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I have looked at various questions on this SE site regarding this topic but I have not really found a satisfactory answer. Some comments here regarding torque and power in electric motors vs gasoline engines were helpful. Additional info also found here.

Underlying my question is the assumption that battery technology will considerably advance from what is available commercially now. Simply assume that providing the electric motor with enough power is not an issue and that the weight of the battery is equal to fuel.

Is it possible to build an electric aircraft engine that would allow current commercial airplanes the same capabilities as jet engines available today do?

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    $\begingroup$ Assuming the question is about the engines only (and we are gifted infinite battery performance) - the only thing I can think of is electric engines can't do afterburner/reheat. However I believe very few commercial aircraft have ever offered this. $\endgroup$ – Andy Apr 14 '16 at 10:46
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    $\begingroup$ Ill your battery weight be burnt in fly? Even if you pair each property of the engine/fuel system my actual airliner get lighter a few tons/hour $\endgroup$ – jean Apr 14 '16 at 16:17
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    $\begingroup$ I submit that you can't simply handwave battery tech away. Li-ion batteries provide about 250 Wh/l and 350 Wh/Kg. Most petroleum distillates (from gasoline to jet fuel) are around 9000 Wh/l and 13,500 Wh/Kg. And there are good reasons (like its atomic weight and high half-reaction potential) why we're unlikely to do better than lithium. $\endgroup$ – Jamie Hanrahan Apr 14 '16 at 21:29
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    $\begingroup$ No, @user3344003, that's not the problem with electric cars. You're forgetting that an electric car is about five times more efficient at turning stored watt-hours into movement as an ICE car. So even if your charge/discharge round-trip efficiency is only 50% (and it's actually much better, around 80% or more) the electric car will still use fewer watt-hours. But in an airplane the battery mass is a much worse problem, because you have constantly spend energy to keep it airborne. Keeping the electric car (and its battery) rolling on the ground is not nearly as much of a load. $\endgroup$ – Jamie Hanrahan Apr 15 '16 at 4:34
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    $\begingroup$ You still have to generate the electricity and transmit the electricity to the car. Compare the cost of heating a house with gas v. electricity. $\endgroup$ – user3344003 Apr 15 '16 at 17:07

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Not yet.

To look at a medium-range aircraft, let's base the engine on the CFM56 or the IAE V2500. Those engines produce between 100 and 150 kN static thrust. In cruise, their thrust is considerably lower due to the low density in cruise altitude and due to them moving at Mach 0.8. Let's use a value of 25 kN - this is enough so that two of them will comfortably push an A320 class airframe through the thin air at altitude.

The power to produce such an amount of thrust is force times speed. The speed when flying with Mach 0.8 in 35.000 ft is 240 m/s, so the power produced by one engine is 6.0 MW. Now let's look up how big and heavy an electric motor has to be to produce 6 MW continuously. As you can see from the linked Wikipedia page, the results are all over the place. Big, industrial motors come in at less than 1 kW/kg, so our motor would weigh more than 6 tons. The smaller motors for electric airplanes are pushing 10 kW/kg, the power-to-weight ratio of the GE90 turbofan, but will lose some of that when scaled up to size. Remember, even at 98% efficiency the motor will generate 120 kW of heat - this needs to be removed, and operating in thin air doesn't make this easy.

With current technology the motor could achieve maybe 2 to 3 kW/kg - this means the motor driving our hypothetical engine comes in at 2 to 3 tons. Add to this the fan and the fairing of the jet engine (we will not need the high-pressure part and all the turbines), but double the fan weight because we need to compensate for the missing high-energy core flow. This will maybe weigh 50% of the CFM56 / V2500, so we need to add another 1.2 tons.

The electric motor will be twice as heavy as the parts it replaces. There is still some work to do before it can gain an advantage over current jet engines, but it has some potential because it does not dump half of the energy supplied to it overboard in a hot, fast-moving, noisy gas stream.


EDIT:

Since so many people get excited about me omitting the energy density aspect of electric propulsion, even though the question did expressly desire to leave this out, here are two things to consider. Energy density is only half of the problem of electric storage.

