The problems with ekectric cars seem overwhelming-short battery life, limited range, charging times, etc. Why not store the energy mechanically? (in a rotating flywheel). I read about this concept years ago-they were looking at a carbon-fiber disc, rotating at 40,000 RPM. Is such a scheme practical?
Dod anyone ever build a prototype?
I submit for consideration by our deep thinkers to say yeh or neh.
Think of a giant gyroscope in your car, and like a good gyroscope it wants to stay put. Now imagine a car that wants to move in directions that are not where the gyroscope wants to be. Storing large amounts of energy in a flywheel seems like an idea with major hurdles. The weight would be kept down by the high speed and a carbon fiber flywheel, so I guess that was for you don’t use a lot of energy hauling extra weight.
A super conductor could lessen loss by friction if you floated the flywheel, but a lot of electricity can be quickly stored and discharged with a super conductor too. I like the idea of a lightening charge stored in the car. 
A huge problem with these sorts of schemes is that all the stored energy can be released in a very short time in a serious accident. Punching a hole in a gas tank (or battery) isn’t going to release all the energy, punching a hole in a high speed gyroscope (or ultracapacitor) is going to be catastrophic.
You mean awesome.
-FrL-
I was thinking spectacular.
From the right vantage point, preferably 100 yards away, with some bullet proof glass in between me and the show.
Someone else can take the driver’s seat view.
Only in a Michael Bay film.
The gyroscopic issue is signficant; even if you gimbal the flywheel so that it doesn’t impart gyroscopic loads on the vehicle, you still have to isolate the mounts so that you’re not imparting shock loads on the bearings, and then you have a mechanical-electrical-mechanical transmission of load. And the structural incontinence of a massy flywheel spinning at ~50,000RPM or more would indeed be spectacular. I say this from personal experience, having seen the results of a disintegrated aircraft turbine. Flywheels are good for moderating torque impulse (hence, their use on crankshafts of motors that are not inherently balanced) and for large stationary power sources, but not great in long-term storage for general mobile applications.
Stranger
I’ve seen plenty of prototypes when I was a kid. I played with them by revving them up on a table, and when the flywheel got going, I released it on the table and the car zoomed off.
As for a real-life case, you’d need either a giant, or the Incredible Hulk around. Steering might be a problem, unless the driver was also Hulk.
If it’s a high speed flywheel and we want to try punching a hole, I suppose Hulk could be the one doing the punching. He’d probably get a nasty skin burn, if he touched the wheel, though. I suppose that might make Hulk angry.
The idea has been used to a limited degree for public transit, the Swiss “gyrobus” experiment being the most often quoted:
For a city bus, it’s workable - they run regular routes and you can position charging stations where they rev up the flywheel as appropriate. They don’t have to store really huge amounts of energy in the flywheel - just enough to get between the charging stations, which still meant you had a heavy flywheel spinning very fast (the gyrobus is given as a 3 ton flywheel revved up to 3000 rpm). The wiki article also notes another disadvantage:
Back in the late 70s or early 80s, some urban buses were rigged up with flywheels (10,000 maybe 50,000 rpm? ). I guess they used carbon fiber for the wheel material. And they were enclosed with enough shielding that, even if it disintegrated, the container would be a contained fragments (being carbon fiber though I would expect it would become a dust). I think the wheels gave the bus about twenty minutes worth of propulsion, but needed something like 15 minutes (?) to spin up to full capacity. I don’t know if the power source was onboard or not. Never heard about them after that.
I remember this subject as an engineering class discussion.
We we talking about the possibility of electric cars and the fact that battery weight made them uneconomic. (this was in the 1970s) One student mentioned that it made more sense to consider flywheel storage as long as you were willing to carry that much weight.
Somebody observed that gyroscopes have a tendency to torque to a 90 degree axis to any change in motion so a single large gyro with its axis oriented with the cars axles might tip over when the car made a sharp turn.
One student said that the gyro should then be oriented with its axle straight up and down. The prof said that car would be OK on a flat grid but might flip over if the car had to suddenly climb or descend a steep grade.
