4 ms·

Why not make a ducted engine? Just replace the fuel burning part of a jet engine with this motor?

by mrfusion 5y ago

baybal2 5y ago
Why do you need a duct?
- eloff 5y ago
  Jet engines achieve higher speeds and efficiencies.
  - baybal2 5y ago
    No, not efficiencies. Jet engines have lower efficiencies than propellers up to near 0.8 speeds of sound, and terrible ones at low speeds, and low altitudes. Last generation airliners have designed cruise speeds below 0.8 speeds of sound. So, a propeller can still be a viable option at that speed, it will just have to be enormous.
    - eloff 5y ago
      If propellers are more efficient, then why do airlines use jet engines? Noise? Reliability? There's a reason jet engines completely dominate the market for commercial airliners. Would making the propellers electric change that?
      - yetihehe 5y ago
        Because jet engines have better power to weight ratio than cylinder engines.
        xyzzyz 5y ago
        The comparison talked about here is not between jet and piston engines, it’s about turbofans and turboprops.
      - baybal2 5y ago
        > then why do airlines use jet engines? Noise? Reliability? - Noise, yes. EU effectively bans turboprops by noise limits - Reliability, yes. Gearboxes needs frequent, and expensive overhauls. That's acceptable on smaller regional planes, but not airliners costing tens of megabucks. - Size. There is an upper limit on propeller blade size after which aeroelastic effects cannot be overcome by any known material.
        eloff 5y ago
        I would argue then if you can't practically build a propeller to replace a jet engine on an airliner, that being more efficient in theory is meaningless. I'll refine my initial claim to say jet engines are more efficient in many practical uses and configurations. Would you agree with that?
        baybal2 5y ago
        Would agree partially. 5-6 seat abreast planes can be switched to powerful turboprops for practical use. Widebody airliners, not at the current level of technology.
deleted 5y ago
[deleted]
unmole 5y ago
Greater parasitic drag from the duct?
stadium 5y ago
The compression effect of the jet engine blades is what ignites the fuel. I don't think that part is replaceable, it's what makes a jet engine a jet engine. The hot gasses from the fuel burn produce thrust, and also sends rotational force back into the turbine blades to keep the compression effect going.
- Toutouxc 5y ago
  > The compression effect of the jet engine blades is what ignites the fuel Not exactly, the igniter ignites the fuel initially, and after a short while the combustion becomes self-sustaining (i.e. new fuel is evaporated and heated above its autoignition temperature by all the residual heat around it). The igniters are turned off after a while and are only turned on again under certain conditions (high risk of flameout for a variety of reasons). Also the "hot gasses from the fuel burn produce thrust" part has slowly become more and more incorrect, because nowadays in airliners we use what's called high-bypass turbofans where the primary purpose of the hot engine core is not to generate thrust, but to drive a turbine connected to the huge fan in the front, which sends most of the air AROUND the core ("high-bypass") in order to create thrust. The core does produce some thrust, but the air coming out of it is pretty fast and creates more noise, so we try not to have too much of that. And the core itself doesn't really need to burn fuel, you could rip the combustion chamber out and put a nuclear-reactor-fed heat exchanger there and it would work the same. Or a beefy electric motor that would drive the fan and also utilize its waste heat to imitate a very-high-bypass turbofan.
  - stadium 5y ago
    Thanks for the detailed explanation. I am not connected to the aerospace industry. In the context of retrofitting an electric motor where the fossil fuel combustion happens today, it still doesn't seem possible. > Or a beefy electric motor that would drive the fan Isn't that what Weight is doing with their motor? Motor attached to fan blades to produce thrust?
- idiotsecant 5y ago
  The compression is the easiest part to emulate. The hard part is the enormous amount of heat you need to generate and quickly transmit into that compressed fluid. Combustion is great on both accounts for that, but you could do it electrically as well given a sufficient power supply.

Why not make a ducted engine? Just replace the fuel burning part of a jet engine with this motor?

baybal2 5y ago

Why do you need a duct?

eloff 5y ago
Jet engines achieve higher speeds and efficiencies.
- baybal2 5y ago
  No, not efficiencies. Jet engines have lower efficiencies than propellers up to near 0.8 speeds of sound, and terrible ones at low speeds, and low altitudes. Last generation airliners have designed cruise speeds below 0.8 speeds of sound. So, a propeller can still be a viable option at that speed, it will just have to be enormous.
  - eloff 5y ago
    If propellers are more efficient, then why do airlines use jet engines? Noise? Reliability? There's a reason jet engines completely dominate the market for commercial airliners. Would making the propellers electric change that?
    - yetihehe 5y ago
      Because jet engines have better power to weight ratio than cylinder engines.
      - xyzzyz 5y ago
        The comparison talked about here is not between jet and piston engines, it’s about turbofans and turboprops.
    - baybal2 5y ago
      > then why do airlines use jet engines? Noise? Reliability? - Noise, yes. EU effectively bans turboprops by noise limits - Reliability, yes. Gearboxes needs frequent, and expensive overhauls. That's acceptable on smaller regional planes, but not airliners costing tens of megabucks. - Size. There is an upper limit on propeller blade size after which aeroelastic effects cannot be overcome by any known material.
      - eloff 5y ago
        I would argue then if you can't practically build a propeller to replace a jet engine on an airliner, that being more efficient in theory is meaningless. I'll refine my initial claim to say jet engines are more efficient in many practical uses and configurations. Would you agree with that?
        baybal2 5y ago
        Would agree partially. 5-6 seat abreast planes can be switched to powerful turboprops for practical use. Widebody airliners, not at the current level of technology.

deleted 5y ago

[deleted]

unmole 5y ago

Greater parasitic drag from the duct?

stadium 5y ago

The compression effect of the jet engine blades is what ignites the fuel. I don't think that part is replaceable, it's what makes a jet engine a jet engine. The hot gasses from the fuel burn produce thrust, and also sends rotational force back into the turbine blades to keep the compression effect going.

Toutouxc 5y ago
> The compression effect of the jet engine blades is what ignites the fuel Not exactly, the igniter ignites the fuel initially, and after a short while the combustion becomes self-sustaining (i.e. new fuel is evaporated and heated above its autoignition temperature by all the residual heat around it). The igniters are turned off after a while and are only turned on again under certain conditions (high risk of flameout for a variety of reasons). Also the "hot gasses from the fuel burn produce thrust" part has slowly become more and more incorrect, because nowadays in airliners we use what's called high-bypass turbofans where the primary purpose of the hot engine core is not to generate thrust, but to drive a turbine connected to the huge fan in the front, which sends most of the air AROUND the core ("high-bypass") in order to create thrust. The core does produce some thrust, but the air coming out of it is pretty fast and creates more noise, so we try not to have too much of that. And the core itself doesn't really need to burn fuel, you could rip the combustion chamber out and put a nuclear-reactor-fed heat exchanger there and it would work the same. Or a beefy electric motor that would drive the fan and also utilize its waste heat to imitate a very-high-bypass turbofan.
- stadium 5y ago
  Thanks for the detailed explanation. I am not connected to the aerospace industry. In the context of retrofitting an electric motor where the fossil fuel combustion happens today, it still doesn't seem possible. > Or a beefy electric motor that would drive the fan Isn't that what Weight is doing with their motor? Motor attached to fan blades to produce thrust?
idiotsecant 5y ago
The compression is the easiest part to emulate. The hard part is the enormous amount of heat you need to generate and quickly transmit into that compressed fluid. Combustion is great on both accounts for that, but you could do it electrically as well given a sufficient power supply.