  1. The energy density of jet fuel is around 43 MJ/kg, while Lithium polymer batteries achieve not even one MJ/kg. But this comparison is linear thinking - realistically, the current will be produced either by a high-efficiency turbine-generator combination, or by fuel cells, burning hydrogen at twice the efficiency of a conventional jet engine. Since hydrogen packs 142 MJ per kilogram, at twice the efficiency the electric airliner would need only 162 kg of hydrogen for every ton of kerosene in a conventional jet. Yes, I know, even then its volume will still be a problem.
  2. If any form of batteries is used, the fact that empty batteries weigh as much as full ones is the final nail in the coffin of battery-powered flight. While your average long-range jet lands at 60% of its take-off weight, the battery-powered jet would need to schlepp those heavy batteries all the way to the final destination. To be competitive, those hypothetical batteries would need to have twice the energy density of kerosene.
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    $\begingroup$ The engines you've mentioned are built for durability and to withstand some rather violent mishandling. Nobody asked them to be light :) So indeed there's still a road ahead to improve the weght/power ration :) $\endgroup$ – gusto2 Apr 14 '16 at 12:32
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    $\begingroup$ This analysis doesn't consider the weight of the power supply for the electric plane versus the weight of fuel for the jet. $\endgroup$ – Russell Borogove Apr 14 '16 at 14:14
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    $\begingroup$ @GabrielVince: The Prius engine is 1.37 kW/kg. It is built for low weight - it is a car engine, not a stationary industry motor. I assumed 2 - 3 kW/kg, well knowing that weight is a premium in aircraft, but with current technology you will not get any better than that. After all, this motor needs to be certified just like any other component on the aircraft. Things would be different for Formula 1, but we are still looking at civil aviation here. $\endgroup$ – Peter Kämpf Apr 14 '16 at 14:51
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    $\begingroup$ @RussellBorogove from the question Simply assume that providing the electric motor with enough power is not an issue and that the weight of the battery is equal to fuel. No analysis of the weight of the power supply is necessary for this particular question. $\endgroup$ – FreeMan Apr 14 '16 at 14:53
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    $\begingroup$ Turbofan engines manage to dissipate a whole lot of heat from the core. Not all of it goes out the exhaust. I don't know if it's equivalent $\endgroup$ – TomMcW Apr 14 '16 at 18:01
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Engines are great as they are. Electric engines can be fast, powerful and effective. I see two problems:

First - the sheer amount of energy consumed by the commercial airplane. With a single engine giving out 200kN you need a small power-plant attached to the aircraft. Even batteries would be 100% effective and could store enough energy, you need to burn higher amount of fuel to charge them up (you would need a A LOT of alternative energy sources to match the energy given out).

Second - it is all about the energy density. Jet fuel having 34 MJ/l , batteries having up to 120 Wh/kg = 0.36MJ/kg (according to this site ). So you need more than 100x more space to store the same amount of energy.

Just search for the 'electric airplane' and you will get a list of mainly small, ultralight or self-propeled gliders where they don't need to carry a LOT of energy with it.

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    $\begingroup$ Thank you for your answer. As mentioned in my question I am aware of the battery technology issues. I was wondering if there are any inherent characteristics that make electric motors unsuited for use as aircraft engines. The first paragraph of your answer indicates that this is not the case and we could build electric motors with the same capabilities as turbofans - the only (admittedly huge) problem is supplying those electric motors with enough energy. $\endgroup$ – lexeter Apr 14 '16 at 12:13
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    $\begingroup$ "One could put a small nuclear power plant on board" -- if you've got a nuclear power plant, you can use the heat from that directly: en.wikipedia.org/wiki/Nuclear-powered_aircraft $\endgroup$ – Roger Lipscombe Apr 14 '16 at 17:10
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    $\begingroup$ @GdD actually, as a general rule, explosives are not very good stores of energy at all. They make their name not by storing a lot of energy, but by their ability to let all of that energy lose in a remarkably short period of time. $\endgroup$ – Cort Ammon - Reinstate Monica Apr 15 '16 at 4:37
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    $\begingroup$ @abligh No. It really isn't. The problem space is simple. Nuclear reactors are radioactive. Humans and radioactivity do not mix. Shielding is heavy (scientific fact, they need to be to have a large cross-section). Heavy planes don't fly [citation needed]. Don't get me started with neutron radiation and fusion. $\endgroup$ – Aron Apr 15 '16 at 6:26
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    $\begingroup$ And - do not forget the planes crash - a few every year :) You don't want to have a crashed and malfunctioning nuclear powerplant in your garden $\endgroup$ – gusto2 Apr 15 '16 at 7:11
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There is one important disadvantage that batteries will always have compared with fuel combustion for aviation propulsion: the weight remains constant. Airliners (especially those used for long-haul flights) burn off a large percentage of their take-off mass over the course of the flight. Batteries, however, retain their initial mass constantly. This is a problem for a number of reasons:

