3 ms·

Sporty behavior not on a highway is still going to get better consumption than extremely high speed driving, because wind resistance rises as the square of the

by NickM 13y ago

Sporty behavior not on a highway is still going to get better consumption than extremely high speed driving, because wind resistance rises as the square of the velocity. If you double your velocity, then air drag increases by 4x, so if users are quoting 8L/100 on 120-130km/h then you can expect to get about 32L/100 going 250km/h. With a 70L tank at 250km/h you may well exhaust the tank in under an hour. These are worst case numbers though, so if you drive a bit slower, then 1-2 hours between refueling seems like a reasonable estimate.

rdl 13y ago
Actually it doesn't look that bad. http://www.automobile-catalog.com/economy/2014/1900655/bmw_535d.html http://www.automobile-catalog.com/economy/2014/1900655/bmw_5... Fuel consumption: in liter / 100 km Estimated fuel consumption by constant speeds on top gear, steady ride without acceleration or braking, flat concrete or tarmac surface, no wind by 50 km/h (31 mph): 8 l/100 km by 60 km/h (37 mph): 6.8 l/100 km by 70 km/h (44 mph): 6.5 l/100 km by 80 km/h (50 mph): 4.9 l/100 km by 90 km/h (56 mph): 4.9 l/100 km by 100 km/h (62 mph): 4.9 l/100 km by 120 km/h (75 mph): 5.2 l/100 km by 140 km/h (87 mph): 5.6 l/100 km by 160 km/h (100 mph):6 l/100 km by 180 km/h (112 mph):6.5 l/100 km by 200 km/h (124 mph):7 l/100 km by 220 km/h (137 mph):7.6 l/100 km by 240 km/h (149 mph):8.4 l/100 km by 260 km/h (161 mph):9.3 l/100 km Clearly around 200 km/h is where the wind resistance (and gearing) is starting to become a factor, but I think you could probably hit 280 or so without being over 12 l/100km. The other factor is that when I've driven >250 km/h I've gotten tired much faster, as you need to be hyperaware. (I did almost 200mph in a "very nice" Corvette Dubai-Abu Dhabi for about 20 seconds, and could see the fuel dial dropping) 200-250 seems like a reasonable high cruise speed for parts of the Autobahn, though. The smarter option is probably Berlin to Munich/Frankfurt/Prague/etc. by train, of course, but I like having a car.
- deleted 13y ago
  [deleted]
- deleted 13y ago
  [deleted]
- NickM 13y ago
  Your link says that those are "estimated fuel consumptions", but I'm somewhat inclined to doubt the veracity of these estimates: between 120 km/h and 240 km/h wind drag is going to quadruple, and ICEs get lower efficiency at higher RPM. So if the efficiency falls by less than 50% when you double the speed, then that implies that tire drag and internal friction represent the majority of the force you're overcoming at highway speeds, which is definitely incorrect. Something doesn't add up.
  - rdl 13y ago
    Cars have geared transmissions. Cars designed for the Autobahn (most high end German cars) have gearing optimized for decent cruise range at 120-180 km/h. The engine is sized for peak acceleration (and marketing). It has a turbo (in the 535d; the 550d has 3....), which helps somewhat without adding more fuel consumption, but still, a 300hp engine is pretty big. Basically, to cruise along at 120kph in a reasonable car with reasonable Cd probably is about 20-30hp (yay for mixing metric and imperial units...). Wind is probably 20hp of that. The engine is less efficient at low output (specific output), so you won't get fuel consumption at 10% power which is only 10% of peak fuel consumption. Wind resistance does go up, but your engine also gets to operate more within its power band. With the right gearing, you can stay within the optimal RPMs and be close to the design power level of the engine at a reasonable cruise speed. Empirically, I've driven big heavy cars fast for long distances, and gotten reasonable fuel economy; especially big German cars with big diesel engines. What kills them is stop and go, or when you have a smaller car with, say, 5 gears, and you are running it at redline to keep your desired speed. What is awesome about electric cars is you could get much more efficiency in the motor across a range of power levels, plus regen. What is not at all awesome is the power density of lithium ion batteries.
  - mturmon 13y ago
    ...between 120 km/h and 240 km/h wind drag is going to quadruple... Your figures here are an idealization. Real results are not going to match this because air resistance and fuel consumption are not a smooth process. (Some versions of the Tesla S even have active suspension, which really makes things interesting.) I expect you'd need some pretty serious finite element modeling, and engine modeling, to really understand the linkage.
masklinn 13y ago
> Sporty behavior not on a highway is still going to get better consumption than extremely high speed driving, because wind resistance rises as the square of the velocity. It's a heavy car, and one built for highway, ramping up and slowing down all the time is extremely expensive. > With a 70L tank at 250km/h you may well exhaust the tank in under an hour. No. That would be a fuel consumption of above 28L/100, that's twice the worst case I've seen from any user of the car. Hell, you can run long-haul trucks with a better efficiency than that: http://newsroom.scania.com/en-group/2011/05/27/scania-fuel-efficiency-duel-mission-accomplished/ http://newsroom.scania.com/en-group/2011/05/27/scania-fuel-e... > These are worst case numbers though They're closer to lunacy than worst-case numbers (and the autobahn is not free-speed throughout). Although we'd need an actual 535d with an actual driver to know for sure.
- NickM 13y ago
  Calling my numbers "lunacy" will, unfortunately, not change the laws of physics. If you drive twice as fast, air drag multiplies by four, and the energy to overcome that air drag has to come from somewhere. At highway speeds air drag is the only appreciable force at work. Tire drag makes a little bit of difference too, but that will be more than canceled out by the fact that ICE efficiency drops at higher RPMs. Your "long-haul truck" citation is irrelevant to this discussion because those numbers are for drastically slower speeds than the ones you're talking about.
  - masklinn 13y ago
    > Calling my numbers "lunacy" will, unfortunately, not change the laws of physics. If you drive twice as fast, air drag multiplies by four, and the energy to overcome that air drag has to come from somewhere. At highway speeds air drag is the only appreciable force at work. As you will. Not that this changes a thing, the Model S's efficiency will also drop further, thus further lowering its range. The issue remains, even driving at inane speeds, the Tesla will have a significantly lower range than the 535d.

