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So, from what I could understand about conventional Turbine Engines and its various types is that normally the combustion chamber ignites fuel with compressed air, rotating a low pressure fan which rotates the compressor fans and the propeller fan, re-initiating the cycle.

enter image description here

And so, I was wondering the possibility of using electricity only in the compressor (and powered by a battery) while the combustion would be made independently by the fuel and drive the fans or props like in a PropFan turbine.

I can't tell if this idea is good or simply bad, nor I could find any examples of such engine, in fact, I don't even know if this thing could be powered by today's batteries. Normally, hybrid airplanes have a turbine engine generator that powers electric powered fans/props.

The only advantage I could think of is that the compressor could be placed away from the combustion chamber, maybe allowing for VTOL or similar. But I can't tell for sure.

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    $\begingroup$ What do you hope to gain by driving the compressor with an electric motor? $\endgroup$
    – Michael
    Commented Dec 19, 2021 at 9:18
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    $\begingroup$ That idea is never going to fly, but on the ground, something similar does exist. There the idea is to compress air when electricity is plentiful and cheap, then use that compressed air to feed a gas turbine generator when electricity is scarce and expensive/profitable. It's less fuel-efficient but with renewables and finance in the mix it can be useful. $\endgroup$
    – Chris H
    Commented Dec 20, 2021 at 13:37
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    $\begingroup$ This concept was used in race cars, namely Formula 1. Called MGU-H, the hybrid system consisted of a electric motor-generator connected to the turbocharger. Like all hybrids, this was intended to optimize transient behavior, recovering energy with exhaust braking to reduce turbo lag. Worse, it didn't work very well and was largely ignored. $\endgroup$
    – user71659
    Commented Dec 21, 2021 at 8:19

7 Answers 7

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I don't know if such an engine exists, but I can assure you that it wouldn't be a good idea. The reason is that you would be introducing multiple additional failure modes, each of which would result in a complete engine failure. i.e. when the battery runs down, the electric motor fails, relays, switches, etc.

While turbine engines are complex in design, the operating principle is actually quite simple and they tend to be more reliable than their piston counterparts.

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    $\begingroup$ The compressor consumes a huge amount of energy; converting mechanical energy from the turbine to electricity (via a generator) and then back to mechanical energy (via an electric motor spinning the compressor) wouldn't be efficient at all. Totally agree with "wouldn't be a good idea"! $\endgroup$
    – Ralph J ♦
    Commented Dec 18, 2021 at 21:08
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    $\begingroup$ Well, the turboprop and some turboshaft designs have two independent parts that rotate (or counter rotate) and they work just fine. "PT6 Turboprop Tutorial": youtu.be/cHcRiHF6GfU $\endgroup$
    – mandiokai
    Commented Dec 18, 2021 at 22:11
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    $\begingroup$ @mandiokai The PT-6 is wildly different than what's proposed in the question. There is no conversion to electricity & back involved. $\endgroup$
    – Ralph J ♦
    Commented Dec 18, 2021 at 22:39
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    $\begingroup$ @RalphJ The proposed engine in the question also doesn't convert anything to electricity, the electricity is stored in a battery. $\endgroup$
    – mandiokai
    Commented Dec 18, 2021 at 23:45
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    $\begingroup$ @mandiokai Even worse -- the more steps between the initial work, be that the turbine or whatever generates the electricity originally, and the final output (turning the compressor), the greater the efficiency losses. $\endgroup$
    – Ralph J ♦
    Commented Dec 19, 2021 at 2:06
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No, such engines do not exist, as is implied in the question already:

I can't tell if this idea is good or simply bad, nor I could find any examples of such engine.

enter image description here

Pic above is an own photo, used earlier in this question-and answer about the efficiency of turbofan engines. In such an engine, turbines power compressors mounted on the same shaft - kinetic energy is transmitted directly, with only the fluid friction losses in the bearings yielding some efficiency reduction, as explained in @mins answer.