  1. The most obvious reason this is a problem is that more energy is required for the flight. Even if you get a battery that has equal energy density to jet fuel and is also stable (which we are currently quite far from,) the airplane will need to carry the entire mass of the batteries for the entire duration of the flight. Thus, as the flight goes on, far more energy will be used per mile on a battery-powered flight than on a fuel-powered one, even if the batteries have the same energy density as the fuel. This also means that even more battery mass will be needed for the same range, since that extra energy requirement has to come from the batteries.

  2. The other big problem is maximum landing weight. Many airliners are not designed to be able to land at their maximum takeoff weight simply because it isn't needed. This is one of the reasons why fuel sometimes needs to be dumped or burnt off before an aircraft that encounters a problem after takeoff can land again. However, with batteries, you'll still be at take-off weight when you're landing, which means you'll need stronger landing gear and tires, which means yet more weight and design/manufacturing cost. It also means you'll be landing faster (because of the extra weight,) so you'll need more runway length to land as well as needing brakes that can absorb more energy. The kinetic energy of the airplane is equal to half its mass times its speed squared, so the energy that must be absorbed by the brakes during landing increases dramatically as landing weight and speed increases.

  3. A somewhat less important, but still significant problem is more punishment to runway surfaces. With planes now landing near their MTOW, runways surfaces will be damaged more quickly and will need to be resurfaced more often and/or designed for larger loads than they are now. This would also probably mean that the aircraft wouldn't be able to access as many runways as an otherwise-equivalent fuel-powered aircraft would be able to use until those runways were strengthened.

Of course, you could start jettisoning battery cells as they're depleted, but this also (obviously) has a lot of problems:

  1. In order to deplete some batter cells sooner than others, you won't be able to draw on all cells in parallel, which will mean higher power draw levels per cell (and, thus, more heat produced per unit time per active cell, etc.)

  2. You'll need to design the airplane to be able to jettison the cells safely. This is doable, but will require lots of extra cost in design effort and extra weight.

  3. Environmentalists won't be too happy when you start dropping huge batteries all over the place. Neither will property owners. Existing battery chemistries are already quite corrosive and a battery chemistry with the energy density of Jet-A is probably going to be even more corrosive, unstable, and otherwise bad for whatever it's dropped on.

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  • $\begingroup$ Thank you. I was aware of 1 and 2 but not 3. Adds an interesting perspective; I have no idea what the costs for a resurfacing or strengthened runway design could be. However, I think if our civilization started using electric passenger jets the incentive to develop stronger runways would be big enough so that someone would come up with a solution for that particular problem (e.g., cheap new super-strong asphalt formula). $\endgroup$ – lexeter Apr 14 '16 at 15:31
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    $\begingroup$ @lexeter We can build stronger runways now, it's just more expensive and doesn't happen overnight. Airports would have to have significant demand for it before they'd make the investment, especially in the case of airports where shutting a runway down to resurface causes major traffic headaches during the construction. $\endgroup$ – reirab Apr 14 '16 at 15:37
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    $\begingroup$ What about fuel cells? :) $\endgroup$ – Sanchises Apr 14 '16 at 15:53
  • $\begingroup$ @sanchises The question was asking specifically about batteries. Also, why would you use an electric fan with fuel cells instead of just burning the fuel directly in the engine to start with? The latter is almost certainly more efficient, not to mention less complex to design and maintain. $\endgroup$ – reirab Apr 14 '16 at 15:57
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    $\begingroup$ @reirab, a fuel cell approaches 100% efficiency at turning chemical energy into electricity. Combustion is at best around 40% efficient. $\endgroup$ – Mark Apr 14 '16 at 20:06
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The biggest advantage of 'going' electric is that electric fans are way more efficient than a jet turbofan. A jet turbofan creates 75-85% of its thrust from the fan and 25-15% from the 'core' exhaust stream. The principle is that slower the accelerated air is, the more efficiently you generate thrust, as moving a small volume of air very fast means you lose energy in the kinetic energy of the accelerated air mass. So, bigger (or more) fans, accelerating a larger volume of air at a slower speed is much more efficient. Jet engines already do this by connecting a large fan at the front to the compressor shaft behind it, and this is a high bypass jet engine.

