4 ms·

>a high portion of freeway driving, which minimizes the opportunity for regenerative braking, are the most adverse conditions for any electric vehicle That's a

by zeteo 14y ago

>a high portion of freeway driving, which minimizes the opportunity for regenerative braking, are the most adverse conditions for any electric vehicle That's a pretty nasty meme and it seems to be spreading. No, people: regenerative braking mitigates the effects of braking on fuel efficiency! It doesn't overcome or reverse them. The laws of physics say that driving on the freeway with no braking is, and will always be, much better for fuel efficiency, in any conceivable vehicle.

moultano 14y ago
Are slower speeds more efficient overall though, such that driving off the highway could make up for the energy lost to braking? With combustion engines I had always read that speeds around ~55 are at the crossover point where increasing wind resistance wins out over increasing engine efficiency, but I'm wondering if the math works out differently for electric cars?
- seanalltogether 14y ago
  The slowest speed in the highest gear will give you the most bang for your buck. I'm not sure what kind of gearing the tesla has though since electric motors are very different beasts to standard combustion engines.
  - moultano 14y ago
    http://en.wikipedia.org/wiki/Tesla_Roadster#Transmission http://en.wikipedia.org/wiki/Tesla_Roadster#Transmission It's a fixed gear, so this is actually plausible.
  - karolist 14y ago
    In internal combustion engines with manual transmission be sure not to overdo that, high gear and very low RPMs are bad for the engine/transmission due to high load and minimal oil lubrication, it's also not efficient on fuel, there were local magazine tests done, but I don't have references now sorry. On an automatic transmission the transmission control module will figure out the gear for you at given conditions (won't let you get too low in the same gear) and what you say will be true.
- rdl 14y ago
  Yes, slower speeds are substantially more efficient. It's obviously not linear all the way to zero, but if there's no parasitic load from heat/ac/etc., I think 15-20mph would be more efficient than 55.
- epochwolf 14y ago
  I can definitely say that my gas-powered SUV got 19mpg at 55mph for long distance. (Several hours on a rural highway in Wisconsin, using cruise control) It normally gets between 12 and 16 mpg driving around highway and city.
  - baddox 14y ago
    The comparison is unfair unless you take into account the amount of braking done when driving slowly.
- sliverstorm 14y ago
  The answer to your first question would seem to be a clear "yes": http://www.fueleconomy.gov/Feg/bymodel/2012_Toyota_Prius.shtml http://www.fueleconomy.gov/Feg/bymodel/2012_Toyota_Prius.sht...
  - moultano 14y ago
    So all these journalists are practically correct, just not being precise about the mechanism. Off highway driving is more efficient, and it is more efficient (relative to a conventional car) due to regenerative breaking. They really should mention that it's more efficient relative to highway driving due to wind resistance.
- ScottBurson 14y ago
  I think the peak is actually closer to 45 for most gasoline-driven cars. It depends on aerodynamics, though. For a big SUV it might be 35. I suppose it could be as high as 55 for a small, highly aerodynamic car.
  - Someone 14y ago
    Aerodynamics and engine/drive train design. Aerodynamically, the slower you drive, the more efficient you drive. The exact curve will depend on air pressure, amount of moisture in the air, etc. For the engine, it is more complicated. You will want to keep its temperature and rpm at ideal levels. rpm and the gearing in the drive train determine what speed(s) that corresponds to (I guess drive train losses will depend a bit on the gearing, too, but that effect should be minimal) For the two combined, this gets really complicated. Do you drive at the engine-optimal 45 or does the gain due to lower speed offset the loss in engine efficiency at 44? If 44 is more efficient now, will it stay that when it starts to rain? When you enter a different road surface? When your tires heat up and tire pressure increases slightly? When you have a window open? http://en.wikipedia.org/wiki/Fuel_economy_in_automobiles#Speed_and_fuel_economy_studies http://en.wikipedia.org/wiki/Fuel_economy_in_automobiles#Spe... shows how convoluted this can get.
- uvdiv 14y ago
  Yes, efficiency is a monotonic function of speed here (Model S). No crossover point. http://www.teslamotors.com/goelectric/efficiency http://www.teslamotors.com/goelectric/efficiency (the graph)
  - tbjohns 14y ago
    I'm not sure. The y-axis on this graph is just the rate of power consumption. It doesn't do much for you to use minimal energy while traveling 0 mph. Efficiency, in terms on miles/gallon, is always a trade-off between traveling fast enough that the overhead of running the vehicle is less significant but slow enough that wind resistance doesn't dominate. There's a sweet spot in there.
    - uvdiv 14y ago
      The y-axis has units of [energy]/[distance], I think I'm interpreting it correctly? It seems to take everything into account in there.
      - tbjohns 14y ago
        Yeah, I'm definitely confused. I don't understand how a car traveling 0 mph can have a finite value on that graph if the y-axis is [energy]/[distance]. I was hoping the units were [Watts][hours]/[minutes] or something to make everything better.
        uvdiv 14y ago
        I found they have another graph with the same units (written "Wh/mile", so that's unambigious). But it has a different shape, with a minimum at 20-25 mph... http://www.teslamotors.com/blog/model-s-efficiency-and-range http://www.teslamotors.com/blog/model-s-efficiency-and-range
        lutusp 14y ago
        That graph shows a maximum efficiency at around 22 MPH (for the 85 kWh battery), which makes perfect sense -- any faster and air resistance becomes a factor. Any slower and energy uses apart from propulsion begin to eat away at efficiency -- the computer, lights, air conditioning, heat, and so forth.
- deleted 14y ago
  [deleted]
cma 14y ago
But they are saying that with a comparison to fossil fuel powered vehicles in mind aren't they? With a comparative framing it is a perfectly valid statement. There is an actual head smacking quote in the article where he says he took a country road instead of the highway in hopes of getting more range by taking advantage of regenerative braking... How can an EV car reviewer for Consumer Reports be so ignorant?
seanp2k2 14y ago
Yeah; this misconception manifesting as "taking the rural roads [will save power]" bothered me too.

