thermodynamic cycle. For a true jet engine, you need compression, heating, and then expansion. Using the heat of compression doesn't count; it's thermodynamically equivalent to a propeller (just less efficient because not all of the work is being converted directly into kinetic energy, but rather a bunch is converted into heat). One could however make an electric jet engine. You "just" need to replace the fuel-based heating element with an electric-based one. Then the front compressor would be electric, and you'd have no use for the turbine, since its sole purpose in conventional jet engines is to extract energy from the combustion to power the compressor. I have this recurring daydream of an electrodynamic jet engine. The inlet ionizes the airflow, the compressor is electromagnetic, the heating element inductive. Just one fancy-shaped tube with a bunch of complicated electronics around it. That would be really interesting from a whole lot of perspectives -- for example, noise reduction. It also would open up a lot more flexibility in terms of power density (which is more or less determined by the amount of heat you can dump into the engine, which is more or less limited by the melting point of the turbine blades). Something else that's interesting about that idea is that it doesn't depend on oxygen in the air, which means you could use it on Mars and Venus. I've never looked into the numbers for Venus, but as I remember, the local speed of sound is a significant challenge for rotorcraft on Mars, so having a rotorless, purely-electric jet engine there could open up a lot of flexibility in terms of aircraft performance.
- idiotsecant 5y ago
  That would be interesting! The only way I know to ionize air en masse is a strong electric arc. These planes would sound like science fair Tesla towers!
- guntars 5y ago
  That's funny, I just wrote about the exact same idea! I literally had not heard about it until now, but clearly many people have had the thought. Do you know of anyone actually attempting to build one, toy scale or otherwise?
  - bityard 5y ago
    Someone get Colin Furze in here

akira2501 5y ago

People seem to be forgetting that the fuel is also a heat sink, and is often used to dump heat from the air conditioning systems. Planes can and do take off in +100 degF weather, I'm not entirely sure how you're going to manage all that heat on an electrical plane with no readily available sink like a wing full of fuel.