Even so, modern turbofans achieve under 2 Newtons of thrust per kW of energy. This is because the engine itself has low thermodynamic efficiency coupled with the fan being sub-optimised by various constraints that do not apply to an electric fan design. For example, the fans diameter is limited by ground clearance and by the RPM of the compressor drive shaft. It still rotates much too quickly and the tip velocity is capable of going supersonic. This makes for drastic drag loses and noise issues. Consequently the bypass ratio is far too low for really high efficiency, which can only really be resolved by having multiple fans. having additional contra-rotating open blades electric fans for example around the rear of the fusilage can ingest slow air from the body of the aircraft which is more efficient, and they can be placed at multiple points along the wing and tail sections.

Electric fans can, due to the approx 4 times less thermodynamic energy loses and slower tip speed, optimal RPM and slower air exit velocity potentially exceed 20N per kW, and probably get to 35N per kW.At high speeds though I don't know what performance they would achieve but it is safe to say it is going to be a LOT better than a turbofan. Consequently a battery can potentially be competitive at around 500Wh/kg, including power electronics and wiring.

Motor weights depend on power required, since as pointed out its harder to cool a large core. However you wouldn't want to try to replace the fan on a current jet engine but have multiple lower power fans, which means that the power density in kW/kg is going to be higher than in the cases listed above, superconductors not withstanding. Smaller fans also can rotate faster, suiting these kind of motors.

As pointed out above, the REAL issue is not the battery energy density but the battery POWER density - to have not only sufficient power at take-off but also to recharge within a turn around of 20-50 minutes. Since electric aircraft would first be competitive only against short to medium haul flights, a lot of which are internal and have fast turn around times, a power density of around 1kW per kg is needed, and that exceeds the current capability of high energy density batteries by a large margin.

Theoretically, if we can gain greater thrust efficiency (say 60 Newtons per kW) then we could use a lot less energy, therefore we would only need to cycle a portion of the battery capacity and could get away with, say 500W per kg charge/discharge power). In practice the power density stated is a maximum value but occurs at lower energy efficiency and tends to shorten battery life, so the battery would need to probably have a stated power density 50% more to operate efficiently this way.

110 Newtons of thrust per kW of power has been demonstrated with electrostatic ion thrusters (the type used in 'lifters' which you can see on youtube) but this have low thrust density so you have to factor in weight. Increasing the voltage will help with that.

The issue of the plane not becoming lighter as you fly is to some extent important, but the cost saving in fuel and the potential to have many fans utilised for example to assist in airflow around the wing can increase lift at low speed and thereby compensate for increased mass through out the flight envelope. The likely embodiment of contra-rotating propellers each with electrically operated variable pitch blades that can optimise both tip velocity and angle to the conditions along with accelerating a much larger volume of air more slowly will greatly increase the overall efficiency. Electric propulsion in contra-rotating fans is much easier mechanically than one hooked to a diesel or jet turbine, and can suit the high velocity of commercial aircraft (see https://en.wikipedia.org/wiki/Propfan) which shows that contra-rotating ductless fans can offer efficiency advantages. Noise issues are a function of having to link these propellers to individual jet engines which again means high tip velocity as a product of the limitations on diameter and high RPM of the engine. When electrically powered, more fans can be used at much slower tip velocity, this slashes the noise produced.

The weight of the extra fans is partially offset by the gains from taking away the cowling both in weight and drag.

Because of the recharge issue, the likely embodiment will be advanced, higher efficiency engines that recharge the batteries once in cruise and descent, and top-up the energy required during climb. These could use superconducting generators and with an adequate battery reserve risk of catastrophic generator failures should be mitigated.