Powering the compressor with electrical power would not solve any problem, and introduces many others:

  1. That is a lot of power that needs to be created by an additional electrical pump. Even without the batteries (what would they be for?) adding a lot of weight.
  2. Transmission losses. Converting one form of energy into another always comes at some loss in the form of heat. With the proposed set-up, mechanical energy is transmitted into electrical energy into mechanical energy into compression energy. With induction losses in the electric cables, heat loss in the electromotor of the compressor, friction losses in the pneumatic tubing. Losses, losses, losses, for what gain?
  3. Extra failure modes, as @MichaelHall mentions. There may not be any catastrophic loss from system failure. Fewer systems means fewer failure rates.

The only problem that sometimes needs to be fixed with turbine engines is in rotational energy transmission - gearing. The rpm's of the axes are very high, which particularly for large turbofans creates a blade tip speed imbalance. The long fan blades impose a tip speed limit that is slightly supersonic, the much shorter turbine blades must make do with much lower tip speeds. An actual problem looking for a solution:

  • In aircraft turbofans (described in this answer), multiple concentric axes can be mounted, and these axes are not mechanically coupled to each other. Allowing a lower rpm for the longer fan blades.
  • In aircraft turboprops and turboshafts, the thrust generating blades are even longer, requiring mechanical gearing. Introducing weight & friction losses, for a gain in propulsive efficiency.Pic below: mechanical gearing contained in the light green frame at the front.accreditation and license in the link.
  • In trains and ships where the earth carries the weight without induced power loss, turbo-electric transmission can take the place of the mechanical gearbox. Particularly useful if the fast spinning turbine is running while the output propulsive means in stationary. The train wheels in the station, the battleship propellers in the harbour. Note that in this set-up the turbine engine is fully mechanically coupled on the drive shafts, it is only the output of the engine that is converted into propulsive energy.

When does the battery stack pose a solution and not only a problem? In hybrid vehicles, converting deceleration into electrical power (brake heat is fully lost kinetic energy). In ships and trains, enabling silent departure and starting the noisy turbines further away. Otherwise: no problem to fix, which is why any examples of the engines described in OP could not be found.

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    $\begingroup$ This is a very good, comprehensive answer. The only thing I can add is that electrical motors and generators are actually very efficient (>90%) $\endgroup$
    – Abdullah is not an Amalekite
    Commented Dec 19, 2021 at 5:19
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    $\begingroup$ @Abdullah yes they are, it means an additional 10% loss though. $\endgroup$
    – Koyovis
    Commented Dec 19, 2021 at 5:22
  • $\begingroup$ Another important thing I completely forgot: vehicles and vessels do have induced power loss. It shows up as increased friction in the wheels due to higher reaction forces and increased hydrodynamic drag from the extra submerged hull area $\endgroup$
    – Abdullah is not an Amalekite
    Commented Dec 19, 2021 at 17:16
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    $\begingroup$ @Abdullah at least in trains the induced loss is negligible. In ships perhaps not, but they get good efficiency by transporting a huge amount of cargo that dwarfs the mass of the drivetrain and by going slow, which aircraft obviously can't do to the same degree. $\endgroup$
    – leftaroundabout
    Commented Dec 19, 2021 at 23:25
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    $\begingroup$ @leftaroundabout train friction is negligible indeed, and the main resistance in ships is from the trailing wave pattern which is a function of ship length and speed. $\endgroup$
    – Koyovis
    Commented Dec 20, 2021 at 0:43
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I am an A&P IA and pilot. Helicopters mostly. Pretty much all helicopter turbine engines are modular. Compressor/Turbine and gearbox. Rolls Royce 250 series Combustion turbine Assy with the gearbox would be a good bet for your project. Two compressor discharge tubes feed the combustion chamber which is a reverse flow into the turbine. All fuel and oil systems are driven by the gearbox. Two centrifugal compressors driven by one or two electric motors feeding the two compressor discharge tubes and into the combustion chamber might work. Look up Rolls Royce 250 series. My two cents. The power to compress the air remains the same for a given horsepower, and the electric compressor simply adds complexity. One advantage I can see is the ability to intercool the charge air for increased density. More complexity and weight to be integrated and carried aloft. Good luck

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Your design:

  • Charge batteries with electricity,
  • Use batteries to rotate a compressor,
  • Use compressed air to burn fuel and rotate a turbine,
  • Use turbine to produce electricity to charge batteries and do actual productive stuff.