The conclusion of the discussion was that it would either require a mechanism that allowed one gyro to “float” to any position while still maintaining a connection to the power train (doable but probably elaborate and expensive) or an arrangement of 6 gyros (some argued 4) in a “box” configuration so that their torque would cancel out during reorientation (Another elaborate and expensive scheme).
This wasn’t the finest minds in the business, just a bunch of college geeks but I think it demonstrates some of the obstacles that would have to be overcome.
Gyrobus (scroll down). The flywheels (which were cast, not carbon fiber composite) spun at 3,000RPM, and drove the buses about 3-4 miles at tram speeds (20-30 mph), recharging at stops from overhead booms. They weren’t even particularly successful as public transit vehicles, and doesn’t suggest that they’d be useful in general purpose private commuter application.
Stranger
You should be able to do it with two counter-rotating flywheels mounted on a common shaft, provided that the shaft and bearings can take the intermediate induced torque. However, you are going to have space/material strength/bearing load limitations that might dictate using multiple sets of flywheels for batter packaging.
Stranger
I was under the (possibly mistaken) impression when I was growing up that some regular combustion engine buses had been rigged with flywheel braking, transferring the forward kinetic energy of the bus into the spinning of a wheel with vertical axis (which surely would have to have significant mass to have a realistic number of revs - light carbon fiber would surely spin too fast for regular bearings if helping to slow a bus?) when the bus was slowing, which would spin while the vehicle was at rest, then pour some of the energy back into the drive train when the clutch was released, saving fuel.
True or not, this struck me as a canny idea, and since it was connected to slower vehicles coming to a stop, wouldn’t have as many of the torque concerns that a car would have.
I’m sorry Stranger that I can’t provide a cite (Popular Science or Popular Mechanics?), but I definitely remembered seeing carbon fiber being called out and thinking that was an odd, but really cool application. If I can find the original source, I’ll try to post a link to it. But hey it was like 25 - 30 years ago, and given how crappy my memory is now for all know it could have been carbon copy paper instead… 
Wow. A practical use for those God-awful spinners!
A lot of modern flywheels for energy storage are built with graphite fiber composite because of the considerably higher tensile strength than cast or forged steel allowing for substantially higher angular speed and thus more energy storage (in proportion to the square of the angular speed) in the same size rotor. Only alloys of the refractory metals get anywhere close to the tensile strength of carbon fiber, and aside from cost and availability there are other reasons (manufacturability, dead weight) that they’re not general used in this application except for rare instances. Long chain carbon fullerenes could increase hoop strength by an order of magnitude or more.
However, I’d be surprised to see a carbon fiber flywheel on a device thirty years old; it’s not impossible but it wouldn’t be a common application at that time. It’s certainly possible to use flywheels for transitory energy storage as jjimm suggests and I’ve seen several proposed systems for use in hybrid drivetrains, but I don’t know of any production vehicle or trolley/bus system where this has been used; all the brakegen systems I know either use electric motors in reverse to charge batteries or capacitor banks, or use a hydraulic accumulator and reversible pump to store braking energy. I don’t claim to have an encyclopedic knowledge of this field so its possible that flywheels have been used, but I can’t think of anything offhand or find anything from a brief online search.
Stranger
You might be remembering the early-1980s tests done using a modified Leyland bus:
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ETA: this idea is being resuscitated in Blackburn College’s Kestrel Project
Cool, so I’m not living with competely fabricated memories! Glad to see someone else is picking up on this - they’re pretty impressive fuel savings.
The central problem of high-speed flywheels, as been said, is the flywheel exploding. (Or rather, a carbonfiber flywheel spontaneously turns into hot magma. There’s no chunks.) This isn’t insurmountable, you just need more shielding. The real problem is you end up with a low energy/weight ratio. If you use low-speed flywheels (like the gyrobus), you end up in the same place.
I actually had an interesting idea on containing this energy. Lining the insides with a light material with the right melting point and a high heat of fusion, it would soak up tremendous energy.