7 ms·

Why did the flywheel hybrid system never catch on for road cars?

16 points by elsewhen 6y ago

Turing_Machine 6y ago
In a now-dead comment, someone suggested that safety was a concern. While it's true that a disintegrating flywheel can be highly dangerous, it's also true that sitting on a tank full of gasoline or a gigantic LiPo battery aren't as safe as lying in bed, either. If you have enough stored energy to move a vehicle at highway speeds, you've got enough energy to create a Very Bad Day if things go south and all of that energy comes out at once. There's pretty much no way around that. The alternative is some sort of power source that doesn't require onboard storage -- catenaries, third-rail systems, beamed power... all of which have their own issues, and none of which have the convenience and versatility of an on-board supply.
- jdlshore 6y ago
  The “if” in “if it all comes out” is the key here. Nobody’s disputing that releasing all that energy at once is catastrophic. The question is, how hard is it to prevent it from releasing all at once? My understanding is that flywheels are far more dangerous in this respect, and require heavy containment vessels to mitigate the risk. That heavy containment is part of what makes flywheels impractical. But that’s just the impression I have. I haven’t studied it.
  - deleted 6y ago
    [deleted]
  - Turing_Machine 6y ago
    I'm not an expert, either, but I've seen mentions of "frangible" flywheel materials that basically turn into a powder when they fail, so you get a housing full of what one article described as "red-hot sand".
    - Nextgrid 6y ago
      I find it hard to believe such a thing is possible. You'd want a material that is both strong enough to stay in one piece despite the centrifugal force when it is spinning in normal operation, but as soon as it fails you'd want all of it to disintegrate into powder (instead of in several chunks, which could make it more dangerous - essentially a fragmentation grenade). You'd want a material that is solid but has a specific trigger that immediately causes it to break entirely and propagate said trigger onwards. Maybe something like a Prince Rupert's drop (a bit of glass which is super strong, but breaking its tail - the weak spot - causes the entire thing to disintegrate due to internal forces)?
      - Turing_Machine 6y ago
        Yes. Another example might be the safety glass used in automobile windshields. It's pretty tough, but when it does shatter, it doesn't produce the dagger-like shards of regular glass. The key would be having no individual fragment with both the momentum and the "pointiness" to penetrate the enclosure.
- hakfoo 6y ago
  I so wish we could get the momentum for a catenary or third-rail system. Aside from solving the efficiency and range-anxiety problems, it would offer some amazing system-control power. We have problems with people driving into flooded riverbeds every year, but if we could cut the external power on a block by block level, that could isolate the dangerous areas. You could also block off temporary pedestrianized zones or areas around VIP movements for security purposes. It could also improve overall road safety, by limiting the opportunity to make unpredictable turns or go wrong-way-- you can only change lanes at specific points where the third-rail or cantenary splits.
- Nextgrid 6y ago
  A fuel tank doesn't release its energy all at once. Fuel by itself will not burn unless it's mixed with the right amount of oxygen, so in a fuel tank fire most of the fuel stays put and only the top layer (where it can aerosolize and mix with oxygen) combusts. You'd have to spread it over a huge area for it to burn all at once. Batteries are similar. Battery fires progress slowly as each battery fails and overheats the one next to it and the cycle continues, so even though the fire from a single battery is scary, it's typically "safe" enough in the sense that it's not an explosion and you have some time to get away before more ignite. A flywheel by comparison releases all its energy in the form of heavy chunks of metal flying at high speeds, and I don't see an easy way around that beyond just a containment vessel which could add so much weight as to make the whole system pointless.
narogab 6y ago
Looking at the diagram I'd judge that the proximity of a !30,000 rpm flywheel to the driver's family jewels might have something to do with it.
yowlingcat 6y ago
Any of the gearheads here know if there's been cross-pollination between flywheel hybrid systems and regenerative braking tech in EVs?
- Turing_Machine 6y ago
  I have no idea, but it seems like you should be able to use a direct mechanical coupling to put the braking energy right back in the flywheel, thus avoiding two conversion steps and their associated losses (mechanical -> electricity, then electricity -> mechanical). Whether the savings would be worth the added transmission complexity? Not a clue.
linsomniac 6y ago
One issue that I never heard them overcome was resistance to turning along one axis. Industrial flywheels are fine, because they're usually fixed in place. Cars tend to want freedom along every axis. My distant memory recalls some discussion of buses fitted with flywheels for regenerative braking, but they had issues with hills.

