15

Is there anything functionally equivalent to a turbo for electric motors?

Obviously, I am not asking for something that reuses exhaust gases to force in more air, because there are no exhaust gases nor the need for air.

What I am asking is if there is something that can use "waste energy" to give an IMMEDIATE POWER BOOST to an electric motor, which is what the turbo of combustion engines do, in functional terms.

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    Are you talking about on a fully electric vehicle? – JPhi1618 Mar 30 '16 at 17:23
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    "output force to reuse it as input", well, a turbo uses wasted exhaust gas energy (which isn't the output of our system) and hence for electric vehicles, I'm not sure what the analogical equivalent would be. – chilljeet Mar 30 '16 at 17:29
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    I'm not sure what exactly you're trying to ask. Forced induction improves the efficiency and power of the engine by allowing more air and fuel to react at higher pressures. The fact that the forced induction is powered by the exhaust gas pressure is relatively minor compared to that - it just makes for a simpler and cheaper device. In an electric motor, there isn't much of either - the efficiency of the drive is very high (and in practice driven by economy), and the only waste is a tiny amount of heat compared to combustion engines. The batteries are a different matter, though :P – Luaan Mar 31 '16 at 8:59
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    it would be a terribly-designed system, but... if you had induction motors that didn't already have capacitors to improve power factor, you could see an improvement in energy efficiency in adding capacitance to the system (though I doubt it would have an effect on max power) – costrom Mar 31 '16 at 20:39
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    The 'immediate power boost' would be more analogous to a pressurized nitrous 'squirt' than a 'turbo'. Exhaust driven intake compressors suffer from turbo lag. – Jeeped Apr 1 '16 at 22:47
20

The "take some of the output force to reuse it as input" can be interpreted as regenerative braking, but the big differences are:

  1. Regenerative braking takes power back from the wheels while turbo take power from the engine itself, that would be otherwise wasted.
  2. The power of the turbo adds to the normal power of the engine while the power of regenerative braking goes to storage to be used by the engine normally without any performance increase.

If you see a turbo as a supplementary power to the max power output of the engine I would see supercapacitors as something close too. Supercapacitors can deliver a high current to the engine (thus high power) that the batteries can not deliver for a short time, thus making the car go faster for a short period (more akin to what a nitrous oxide injection would do) at expense of overheating, decreased efficiency, and otherwise reducing the lifetime of the electric engine.

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    In a (Toyota-style drivetrain) hybrid, using the combustion engine and the electric motor at the same time for maximum acceleration is a performance increase over just using the combustion engine itself. Unless it's a plug-in hybrid, all of the electrical power must come from the combustion engine, so regenerative braking indirectly does provide at least a short-term boost in performance. – Monty Harder Mar 30 '16 at 21:56
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    “The power of the turbo adds to the normal power of the engine” – that's not really correct. Rather, the turbo just makes the engine itself capable of delivering more power. It does so without consuming any fuel or other resources, so this is not akin to what a supercapacitor could do (that would rather be an analogue to a jet engine's afterburner). – leftaroundabout Mar 30 '16 at 23:01
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    @leftaroundabout That's not true. The extra power that a turbo delivers always comes at cost of some extra fuel consumption compared to the same engine and same displacement without turbo. The main difference is that, in a turbocharged engine, this extra power usually comes more efficiently than an equivalent increase of power by increasing the displacement of the engine. – Gabriel Diego Mar 31 '16 at 2:26
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    @MontyHarder. The original question was about full electric engines. In hybrid engines I agree that this is the practical effect. – Gabriel Diego Mar 31 '16 at 2:28
  • @gabrieldiego: “extra power...comes at cost of some extra fuel consumption” that's a tautology. Power means energy translated per time; with any given efficiency, increasing power does always increase resource demand. Just, as you say, increasing the engine power using a turbo actually improves the efficiency, unlike increasing it with additional displacement. – leftaroundabout Mar 31 '16 at 10:08
17

regenerative braking

This question and answer regarding the subject matter has some very good information in it as well the answer reveals a mathematical paradox with regenerative braking