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  • $\begingroup$ You can reap the increased efficiency of "electric fans" primarily by flying more slowly. Then do away with the shroud and you end up with regular propellers. Then it all makes sense. $\endgroup$ – Peter Kämpf Nov 20 '16 at 9:38
  • $\begingroup$ Most batteries are able to fully recharge in less than 1 hour. Many in 30 minutes. 10 minutes to 80% charged is not out of question with adequate thermal management. I don't see this changing with increased energy density, so I don't see why power density would be an issue. But yeah, as Peter said, I would be interested in your thrust per kW at mach 0.85 speeds. $\endgroup$ – ReneSac Feb 5 '17 at 21:33
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All comments very true and valid. I would only like to add that Siemens has made and flight tested a 260kW aeroplane engine with a 5kW/kg power to weight ratio specifically for the single engine ICE type of aircraft and believes the design is scalable such that 100 seater regional series hybrids could be a reality soon. Important to note here is that hybrids address battery energy density issues as well as take-off vs landing weight and the electric motors dramatically improve the safety over conventional ICE's (internal combustion engines)

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    $\begingroup$ Hi, and welcome to Aviation.SE. Thanks for a good first answer. Adding references to this answer would make it even better. $\endgroup$ – Simon Oct 1 '16 at 12:10
  • $\begingroup$ Here's the link to Siemens' website regarding the mentioned aeroplane engine and its use in an Extra 330LE aerobatic airplane. siemens.com/press/en/feature/2015/corporate/… $\endgroup$ – lexeter Oct 3 '16 at 9:28
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(While this question has long been answered, I feel there's a bit that can be added as it keeps getting asked and the technology is not entirely static.)

Let's first look at the power/weight ratios. The highest number for Tesla's car motors is 8.5 kW/kg. The electric-aircraft-specific Emrax 268 delivers about 11.6 kW/kg.

In comparison, the Trent XWB delivers 430 kN of thrust at 300m/s flow rate, which equates to 64.5 MW of power, in a 7,550 kg package - a power/weight ratio of 8.5 kW/kg. However this isn't apples to oranges: this ratio is for the whole package, engine and fan, and measures useful output, like wheel power for a car.

In short, turbine engines are still lighter than electric motors, but the difference is not dramatic. Where all-electric powerplants fail to stack up is range. I've elaborated on it in response to another question - Are there any hybrid electric planes?. The short version is that the maximum possible range of an electric aircraft is 10 nmi for every % of its weight dedicated to the battery. This limits the range of electric aircraft to 300-450 nmi, if sticking with the fuel fractions of known airliners.

But there are aviation applications where this is enough. The most important non-renewable resource consumed by modern aviation - the supply of 1960s Cessnas and Pipers, without which no one could afford to become a pilot - will not last forever. Should authorities permit it, mass-produced Tesla powerplants could power trainers and GA planes at a fraction of the ownership cost of a certified avgas engine.

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Yes. What your question boils down to is, essentially:

Ignoring power input, can an electrical engine produce equivalent output to a jet engine within the size and weight of that jet engine?

So:

Is the power to weight ratio of a jet engine greater than electric engines?

and

Is the power to volume ratio of a jet engine greater than electric engines?

https://en.wikipedia.org/wiki/Power-to-weight_ratio#Electric_motors.2FElectromotive_generators

The GE90-115B Brayton turbofan jet engine used on the Boeing 777 has a power to weight ratio of 10.0 kW/kg.

An electric motor made for aviation, the EMRAX268, has significantly lower power, but attains 10.0 5kW/kg.

Some will be concerned about whether the motors can scale up but as can be seen in the electric vehicle industry fast electric cars are readily available, and the size and volume of of just the motor and its required components (cooling, control) is smaller and lighter than the gas engines for those vehicles that can compete in terms of acceleration and top speed.

Even more telling is that the electric motor and its components are not only lighter and smaller, but they are cheaper as well.

The only limiting factor to electric aviation is the power source, and as a step forward every major manufacturer is already designing hybrid electric planes. As powerful as jet engines are, they are still not as fuel efficient (and thus emissions efficient) as they could be. A jet fuel powered generators powering electric engines may be available in the market within a decade.

These hybrid planes wouldn't be possible if the electric engines couldn't hold their own in size, weight, and power relative to a jet engine.

