7 ms·

Show HN: lapetitejort goes hiking, discovers perpetual motion

lapetitejort 5y ago
That would be a neat test. Find a steep hill, preferably as straight as possible, and test how many times an electric car can traverse it before running out of batteries. Would the distance traveled exceed that on a flat road with no generation? By how much? I'm sure someone in San Fran has already tested this.
- cj 5y ago
  My intuition would say you'd never have longer range driving up and down a hill compared to driving flat in a straight line (and never braking). I would imagine that at best, you could maybe match the range. If there's any scenario where going up and down a hill would yield more range than driving flat, I'd be very interested in how/why.
  - lapetitejort 5y ago
    Yeah, rethinking it, assuming you start and end at the same spot, the milage at the beginning is the max you can achieve, full stop. Simple conservation of energy. So the question becomes, how many miles do you lose at the end?
  - dreamcompiler 5y ago
    Going up/down at 20 MPH and going flat at 80 MPH might be this scenario. If the velocity difference is big enough, wind resistance will have a bigger impact on range than the thermodynamic inefficiency of regen braking.
  - saltcured 5y ago
    There is a potential benefit, and it is not really due to regenerative braking. It would require that your traction system is more efficient at converting stored charge into kinetic energy at high loads than at medium loads and that it also has a very efficient "coasting" mode to cut losses when almost no output is required. Cruising on level terrain is a mediocre load on the traction system to offset the rolling and aerodynamic friction. Climbing a steep grade at the same speed will increase the load significantly, but that extra kinetic output energy is being stored as gravitational potential energy rather than lost like the baseline cruising load. Then, when you descend the mountain on the other side, you recover that gravitational energy to offset the rolling and aerodynamic friction. You want to be "falling" down the mountain grade at your terminal velocity where no braking and no traction force is required to maintain your cruising speed. It will not work well for winding descents where you need to brake for turns. I've seen this work with turbocharged ICE cars with elevation gains of 4k foot or more and hundreds of miles distance. I've repeated on many trips to prove to me that it isn't simply a fluke (such as extreme headwind or tailwind). Going over a pass yields me a better trip MPG than covering the same several hundred miles on relatively flat ground. I could see this benefit for a hybrid car too, since I believe they mostly handle highway cruise on ICE power. However, it seems unlikely for a BEV car unless there is something about battery and power electronics that I do not understand, that would give them a significant efficiency boost at high power.
  - adrianmonk 5y ago
    It seems possible in theory. The air is thinner at higher altitudes, and wind resistance is a big part of fuel economy, isn't it? Suppose you're going to make a 100 mile (160 km) trip. You have two possible routes. One is a straight shot on level ground. The other is straight, too, but it takes you up a 10% grade for the first 10 miles (gaining 1 mile in altitude), then you continue on level ground for 80 miles, and then you descend a matching 10% grade at the end. It seems like the 80 miles of cruising on level ground at high altitude would use less battery than 80 miles of cruising at sea level (if the speeds are the same). There will be some losses due to climbing and descending. Maybe climbing and descending is a little less efficient. Also, you're definitely traveling a very slightly longer distance. But those losses might be made up for spending the bulk of your trip in thinner air.
- Workaccount2 5y ago