This Q&A is a bit off your topic but has breadcrumbs regarding recapturing lost energy through a turbo to charge a batter and kinetic energy recovery through braking in Formula 1

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    I think this is a great interpretation of the question. An electric motor is tied directly to the wheel, so there's not really anywhere "waste" could be captured. I think re-using the momentum the motors put into the car is as close as the OP is going to get. – JPhi1618 Mar 30 '16 at 19:09
  • I think this is the WRONG interpretation of the question. I already knew what regenerative braking is, and what I meant with my question is to take a part of the output and reuse it as input to give an IMMEDIATE POWER BOOST. That is what a turbo does, in practical terms. – sergiol Apr 1 '16 at 11:11
  • Downvotes work better than comments. Love your passion. – DucatiKiller Apr 1 '16 at 15:35
  • @sergiol You changed the interpretation of the question (which already gives way to a number of interpretations). If someone didn't get it the way you meant, you don't have to use all caps and bold text to point it out because this looks impolite and rude. – Gabriel Diego Apr 1 '16 at 17:50
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    It's all good. It's his passion for the truth. I understand. That being said we really try to be different from the SO flame wars. @sergiol I totally respect your view. It was my interpretation when no one had answered your question. I think Gabriel's answer is certainly better from a performance alignment with your question. From a harvesting perspective I thought my answer did OK from that perspective. Cheers to you, cheers to passion and welcome to the mechanics site if I haven't already said that to you in the past. – DucatiKiller Apr 1 '16 at 17:57
17

No, there isn't any equivalent. A turbo is used because combustion engines are inherently inefficient: they convert chemical energy into mechanical energy, using an awkward detour via heat. Unfortunately, heat is pretty much the worst possible way to store energy: by the laws of thermodynamics, you can only convert it to other forms of energy if you also increase entropy. If you do the physics, you find out that the maximum efficiency is the Carnot efficiency

η = 1 − TC/TH

where TC and TH are the cold and hot temperature points of the engine cycle, i.e. surrounding air vs. combustion temperature. Note that the fraction approaches zero as TH rises*, i.e. the loss can be made quite small by letting the combustion happen at high temperature. But you can't make the temperature infinitely high, and therefore some energy is inevitably lost.

You can consider the turbo as a device that retrieves back some part of the lost energy or, more to the point, you can just see it as a means of increasing the operation pressure and thus temperature, and thereby reducing the loss somewhat. At any rate, a turbo is just a means to adress the problem that a combustion engine is not efficient. (In practice, you won't find any engine with efficiency better than 30%.)

There's no need to do that for an electric motor – because these are efficient! They convert electric energy to mechanical via magnetic fields, and that process is far better controlled. You can approach 100% efficiency without needing to have any temperature or so approach infinity.
Sure, there are some small losses in the copper windings' electrical resistivity, in eddy currents and bearing friction, but these can be made very small by precision design.

If anything, it might make sense to search for a turbo-analogue for batteries, because these are actually the weak part of an electric car, efficiency-wise. Perhaps it would make sense to tack on some kind of waste-heat retrieval to these.


*In case you remark that the loss also vanishes if TC becomes zero: correct, but there's not much you can do about TC. Cooling the air below ambient temperature would require a giant fridge, which would of course overall just waste even more energy. Cooling after a supercharger does make sense though, because the temperature is already higher than ambient here, i.e. this can be done passively.

In the end, “retrieving waste energy” is moot: you can always consider the motor and turbo together as one thermodynamic engine, and its total efficiency can not be better than Carnot.

