3 ms·

I agree with most of what you said, however, internal combustion engine is nowhere near as efficient or complete combustion as a jet engine which can get to hig

I agree with most of what you said, however, internal combustion engine is nowhere near as efficient or complete combustion as a jet engine which can get to high 60s with heat recovery. I with somebody had thought of making a series hybrid with a small but optimal turbine based battery charger. the other benefit is the weight of gas goes down as the flight progresses which is about 1/3 of the total weight so make the design a bit worse but I believe it would still be compensated by cheapness of solar electricity based charging. on a related note, IMO this is the reason the whole fuel cell shtick from toyota is BS. Because if it were anywhere near economical & scalable then we would be seeing fuel cell powered drones all over.

korethr 7y ago
My internal pedant compels me to note that a turbine-based engine (which all aviation jet engines are, AFAIK) is still an internal-combustion-engine. Not all internal combustion designs rely on reciprocating pistons. Though you are quite correct that turbine-based designs can achieve efficiencies that most piston-based designs typically can't. I was thinking in terms of passenger cars for some reason (despite the word "airplane" being in the title), where piston engines are far more common than turbine engines. That said, I stand by the main point of my post, which is that you can't just consider the energy density of your energy source in isolation, you have to also account for the efficiency by which you convert that energy source to useful work. A thing I find myself wondering about now is the viability of an arrangement similar to that of cargo trains. IIRC, the locomotive of cargo trains is a hybrid of sorts. An internal combustion engine running at relatively high efficiency drives a generator, which in turn powers electric motors which drive the wheels. This sound similar to your hybrid idea, perhaps with the addition of batteries. I think one of the biggest challenges to hybrid systems is to make sure the overall system efficiency doesn't drop below that of the current typical systems which it is intended to replace. Say you have a plane with turbojet engines running at 60% efficiency. And we replace it with a charging turbine-engine at 70% efficiency, coupled to an 80%-efficient charging system, coupled to a 90%-efficient electrical engine. In isolation, each of those numbers sound better than the 60% efficiency of the turbojet engine. However, efficiencies combine multiplicatively, and so 70% * 80% * 90% = 50.4%. That's overall worse than the turbojet we started with. This new system better be much lighter than a turbojet to be worth it.
- SigmundA 7y ago
  Yes a turbine engine is a internal combustion engine, specifically a Brayton cycle engine vs a Otto or Diesel cycle piston engine. Turbines are not used in cars because they have high rpm and low torque requiring large transmissions to gear them down properly. They also are not very responsive to throttle requiring spin up, very noisy and have have an abundance of hot exhaust to deal with. You can't muffle them as easily. Series hybrid diesel electric trains are actually less efficient than diesel mechanical drive trains. They are used because of the need for precise traction control to prevent wheel slip, they sacrifice efficiency at speed though to attain it. Double conversion from mechanical to electrical back to mechanical will always be less efficient than a straight through mechanical drive once up to speed, this is why nearly all hybrid cars are parallel and go full mechanically coupled at highway speed.
  - korethr 7y ago
    So, how would you design a hybrid powerplant for an airplane? Electric engine at low prop/fan shaft speeds, combustion engine takes over at higher speeds?

korethr 7y ago

My internal pedant compels me to note that a turbine-based engine (which all aviation jet engines are, AFAIK) is still an internal-combustion-engine. Not all internal combustion designs rely on reciprocating pistons. Though you are quite correct that turbine-based designs can achieve efficiencies that most piston-based designs typically can't. I was thinking in terms of passenger cars for some reason (despite the word "airplane" being in the title), where piston engines are far more common than turbine engines. That said, I stand by the main point of my post, which is that you can't just consider the energy density of your energy source in isolation, you have to also account for the efficiency by which you convert that energy source to useful work. A thing I find myself wondering about now is the viability of an arrangement similar to that of cargo trains. IIRC, the locomotive of cargo trains is a hybrid of sorts. An internal combustion engine running at relatively high efficiency drives a generator, which in turn powers electric motors which drive the wheels. This sound similar to your hybrid idea, perhaps with the addition of batteries. I think one of the biggest challenges to hybrid systems is to make sure the overall system efficiency doesn't drop below that of the current typical systems which it is intended to replace. Say you have a plane with turbojet engines running at 60% efficiency. And we replace it with a charging turbine-engine at 70% efficiency, coupled to an 80%-efficient charging system, coupled to a 90%-efficient electrical engine. In isolation, each of those numbers sound better than the 60% efficiency of the turbojet engine. However, efficiencies combine multiplicatively, and so 70% * 80% * 90% = 50.4%. That's overall worse than the turbojet we started with. This new system better be much lighter than a turbojet to be worth it.

SigmundA 7y ago
Yes a turbine engine is a internal combustion engine, specifically a Brayton cycle engine vs a Otto or Diesel cycle piston engine. Turbines are not used in cars because they have high rpm and low torque requiring large transmissions to gear them down properly. They also are not very responsive to throttle requiring spin up, very noisy and have have an abundance of hot exhaust to deal with. You can't muffle them as easily. Series hybrid diesel electric trains are actually less efficient than diesel mechanical drive trains. They are used because of the need for precise traction control to prevent wheel slip, they sacrifice efficiency at speed though to attain it. Double conversion from mechanical to electrical back to mechanical will always be less efficient than a straight through mechanical drive once up to speed, this is why nearly all hybrid cars are parallel and go full mechanically coupled at highway speed.
- korethr 7y ago
  So, how would you design a hybrid powerplant for an airplane? Electric engine at low prop/fan shaft speeds, combustion engine takes over at higher speeds?