  In theory they would be identical, but in practice a flat road would be better. There are more real world inefficiencies with going up and down a hill.
- codeulike 5y ago
  Would the distance traveled exceed that on a flat road with no generation? No. Climbing the hill the car uses extra energy (compared to a flat road) because its fighting gravity. On the way back down the hill regen will recover some of that gravity-fighting energy but nowhere near all of it.
  - hourislate 5y ago
    This scenario is a little different than a car but a Swiss company is experimenting with a Komatsu Dump Truck that basically recharges its battery using regen braking on it's trip down so it has enough power for the trip up. It actually generates a surplus of an extra 10kwh because the truck is carrying a full load down. https://phys.org/news/2017-09-e-dumper-world-largest-electric-vehicle.html https://phys.org/news/2017-09-e-dumper-world-largest-electri...
    - monkeybutton 5y ago
      And going up it is empty. Its basically exploiting the fact that rocks at the top of a hill have higher potential energy than at the bottom. If you charged the trucks with wind/solar at peak times and used them to carry the rocks back uphill you could have yourself a very convoluted and mechanically fraught battery!
      - UncleOxidant 5y ago
        A good visual demonstration of entropy. Heat death occurs when all of the mountains have been leveled and there are no altitude differences to exploit.
        EvanAnderson 5y ago
        That's excellent! I'm stealing that.
      - codeulike 5y ago
        Like Pumped Storage Hydroelectricity https://en.wikipedia.org/wiki/Pumped-storage_hydroelectricity https://en.wikipedia.org/wiki/Pumped-storage_hydroelectricit...
- hamburglar 5y ago
  I’ve tested this on an extremely steep block in seattle. This was about 8 years ago in a plug-in Prius. Distance around the block was about 0.25 miles, one side generating a lot of range, the other side eating it up because I’m going back uphill. On each cycle, I would lose an extra 0.1miles of range over what I experienced on flat ground. It’s actually very interesting to me that while driving that car, I developed a sort of feel for how trading potential and kinetic energy affected my available range, and I had an imaginary boundary drawn in my head that defined all the places where I’d be able to make it home “for free” that was kind of like a topo map. In particular, if I managed to crest the hill where Canlis sits on highway 99, I knew that despite all the ups and downs in between me and home, I’d be able to make it there without having to fire up the gas engine. :D
ineedasername 5y ago
That's just pseudoscience: The earth is flat so there's no such thing as going up hills.
seanmcdirmid 5y ago
+3 miles hopefully didn’t include the energy spent going up, or the law of energy conservation would have been broken.
- TheSoftwareGuy 5y ago
  I think the wind at his back is key here. His car was acting like a wind turbine
  - croon 5y ago
    I think OP meant winding and not windy, but even if not, Teslas (and any other car) are designed to be aerodynamic, not the opposite, so it's quite impossible to yield more energy than it cost in that scenario, even ignoring motor efficiency, regenerative braking efficiency, and every other real world inefficiency.
    - underwater 5y ago
      Windy is a perfectly valid descriptive word for a twisting road.
      - tomjakubowski 5y ago
        the confusion in this thread is that windy, as written, could mean there's lots of wind (said "win-dee") or lots of bends (or "winds", said "whine-dee"). "winding" is less ambiguous
        underwater 5y ago
        Obviously. But `croon` seemed to think that windy was was incorrectly, not just ambiguously: "I think OP meant winding and not windy"
    - wolverine876 5y ago
      > Teslas (and any other car) are designed to be aerodynamic Why not design them to be aerodynamic for headwinds, and maximize wind-resistance for tailwinds? I wonder how much energy would be gained.

