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Jet enegines have a combustion efficiency of 100% at low altitude and 98% at cruising altitudes. High bypass jet engines will operate 90%+ propulsive efficiency

Jet enegines have a combustion efficiency of 100% at low altitude and 98% at cruising altitudes. High bypass jet engines will operate 90%+ propulsive efficiency when you reach cruising speed and altitude. This can include torbo props. https://en.m.wikipedia.org/wiki/Propulsive_efficiency https://en.m.wikipedia.org/wiki/Propulsive_efficiency Fuel has also a very big advantage over batteries and that is that you shed off weight when you use it while an empty battery will weight as much as a full one so you’ll end up carrying much more dead weight for the entire flight. Fuel cells might be a possibility but I don’t think airplanes with hydrogen tanks are a good idea.

montenegrohugo 8y ago
You are confusing combustion efficiency with thermal efficiency. Having a combustion efficiency of 100% does not mean that 100% of the energy stored in the propellant is transformed into kinetic energy (the thing the plane is interested in). The ICE engine with highest thermal efficiency reaches just 50%: The Wärtsilä-Sulzer RTA96-C turbocharged 2-stroke Diesel [1] [1]https://en.wikipedia.org/wiki/Thermal_efficiency https://en.wikipedia.org/wiki/Thermal_efficiency
- tim333 8y ago
  That's quite an engine - I just looked up a picture of it https://pbs.twimg.com/media/DQv24mYUMAAmvQs.jpg https://pbs.twimg.com/media/DQv24mYUMAAmvQs.jpg
- aunty_helen 8y ago
  The latest F1 tech has started to exceed this using their jet ignition system. https://www.autosport.com/f1/news/131772/mercedes-engine-hits-remarkable-dyno-target https://www.autosport.com/f1/news/131772/mercedes-engine-hit... https://www.enginelabs.com/news/f1-tech-what-is-turbulent-jet-ignition/ https://www.enginelabs.com/news/f1-tech-what-is-turbulent-je...
- aerodude 8y ago
  Combined cycle gas turbines are actually higher efficiency than this. The current state-of-the-art is somewhere around 63% thermal efficiency.
  - montenegrohugo 8y ago
    True, but those are usually not on a moving vehicle and have the luxury of being as heavy and big as desired. Engines on ships instead are limited by weight and size, engines on cars/trucks a little bit more, engines on planes much more and at the edge of technology we have the engines on rockets which have to be as light as humanly possible whilst still providing high thrust-specific fuel consumption
fdej 8y ago
The propulsive efficiency as defined in the Wikipedia article is the product of the cycle efficiency (the efficiency turning heat into mechanical power) and the mechanical efficiency (the efficiency turning the mechanical power generated by the engine into movement of the airplane). The mechanical efficiency can be close to 100%, but the cycle efficiency is only 40% or so.
Firadeoclus 8y ago
You could use fuel cells that work with more convenient fuels.
selimthegrim 8y ago
Would Li-air batteries be advantageous in this regard if they get them working?
peterlk 8y ago
Out of my depth here. Why are tanks of Kerosene safer than tanks of hydrogen? Pressure?
- montenegrohugo 8y ago
  Hydrogen is gaseous at normal temperatures. Both Kerosene and hydrogen are flammable, but being gaseous means that the resulting exothermic reaction will happen much faster and thus the energy will be released much more violently (think:explosion). Additionally, as you said, storing hydrogen densely enough means pressured containers, or extreme cooling (like for rockets).
- gambiting 8y ago
  Hydrogen leaks out of literally any container it's stored in, if you fill up a pure lead bottle(a 25kg bottle only holds 1 litre of hydrogen) it will be empty after 2-3 weeks of doing absolutely nothing - hydrogen molecules are so small they just permeate through any safe to handle material. Obviously planes couldn't use tanks made out of lead because of weight, so whatever material they used would leak even faster - and as hydrogen forms an explosive mixture with air, that's not a good thing.
  - gst 8y ago
    How do consumer-level hydrogen fuel-cell vehicles solve this issue? Does this mean you can't store them in a garage, because the leaking hydrogen could cause an explosion?
    - gambiting 8y ago
      They do not - you're not supposed to keep them in enclosed spaces because of this exact issue.
  - icebraining 8y ago
    Can't you just vent to outside of the plane? It's not like they are usually in enclosed spaces.