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Roughly, potentially, but there are some key differences in the comparison of a jet engine and the theoretical 'electric jet engine', that are very different from the comparison of a car engine to a motor-driven EV.

Most notably, as previously mentioned, is the turbo-fan is mechanically driven by the combustion heat-driven expansion of the air compressed by it's compressor. At cruising speeds (where the jet engine is optimized), this is a much more fuel efficient arrangement than the cruising speed operation of an automobile combustion engine.

Basically, there are two places where the released heat is converted to mechanical energy-- first, much of the heat-release of combustion is captured by the turbine that drives the compressor. Second, the exhaust nozzle also converts heat not captured by the turbine into kinetic energy by accelerating the mass-flow through the engine, converting a pressure delta generated by heat expansion into a velocity delta through nozzle geometry. By comparison, the combustion engine converts the exhaust gas heat expansion into mechanical energy by driving a linear piston, and gains no mechanical energy by exhaust. Generally, turbines are more efficient than pistons at mechanical energy conversion. There's also a tertiary efficiency-- namely that combustion at high pressures more efficiently converts heat to pressure as the gaseous density is higher, so more of the chemical energy of the fuel is converted to kinetic energy in a jet engine than a combustion engine, simply by virtue of the higher pressure of the combustion reaction in the jet engine. The 'downside' for the jet engine is that to make the whole arrangement work fuel efficiently you have to be operating at a significant fraction of Mach, much faster than ground transport can manage safely. Hence, combustion engines rule the earth and jet engines rule the sky in the current paradigm.

So, even assuming unlimited power supply, you would still have to have to have a very efficient motor on an energy cost-efficiency basis. To boot, you would have to have an engine that operated at similar cruise velocities. Even leaving aside infinite power generation, we can still consider that more time in the air is a longer time-frame along which the aircraft must be energy self-sufficient, generally equating to more mass in battery and/or power generation. More mass lowers the mechanical efficiency on an aircraft operation basis, because it is more energy you have to spend to accelerate and decelerate the extra mass.

So in an electric motor driven equivalent, you probably still have something resembling a turbo-fan. Except that your motor is primarily driving your compressor fan, and the turbine is there mostly to recapture some of the energy of compression (which also generates heat) into energy to drive certain engine functions like coolant and lubrication circulation, possibly some power regeneration. So probably a smaller turbine, but this puts you up against the inconvenient fact that compressing air is not very energy efficient as a means of generating thrust. If it were, we'd be running aircraft off compressed air.

What this generically gets at is that the electrification of air travel is likely not to resemble current jet-era technology. It's within the bounds of known technology to apply efficiency of electric motors to the problem of air transport, but the resulting architecture is likely to be very different, much as the fundamental architecture of a full EV is different than a gas automobile. This will likely in addition mean some fundamentally different infrastructure.

E.g. much of the energy of a flight is taken up in the initial acceleration, so it's possible that an Aerial EV would take off from a runway that more resembles that of an aircraft carrier than a flat road, with an assisted launch. Similarly, recapturing the energy on landing could again utilize a system more similar to those seen on aircraft carriers, only dedicated to regenerative capture rather than rapid deceleration.

More directly, though, the fundamental problem is generating thrust at near-Mach speeds. The efficiency of electrical motors at turning electrical power into rotational mechanical power is somewhat mitigated by subsonic and supersonic fluid mechanics, because an aircraft has to generate thrust by accelerating an airflow, or 'pushing' against air in some way or another. At these speeds, propellers basically start to lose their efficiency, and propulsion methods above these speeds therefore rely on the expansion of gasses with the transfer of heat into the gas. So to compete in these speed areas, an energy efficient means of transferring heat to (compressed) airflow has to be devised, which is very different than simply applying known electric motor technology.