In that case there are conversion losses between electric, chemical and mechanical energy that can be prevented using the turbine to mechanically rotate the compressor, removing electrical and chemical energy.

The advantages are less failure points, less maintenance, better efficiency and less CO2 created.

Answering your comment "the design doesn't include [producing electricity]": If your plan is to charge the batteries with an external source, you're still creating losses (10-20% according to Wikipedia) when converting electricity into chemical energy, then back into electricity. Not mentioning the charging time and the required capacity.

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  • $\begingroup$ "Use turbine to produce electricity and do useful stuff.", their design doesn't include this last part. $\endgroup$
    – Fulano
    Commented Dec 18, 2021 at 22:50
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There is a hybrid design that uses a small helium gas reactor turbine to drive the air compressor of a combustion turbine. Essentially doubles electrical output of combustion turbine, while drastically reduces emissions. The nuclear fuel is much cheaper than natural gas but the cost of the reactor is not small. The economics really only work with big combustion turbines. The hybrid’s output is +1000 megawatts electric with the reactor about 630 megawatts thermal. See hybridpwr.com

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    $\begingroup$ Not really an aviation application then. $\endgroup$
    – Koyovis
    Commented Dec 21, 2021 at 5:38
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What you're describing sounds like a motorjet / turboshaft hybrid. There has been some recent interest in these for use in drones. In fact, last year I saw an ad for one on Twitter.

By combining a generator powered by a turbine, a battery, and a compressor powered by an electric motor, the two halves of the traditional turbojet engine effectively become decoupled. This allows it to choose what fraction of its output is used to produce thrust or electricity. For example, a VTOL could power a set of lift fans by prioritizing electricity during takeoff and landing, then transition to level flight by prioritizing thrust.

The disadvantage is that replacing a simple drive shaft with a bunch of electronics reduces efficiency while significantly increasing mass. However, in some applications the extra flexibility it provides would potentially increase efficiency compared to a more traditional system. Similar to how plug-in EVs lose efficiency on highways, but are extremely efficient in cities.

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    – Mark Jones Jr.
    Commented Jul 22, 2023 at 22:24
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Turbo electric engines use the turbine as an electricity generator. The propellers are driven by electric motors. It's currently used on ships, and there are studies to use it on aircraft. It has the advantage you mentioned

The (big!) difference is that the propeller, not the compressor, is driven electrically.

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    $\begingroup$ I don't think those motors you refer to use the generated electricity to power the compressor, which is the key point of the question. Using generated electricity to run propellers is entirely different than using it to run the compressor. $\endgroup$
    – Ralph J ♦
    Commented Dec 18, 2021 at 21:11
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    $\begingroup$ "use the turbine as an electricity generator": I can't understand how this answer matches the question about engines: "where only the compressor is electric driven?". Is the compressor electric? And by the way the turbine is not an electricity generator. The turbine drives a generator, like it could drive a propeller. $\endgroup$
    – mins
    Commented Dec 18, 2021 at 22:07
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    $\begingroup$ Only the engine output is converted into electrical energy, none of the simple and reliable internal connections are replaced. $\endgroup$
    – Koyovis
    Commented Dec 19, 2021 at 2:28
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    $\begingroup$ @RalphJ I know. I suppose I should have emphasized this difference more $\endgroup$
    – Abdullah is not an Amalekite
    Commented Dec 19, 2021 at 4:57
  • $\begingroup$ @mins see above $\endgroup$
    – Abdullah is not an Amalekite
    Commented Dec 19, 2021 at 4:58

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