That would be a neat test. Find a steep hill, preferably as straight as possible, and test how many times an electric car can traverse it before running out of batteries. Would the distance traveled exceed that on a flat road with no generation? By how much? I'm sure someone in San Fran has already tested this.

cj 5y ago
My intuition would say you'd never have longer range driving up and down a hill compared to driving flat in a straight line (and never braking). I would imagine that at best, you could maybe match the range. If there's any scenario where going up and down a hill would yield more range than driving flat, I'd be very interested in how/why.
- lapetitejort 5y ago
  Yeah, rethinking it, assuming you start and end at the same spot, the milage at the beginning is the max you can achieve, full stop. Simple conservation of energy. So the question becomes, how many miles do you lose at the end?
- dreamcompiler 5y ago
  Going up/down at 20 MPH and going flat at 80 MPH might be this scenario. If the velocity difference is big enough, wind resistance will have a bigger impact on range than the thermodynamic inefficiency of regen braking.
- saltcured 5y ago
  There is a potential benefit, and it is not really due to regenerative braking. It would require that your traction system is more efficient at converting stored charge into kinetic energy at high loads than at medium loads and that it also has a very efficient "coasting" mode to cut losses when almost no output is required. Cruising on level terrain is a mediocre load on the traction system to offset the rolling and aerodynamic friction. Climbing a steep grade at the same speed will increase the load significantly, but that extra kinetic output energy is being stored as gravitational potential energy rather than lost like the baseline cruising load. Then, when you descend the mountain on the other side, you recover that gravitational energy to offset the rolling and aerodynamic friction. You want to be "falling" down the mountain grade at your terminal velocity where no braking and no traction force is required to maintain your cruising speed. It will not work well for winding descents where you need to brake for turns. I've seen this work with turbocharged ICE cars with elevation gains of 4k foot or more and hundreds of miles distance. I've repeated on many trips to prove to me that it isn't simply a fluke (such as extreme headwind or tailwind). Going over a pass yields me a better trip MPG than covering the same several hundred miles on relatively flat ground. I could see this benefit for a hybrid car too, since I believe they mostly handle highway cruise on ICE power. However, it seems unlikely for a BEV car unless there is something about battery and power electronics that I do not understand, that would give them a significant efficiency boost at high power.
- adrianmonk 5y ago
  It seems possible in theory. The air is thinner at higher altitudes, and wind resistance is a big part of fuel economy, isn't it? Suppose you're going to make a 100 mile (160 km) trip. You have two possible routes. One is a straight shot on level ground. The other is straight, too, but it takes you up a 10% grade for the first 10 miles (gaining 1 mile in altitude), then you continue on level ground for 80 miles, and then you descend a matching 10% grade at the end. It seems like the 80 miles of cruising on level ground at high altitude would use less battery than 80 miles of cruising at sea level (if the speeds are the same). There will be some losses due to climbing and descending. Maybe climbing and descending is a little less efficient. Also, you're definitely traveling a very slightly longer distance. But those losses might be made up for spending the bulk of your trip in thinner air.
Workaccount2 5y ago
In theory they would be identical, but in practice a flat road would be better. There are more real world inefficiencies with going up and down a hill.
codeulike 5y ago
Would the distance traveled exceed that on a flat road with no generation? No. Climbing the hill the car uses extra energy (compared to a flat road) because its fighting gravity. On the way back down the hill regen will recover some of that gravity-fighting energy but nowhere near all of it.
- hourislate 5y ago
  This scenario is a little different than a car but a Swiss company is experimenting with a Komatsu Dump Truck that basically recharges its battery using regen braking on it's trip down so it has enough power for the trip up. It actually generates a surplus of an extra 10kwh because the truck is carrying a full load down. https://phys.org/news/2017-09-e-dumper-world-largest-electric-vehicle.html https://phys.org/news/2017-09-e-dumper-world-largest-electri...
  - monkeybutton 5y ago
    And going up it is empty. Its basically exploiting the fact that rocks at the top of a hill have higher potential energy than at the bottom. If you charged the trucks with wind/solar at peak times and used them to carry the rocks back uphill you could have yourself a very convoluted and mechanically fraught battery!
    - UncleOxidant 5y ago
      A good visual demonstration of entropy. Heat death occurs when all of the mountains have been leveled and there are no altitude differences to exploit.
      - EvanAnderson 5y ago
        That's excellent! I'm stealing that.
    - codeulike 5y ago
      Like Pumped Storage Hydroelectricity https://en.wikipedia.org/wiki/Pumped-storage_hydroelectricity https://en.wikipedia.org/wiki/Pumped-storage_hydroelectricit...
hamburglar 5y ago
I’ve tested this on an extremely steep block in seattle. This was about 8 years ago in a plug-in Prius. Distance around the block was about 0.25 miles, one side generating a lot of range, the other side eating it up because I’m going back uphill. On each cycle, I would lose an extra 0.1miles of range over what I experienced on flat ground. It’s actually very interesting to me that while driving that car, I developed a sort of feel for how trading potential and kinetic energy affected my available range, and I had an imaginary boundary drawn in my head that defined all the places where I’d be able to make it home “for free” that was kind of like a topo map. In particular, if I managed to crest the hill where Canlis sits on highway 99, I knew that despite all the ups and downs in between me and home, I’d be able to make it there without having to fire up the gas engine. :D

ineedasername 5y ago

That's just pseudoscience: The earth is flat so there's no such thing as going up hills.

seanmcdirmid 5y ago

+3 miles hopefully didn’t include the energy spent going up, or the law of energy conservation would have been broken.

TheSoftwareGuy 5y ago
I think the wind at his back is key here. His car was acting like a wind turbine
- croon 5y ago
  I think OP meant winding and not windy, but even if not, Teslas (and any other car) are designed to be aerodynamic, not the opposite, so it's quite impossible to yield more energy than it cost in that scenario, even ignoring motor efficiency, regenerative braking efficiency, and every other real world inefficiency.
  - underwater 5y ago
    Windy is a perfectly valid descriptive word for a twisting road.
    - tomjakubowski 5y ago
      the confusion in this thread is that windy, as written, could mean there's lots of wind (said "win-dee") or lots of bends (or "winds", said "whine-dee"). "winding" is less ambiguous
      - underwater 5y ago
        Obviously. But `croon` seemed to think that windy was was incorrectly, not just ambiguously: "I think OP meant winding and not windy"
  - wolverine876 5y ago
    > Teslas (and any other car) are designed to be aerodynamic Why not design them to be aerodynamic for headwinds, and maximize wind-resistance for tailwinds? I wonder how much energy would be gained.