  • 1
    Nice answer...... +1 – DucatiKiller Mar 30 '16 at 22:41
  • I have to praise all the effort for the technical description you put in the answer. The question was much simpler than that though, the OP just wanted to know the practical equivalent, something that one would feel in the pedal (and when refilling/recharging). – Gabriel Diego Mar 31 '16 at 2:36
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    @gabrieldiego The question doesn't seem as specific as the way you seem to read it. This answer of 'No' is a better answer. Regenerative braking is collecting energy when slowing down. A turbo is using waste energy when the engine is driving the vehicle. – HandyHowie Mar 31 '16 at 7:39
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    I think you can add approximate numbers: typical gas engine operates at 20% efficiency, typical electric engine at 90%. This at first glance shows how little energy there is left for reuse. – Agent_L Mar 31 '16 at 9:13
  • Does a turbo actually improve the efficiency of an engine, or does it merely increase its peak power (which may in turn allow use of a smaller engine which would be more efficient when supplying lower energy demands)? – supercat Mar 31 '16 at 13:31
4

You could capture the heat from the electric motor, and convert that into more energy using a thermoelectric device. University of Florida research

  • Sounds interesting. I'd be great if you could add the full title of the paper and a quote that gets to the gist of it in case that link dies in the future. – JPhi1618 Mar 30 '16 at 21:10
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    This is indeed very interesting, but that project is evidently supposed to capture waste heat from combustion engines. An electric motor produces much, much less waste heat, and in particular at a far lower temperature, hence there's hardly any energy you could capture here. If anything, it might make sense to capture waste heat from the batteries. – leftaroundabout Mar 30 '16 at 22:39
2

There are some good answers here already that pretty much entirely cover the topic. One thing that wasn't mentioned however is KERS - kinetic energy recovery systems. Effectively you have a large mass (flywheel) that spins up as the vehicle is in motion. Generally under brakes or vac (no throttle) the driveline feeds energy into this flywheel. When needed, the flywheel then engages via a clutch and can feed that energy back into the driveline.

While not strictly an EV technology (and, in fact, I'm not sure if any EV's use KERS), it's another possible avenue.

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    KERS is just a particular implementation of regenerative braking, mentioned by DucatiKiller. And half of your answer doesn't even apply here, as electric motor has no "no throttle" operation mode. – Agent_L Mar 31 '16 at 9:17
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    No, the motor doesn't, however the controller certainly does have a coasting state in most regards. – Aaron Lavers Apr 1 '16 at 3:52
  • Also, here's an application for KERS in an EV. hybridcars.com/kers-equipped-twizy-ev-concept and a company who sells components kerstech.com, and an Intech pdf which in part discusses another implementation. cdn.intechopen.com/pdfs-wm/41416.pdf So yes, it somewhat applies here. For these systems to operate they take feedback from the controller. – Aaron Lavers Apr 1 '16 at 3:59
  • @Aron Lavers Dude! the question was about electric motors. Hybrids can have regular turbo because they have regular gas engines. The "kers" put in Twizzy is just another implementation of regenerative braking - exactly what DucatiKiller described way better than you. You should learn what "kers" is from textbooks, not form manufacturer advertisements. – Agent_L Apr 1 '16 at 7:54
  • @Agent_L I feel you're pushing the etiquette here. DucatiKiller linked two articles, however described little about it in his post. I don't believe my run-down of how a kinetic recovery system works is incorrect. I've given context as to how it works in a conventional application (i.e. under vac in a conventional car) and as such how it "could" adapt to this situation. Then provided some real-world implementations. Telling me where I should learn concepts from is a little beyond your position and beyond what's considered above-board here however.. – Aaron Lavers Apr 1 '16 at 8:39
1

Most Electric traction systems Buck down the voltage that goes to the electric motor from the battery .For example a 48V electric cart motor controller will give the motor Up to 48V but no more .Summarising and boiling down electric motor theory for this stack voltage is speed and current is torque .The jargon term TURBO is used when the controller is modified to sometimes act as a Voltage Amplifier giving more volts than the battery makes .This gives more speed but not more torque .This is how electric carts can be and are modded to go faster while keeping the same motor and the same battery pack .You have to know what you are doing otherwise you will blow up the motor just like adding turbo to a petrol engine .The degree of boost is the amount of voltage amplification that the controller is set up for .25% is a ball park reasonable figure.

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