by zeteo 14y ago

moultano 14y ago

Are slower speeds more efficient overall though, such that driving off the highway could make up for the energy lost to braking? With combustion engines I had always read that speeds around ~55 are at the crossover point where increasing wind resistance wins out over increasing engine efficiency, but I'm wondering if the math works out differently for electric cars?

seanalltogether 14y ago
The slowest speed in the highest gear will give you the most bang for your buck. I'm not sure what kind of gearing the tesla has though since electric motors are very different beasts to standard combustion engines.
- moultano 14y ago
  http://en.wikipedia.org/wiki/Tesla_Roadster#Transmission http://en.wikipedia.org/wiki/Tesla_Roadster#Transmission It's a fixed gear, so this is actually plausible.
- karolist 14y ago
  In internal combustion engines with manual transmission be sure not to overdo that, high gear and very low RPMs are bad for the engine/transmission due to high load and minimal oil lubrication, it's also not efficient on fuel, there were local magazine tests done, but I don't have references now sorry. On an automatic transmission the transmission control module will figure out the gear for you at given conditions (won't let you get too low in the same gear) and what you say will be true.
rdl 14y ago
Yes, slower speeds are substantially more efficient. It's obviously not linear all the way to zero, but if there's no parasitic load from heat/ac/etc., I think 15-20mph would be more efficient than 55.
epochwolf 14y ago
I can definitely say that my gas-powered SUV got 19mpg at 55mph for long distance. (Several hours on a rural highway in Wisconsin, using cruise control) It normally gets between 12 and 16 mpg driving around highway and city.
- baddox 14y ago
  The comparison is unfair unless you take into account the amount of braking done when driving slowly.
sliverstorm 14y ago
The answer to your first question would seem to be a clear "yes": http://www.fueleconomy.gov/Feg/bymodel/2012_Toyota_Prius.shtml http://www.fueleconomy.gov/Feg/bymodel/2012_Toyota_Prius.sht...
- moultano 14y ago
  So all these journalists are practically correct, just not being precise about the mechanism. Off highway driving is more efficient, and it is more efficient (relative to a conventional car) due to regenerative breaking. They really should mention that it's more efficient relative to highway driving due to wind resistance.
ScottBurson 14y ago
I think the peak is actually closer to 45 for most gasoline-driven cars. It depends on aerodynamics, though. For a big SUV it might be 35. I suppose it could be as high as 55 for a small, highly aerodynamic car.
- Someone 14y ago
  Aerodynamics and engine/drive train design. Aerodynamically, the slower you drive, the more efficient you drive. The exact curve will depend on air pressure, amount of moisture in the air, etc. For the engine, it is more complicated. You will want to keep its temperature and rpm at ideal levels. rpm and the gearing in the drive train determine what speed(s) that corresponds to (I guess drive train losses will depend a bit on the gearing, too, but that effect should be minimal) For the two combined, this gets really complicated. Do you drive at the engine-optimal 45 or does the gain due to lower speed offset the loss in engine efficiency at 44? If 44 is more efficient now, will it stay that when it starts to rain? When you enter a different road surface? When your tires heat up and tire pressure increases slightly? When you have a window open? http://en.wikipedia.org/wiki/Fuel_economy_in_automobiles#Speed_and_fuel_economy_studies http://en.wikipedia.org/wiki/Fuel_economy_in_automobiles#Spe... shows how convoluted this can get.
uvdiv 14y ago
Yes, efficiency is a monotonic function of speed here (Model S). No crossover point. http://www.teslamotors.com/goelectric/efficiency http://www.teslamotors.com/goelectric/efficiency (the graph)
- tbjohns 14y ago
  I'm not sure. The y-axis on this graph is just the rate of power consumption. It doesn't do much for you to use minimal energy while traveling 0 mph. Efficiency, in terms on miles/gallon, is always a trade-off between traveling fast enough that the overhead of running the vehicle is less significant but slow enough that wind resistance doesn't dominate. There's a sweet spot in there.
  - uvdiv 14y ago
    The y-axis has units of [energy]/[distance], I think I'm interpreting it correctly? It seems to take everything into account in there.
    - tbjohns 14y ago
      Yeah, I'm definitely confused. I don't understand how a car traveling 0 mph can have a finite value on that graph if the y-axis is [energy]/[distance]. I was hoping the units were [Watts][hours]/[minutes] or something to make everything better.
      - uvdiv 14y ago
        I found they have another graph with the same units (written "Wh/mile", so that's unambigious). But it has a different shape, with a minimum at 20-25 mph... http://www.teslamotors.com/blog/model-s-efficiency-and-range http://www.teslamotors.com/blog/model-s-efficiency-and-range
        lutusp 14y ago
        That graph shows a maximum efficiency at around 22 MPH (for the 85 kWh battery), which makes perfect sense -- any faster and air resistance becomes a factor. Any slower and energy uses apart from propulsion begin to eat away at efficiency -- the computer, lights, air conditioning, heat, and so forth.
deleted 14y ago
[deleted]

cma 14y ago

But they are saying that with a comparison to fossil fuel powered vehicles in mind aren't they? With a comparative framing it is a perfectly valid statement. There is an actual head smacking quote in the article where he says he took a country road instead of the highway in hopes of getting more range by taking advantage of regenerative braking... How can an EV car reviewer for Consumer Reports be so ignorant?

seanp2k2 14y ago

Yeah; this misconception manifesting as "taking the rural roads [will save power]" bothered me too.