16 points by elsewhen 6y ago

In a now-dead comment, someone suggested that safety was a concern. While it's true that a disintegrating flywheel can be highly dangerous, it's also true that sitting on a tank full of gasoline or a gigantic LiPo battery aren't as safe as lying in bed, either. If you have enough stored energy to move a vehicle at highway speeds, you've got enough energy to create a Very Bad Day if things go south and all of that energy comes out at once. There's pretty much no way around that. The alternative is some sort of power source that doesn't require onboard storage -- catenaries, third-rail systems, beamed power... all of which have their own issues, and none of which have the convenience and versatility of an on-board supply.

jdlshore 6y ago
The “if” in “if it all comes out” is the key here. Nobody’s disputing that releasing all that energy at once is catastrophic. The question is, how hard is it to prevent it from releasing all at once? My understanding is that flywheels are far more dangerous in this respect, and require heavy containment vessels to mitigate the risk. That heavy containment is part of what makes flywheels impractical. But that’s just the impression I have. I haven’t studied it.
- deleted 6y ago
  [deleted]
- Turing_Machine 6y ago
  I'm not an expert, either, but I've seen mentions of "frangible" flywheel materials that basically turn into a powder when they fail, so you get a housing full of what one article described as "red-hot sand".
  - Nextgrid 6y ago
    I find it hard to believe such a thing is possible. You'd want a material that is both strong enough to stay in one piece despite the centrifugal force when it is spinning in normal operation, but as soon as it fails you'd want all of it to disintegrate into powder (instead of in several chunks, which could make it more dangerous - essentially a fragmentation grenade). You'd want a material that is solid but has a specific trigger that immediately causes it to break entirely and propagate said trigger onwards. Maybe something like a Prince Rupert's drop (a bit of glass which is super strong, but breaking its tail - the weak spot - causes the entire thing to disintegrate due to internal forces)?
    - Turing_Machine 6y ago
      Yes. Another example might be the safety glass used in automobile windshields. It's pretty tough, but when it does shatter, it doesn't produce the dagger-like shards of regular glass. The key would be having no individual fragment with both the momentum and the "pointiness" to penetrate the enclosure.
hakfoo 6y ago
I so wish we could get the momentum for a catenary or third-rail system. Aside from solving the efficiency and range-anxiety problems, it would offer some amazing system-control power. We have problems with people driving into flooded riverbeds every year, but if we could cut the external power on a block by block level, that could isolate the dangerous areas. You could also block off temporary pedestrianized zones or areas around VIP movements for security purposes. It could also improve overall road safety, by limiting the opportunity to make unpredictable turns or go wrong-way-- you can only change lanes at specific points where the third-rail or cantenary splits.
Nextgrid 6y ago
A fuel tank doesn't release its energy all at once. Fuel by itself will not burn unless it's mixed with the right amount of oxygen, so in a fuel tank fire most of the fuel stays put and only the top layer (where it can aerosolize and mix with oxygen) combusts. You'd have to spread it over a huge area for it to burn all at once. Batteries are similar. Battery fires progress slowly as each battery fails and overheats the one next to it and the cycle continues, so even though the fire from a single battery is scary, it's typically "safe" enough in the sense that it's not an explosion and you have some time to get away before more ignite. A flywheel by comparison releases all its energy in the form of heavy chunks of metal flying at high speeds, and I don't see an easy way around that beyond just a containment vessel which could add so much weight as to make the whole system pointless.

narogab 6y ago

Looking at the diagram I'd judge that the proximity of a !30,000 rpm flywheel to the driver's family jewels might have something to do with it.

yowlingcat 6y ago

Any of the gearheads here know if there's been cross-pollination between flywheel hybrid systems and regenerative braking tech in EVs?

Turing_Machine 6y ago
I have no idea, but it seems like you should be able to use a direct mechanical coupling to put the braking energy right back in the flywheel, thus avoiding two conversion steps and their associated losses (mechanical -> electricity, then electricity -> mechanical). Whether the savings would be worth the added transmission complexity? Not a clue.

linsomniac 6y ago

One issue that I never heard them overcome was resistance to turning along one axis. Industrial flywheels are fine, because they're usually fixed in place. Cars tend to want freedom along every axis. My distant memory recalls some discussion of buses fitted with flywheels for regenerative braking, but they had issues with hills.