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  • $\begingroup$ Except that your motor is primarily driving your compressor fan but that's already what happens, the turbine is there just to extract the energy needed for the compressor and fan. $\endgroup$ – Federico Aug 27 '16 at 8:17
  • $\begingroup$ much of the energy of a flight is taken up in the initial acceleration - only for very short range, low level flights. For any realistic condition your assumption is plain wrong and belongs in a field called ballistics. $\endgroup$ – Peter Kämpf Aug 27 '16 at 19:32
  • $\begingroup$ @ Fedrico What I'm getting at is that whereas a petro-fuel jet engine is driven by it's turbine, converting heat release from fuel combustion into rotational energy to drive the compressor, an electric motor driven subsonic/hypersonic 'engine' would apply the electric motor to the compressor fan, as the the compressor would be the most mechanically efficient place to apply rotational energy driven by an electric motor. In this arrangement, it still may be energy efficient to recapture some of the energy with a turbine, but the turbine is not driving the engine, the electric motor is. $\endgroup$ – Jason Hubbard Aug 29 '16 at 2:30
  • $\begingroup$ @Peterkampf, the Airbus E-Fan already applies an electric motor on it's main wheel to accelerate during takeoff, getting the aircraft to around 60 KPH. A ground launch assist would achieve similar effect, but reduce mass on the flight vehicle-- that launch-assist motor on the E-fan is dead weight for the rest of the flight. With ground launch assist, you might save more energy, and extend range, of an aerial EV. Particularly if the assisted launch provided 100% of energy for takeoff velocity. The Navy uses assisted launch for short takeoff, no reason it can't be used for energy efficiency. $\endgroup$ – Jason Hubbard Aug 29 '16 at 2:41
  • $\begingroup$ @JasonHubbard: An aircraft with 20% fuel fraction and 25% efficiency (conversion of chemical energy into work) uses 0.0346% of the energy needed for the trip to accelerate to a take-off speed of 150 kts. On a comparable electrical aircraft you will be able to save 0.0346% of the hypothetical battery capacity by using a catapult launch, or you can stretch its range by a similar fraction. You will gain far more efficiency by e.g. choosing favorable winds enroute, or spending the catapult expense on a lighter structure. $\endgroup$ – Peter Kämpf Aug 29 '16 at 8:27
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Battery and motor technologies aside, there is one major problem with the application of electric engines on commercial aircraft and that is recharge time. Commercial aircraft only make money when they're loaded with revenue paying passengers and in the air; when they're on the ground, it's extremely costly. Conventional chemical fuels not only pack a great deal of energy density but are also extremely easy to replenish the supply of. Refueling a passenger plane takes a matter of minutes - in some cases as long as a half hour for say an A-380 or a 747. Batteries currently takes hours to recharge so this would have a huge implication on the delay a passenger or cargo flights.

Personally, I don't see an electric powered commercial aircraft as a viable alternative to current jet engines. Probably the cleanest form of air travel would be Karen high-efficiency jet engines powered by a carbon neutral bio diesel fuel.

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  • $\begingroup$ The question is about engines, not batteries or fuel. Anyway, most lithium batteries accept over 1C charging current w/o significative damage, given good thermal management. That translates in 80% charge in about 30 minutes. Just look at electric cars (tesla, leaf, etc) or smartphones with quick charging. $\endgroup$ – ReneSac Feb 5 '17 at 21:42
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Yes, they could. As you said, assuming the power SUPPLY issue has been solved. At it's heart, a jet engine heats air, harnesses the expansion to drive a compressor and in most turbofans drive the "propeller". While presently all jet engines use burning fuel to produce that heat, the underlying principle of the system shouldn't care where the heat comes from. If you could dump enough energy through electric heaters in the combustion section of an otherwise standard engine, I think you could in theory operate the exact same engine off of electricity.

For a modern turbofan, that'd be about 35MW of power you'd have to dump into air heaters in the "combustion" section. This would be a pretty big engineering challenge, but I don't think it's out of the realm of possibility in theory. One option might be using plasma sparks, like an arc welder. Again, electrode lifespan would be an issue, but not necessarily impossible. Credit for this idea comes from this page: http://contest.techbriefs.com/2013/entries/aerospace-and-defense/3129

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Well the thing we need to start consideration is that jet engines are capable of providing a very high amount of thrust, yet they function on a simple principle of a gas turbine. Somehow it is possible to make an electric engine for propulsion but it will be complex and very heavy and lower powered. The only way an electric engine would work is to replace the jet engine core with some electric motor sort of, that can rotate the fan disk, creating thrust; however, considering how much torque is required to spin it in order to generate reasonable thrust is a nightmare, also the motor needs heavy batteries.

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