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The statement is not false, look at EPA gas mileage numbers for hybrids over the last decade. You're arguing against something I was careful not to say.

lutusp 14y ago
> The statement is not false, look at EPA gas mileage numbers for hybrids over the last decade. That may be true, but it's not the topic. The topic is how efficient regenerative braking is on the Tesla Model S.
- MRSallee 14y ago
  The topic is HN being critical of authors suggesting that regenerative braking is a net positive. I suspect that in both the NYT article and in this CR article, the author is mistaken about the role of regenerative braking on fuel efficiency, but they are not incorrect to point to regenerative braking as the enabler of improved fuel efficiency. Some made up numbers to illustrate the point. I do not know if these apply to the Model S specifically but do apply to other vehicles with regen braking, familiarity with which is likely to have influenced the authors' explanations. City driving vs. highway driving: Lower drag: +20% efficiency Lower engine RPM: +20% efficiency More stop and go with normal braking: -50% efficiency More stop and go with regen braking: -50%(.6) = -30% efficiency = Net city driving with normal braking: -10% efficiency = Net city driving with regen braking: +10% efficiency
  - lutusp 14y ago
    > The topic is HN being critical of authors suggesting that regenerative braking is a net positive. But regenerative braking cannot produce net positive energy compared to not braking at all -- that violates basic physical principles. Regenerative braking is better than no regenerative braking, but regenerative braking is not better than not braking at all. That has been the point all along. > = Net city driving with regen braking: +10% efficiency No, not compared to driving at a steady pace. If the scenario you describe were so, it would represent a gain in energy greater than 100%, and that is not possible. Stop-and-go driving is always less efficient than driving at a steady pace at the same average speed. Regenerative braking can only minimize the loss caused by stop-and-go driving, it can't reverse it -- it can't produce a net gain as you are implying.
    - MRSallee 14y ago
      Dude, you're ignoring what I'm saying.
      - lutusp 14y ago
        No, I'm addressing what you're saying. You tried to say that regenerative braking produced a net advantage, but this is not possible. Here's the source of your error: > More stop and go with normal braking: -50% efficiency Wrong value. Apart from the fact that the number is invented, because each stop turns all the car's kinetic energy into heat, the number should be -100%. Either that or perform a full analysis, with a listing of all the factors. After you recover 20% of the energy with regenerative braking, you end up with -80% instead of -100%.
      - MRSallee 14y ago
        I'm saying that less drag and less engine speed are giving the fuel efficiency gains. I am saying that, typically, braking wipes out all of those gains and more. I am saying that regenerative braking often reduces the negative impact of braking enough that the gains -- from lower drag and engine speed -- aren't completely undone by braking. I ride a low-power motorcycle. On the freeway, I get about 50 mpg. On back roads, with braking and generally higher rpm, I get 60+ mpg. For my low-power bike, the drag at freeway speeds takes more effort to overcome than does the varying speed and braking of back road riding. And that is without regenerative braking. If I had regen braking, I'd get even better economy on back roads.

lutusp 14y ago

> The statement is not false, look at EPA gas mileage numbers for hybrids over the last decade. That may be true, but it's not the topic. The topic is how efficient regenerative braking is on the Tesla Model S.

MRSallee 14y ago
The topic is HN being critical of authors suggesting that regenerative braking is a net positive. I suspect that in both the NYT article and in this CR article, the author is mistaken about the role of regenerative braking on fuel efficiency, but they are not incorrect to point to regenerative braking as the enabler of improved fuel efficiency. Some made up numbers to illustrate the point. I do not know if these apply to the Model S specifically but do apply to other vehicles with regen braking, familiarity with which is likely to have influenced the authors' explanations. City driving vs. highway driving: Lower drag: +20% efficiency Lower engine RPM: +20% efficiency More stop and go with normal braking: -50% efficiency More stop and go with regen braking: -50%(.6) = -30% efficiency = Net city driving with normal braking: -10% efficiency = Net city driving with regen braking: +10% efficiency
- lutusp 14y ago
  > The topic is HN being critical of authors suggesting that regenerative braking is a net positive. But regenerative braking cannot produce net positive energy compared to not braking at all -- that violates basic physical principles. Regenerative braking is better than no regenerative braking, but regenerative braking is not better than not braking at all. That has been the point all along. > = Net city driving with regen braking: +10% efficiency No, not compared to driving at a steady pace. If the scenario you describe were so, it would represent a gain in energy greater than 100%, and that is not possible. Stop-and-go driving is always less efficient than driving at a steady pace at the same average speed. Regenerative braking can only minimize the loss caused by stop-and-go driving, it can't reverse it -- it can't produce a net gain as you are implying.
  - MRSallee 14y ago
    Dude, you're ignoring what I'm saying.
    - lutusp 14y ago
      No, I'm addressing what you're saying. You tried to say that regenerative braking produced a net advantage, but this is not possible. Here's the source of your error: > More stop and go with normal braking: -50% efficiency Wrong value. Apart from the fact that the number is invented, because each stop turns all the car's kinetic energy into heat, the number should be -100%. Either that or perform a full analysis, with a listing of all the factors. After you recover 20% of the energy with regenerative braking, you end up with -80% instead of -100%.
    - MRSallee 14y ago
      I'm saying that less drag and less engine speed are giving the fuel efficiency gains. I am saying that, typically, braking wipes out all of those gains and more. I am saying that regenerative braking often reduces the negative impact of braking enough that the gains -- from lower drag and engine speed -- aren't completely undone by braking. I ride a low-power motorcycle. On the freeway, I get about 50 mpg. On back roads, with braking and generally higher rpm, I get 60+ mpg. For my low-power bike, the drag at freeway speeds takes more effort to overcome than does the varying speed and braking of back road riding. And that is without regenerative braking. If I had regen braking, I'd get even better economy on back roads.