montenegrohugo 8y ago

You are confusing combustion efficiency with thermal efficiency. Having a combustion efficiency of 100% does not mean that 100% of the energy stored in the propellant is transformed into kinetic energy (the thing the plane is interested in). The ICE engine with highest thermal efficiency reaches just 50%: The Wärtsilä-Sulzer RTA96-C turbocharged 2-stroke Diesel [1] [1]https://en.wikipedia.org/wiki/Thermal_efficiency https://en.wikipedia.org/wiki/Thermal_efficiency

tim333 8y ago
That's quite an engine - I just looked up a picture of it https://pbs.twimg.com/media/DQv24mYUMAAmvQs.jpg https://pbs.twimg.com/media/DQv24mYUMAAmvQs.jpg
aunty_helen 8y ago
The latest F1 tech has started to exceed this using their jet ignition system. https://www.autosport.com/f1/news/131772/mercedes-engine-hits-remarkable-dyno-target https://www.autosport.com/f1/news/131772/mercedes-engine-hit... https://www.enginelabs.com/news/f1-tech-what-is-turbulent-jet-ignition/ https://www.enginelabs.com/news/f1-tech-what-is-turbulent-je...
aerodude 8y ago
Combined cycle gas turbines are actually higher efficiency than this. The current state-of-the-art is somewhere around 63% thermal efficiency.
- montenegrohugo 8y ago
  True, but those are usually not on a moving vehicle and have the luxury of being as heavy and big as desired. Engines on ships instead are limited by weight and size, engines on cars/trucks a little bit more, engines on planes much more and at the edge of technology we have the engines on rockets which have to be as light as humanly possible whilst still providing high thrust-specific fuel consumption

fdej 8y ago

The propulsive efficiency as defined in the Wikipedia article is the product of the cycle efficiency (the efficiency turning heat into mechanical power) and the mechanical efficiency (the efficiency turning the mechanical power generated by the engine into movement of the airplane). The mechanical efficiency can be close to 100%, but the cycle efficiency is only 40% or so.

Firadeoclus 8y ago

You could use fuel cells that work with more convenient fuels.

selimthegrim 8y ago

Would Li-air batteries be advantageous in this regard if they get them working?

peterlk 8y ago

Out of my depth here. Why are tanks of Kerosene safer than tanks of hydrogen? Pressure?

montenegrohugo 8y ago
Hydrogen is gaseous at normal temperatures. Both Kerosene and hydrogen are flammable, but being gaseous means that the resulting exothermic reaction will happen much faster and thus the energy will be released much more violently (think:explosion). Additionally, as you said, storing hydrogen densely enough means pressured containers, or extreme cooling (like for rockets).
gambiting 8y ago
Hydrogen leaks out of literally any container it's stored in, if you fill up a pure lead bottle(a 25kg bottle only holds 1 litre of hydrogen) it will be empty after 2-3 weeks of doing absolutely nothing - hydrogen molecules are so small they just permeate through any safe to handle material. Obviously planes couldn't use tanks made out of lead because of weight, so whatever material they used would leak even faster - and as hydrogen forms an explosive mixture with air, that's not a good thing.
- gst 8y ago
  How do consumer-level hydrogen fuel-cell vehicles solve this issue? Does this mean you can't store them in a garage, because the leaking hydrogen could cause an explosion?
  - gambiting 8y ago
    They do not - you're not supposed to keep them in enclosed spaces because of this exact issue.
- icebraining 8y ago
  Can't you just vent to outside of the plane? It's not like they are usually in enclosed spaces.