by NickM 13y ago

rdl 13y ago

Actually it doesn't look that bad. http://www.automobile-catalog.com/economy/2014/1900655/bmw_535d.html http://www.automobile-catalog.com/economy/2014/1900655/bmw_5... Fuel consumption: in liter / 100 km Estimated fuel consumption by constant speeds on top gear, steady ride without acceleration or braking, flat concrete or tarmac surface, no wind by 50 km/h (31 mph): 8 l/100 km by 60 km/h (37 mph): 6.8 l/100 km by 70 km/h (44 mph): 6.5 l/100 km by 80 km/h (50 mph): 4.9 l/100 km by 90 km/h (56 mph): 4.9 l/100 km by 100 km/h (62 mph): 4.9 l/100 km by 120 km/h (75 mph): 5.2 l/100 km by 140 km/h (87 mph): 5.6 l/100 km by 160 km/h (100 mph):6 l/100 km by 180 km/h (112 mph):6.5 l/100 km by 200 km/h (124 mph):7 l/100 km by 220 km/h (137 mph):7.6 l/100 km by 240 km/h (149 mph):8.4 l/100 km by 260 km/h (161 mph):9.3 l/100 km Clearly around 200 km/h is where the wind resistance (and gearing) is starting to become a factor, but I think you could probably hit 280 or so without being over 12 l/100km. The other factor is that when I've driven >250 km/h I've gotten tired much faster, as you need to be hyperaware. (I did almost 200mph in a "very nice" Corvette Dubai-Abu Dhabi for about 20 seconds, and could see the fuel dial dropping) 200-250 seems like a reasonable high cruise speed for parts of the Autobahn, though. The smarter option is probably Berlin to Munich/Frankfurt/Prague/etc. by train, of course, but I like having a car.

deleted 13y ago
[deleted]
deleted 13y ago
[deleted]
NickM 13y ago
Your link says that those are "estimated fuel consumptions", but I'm somewhat inclined to doubt the veracity of these estimates: between 120 km/h and 240 km/h wind drag is going to quadruple, and ICEs get lower efficiency at higher RPM. So if the efficiency falls by less than 50% when you double the speed, then that implies that tire drag and internal friction represent the majority of the force you're overcoming at highway speeds, which is definitely incorrect. Something doesn't add up.
- rdl 13y ago
  Cars have geared transmissions. Cars designed for the Autobahn (most high end German cars) have gearing optimized for decent cruise range at 120-180 km/h. The engine is sized for peak acceleration (and marketing). It has a turbo (in the 535d; the 550d has 3....), which helps somewhat without adding more fuel consumption, but still, a 300hp engine is pretty big. Basically, to cruise along at 120kph in a reasonable car with reasonable Cd probably is about 20-30hp (yay for mixing metric and imperial units...). Wind is probably 20hp of that. The engine is less efficient at low output (specific output), so you won't get fuel consumption at 10% power which is only 10% of peak fuel consumption. Wind resistance does go up, but your engine also gets to operate more within its power band. With the right gearing, you can stay within the optimal RPMs and be close to the design power level of the engine at a reasonable cruise speed. Empirically, I've driven big heavy cars fast for long distances, and gotten reasonable fuel economy; especially big German cars with big diesel engines. What kills them is stop and go, or when you have a smaller car with, say, 5 gears, and you are running it at redline to keep your desired speed. What is awesome about electric cars is you could get much more efficiency in the motor across a range of power levels, plus regen. What is not at all awesome is the power density of lithium ion batteries.
- mturmon 13y ago
  ...between 120 km/h and 240 km/h wind drag is going to quadruple... Your figures here are an idealization. Real results are not going to match this because air resistance and fuel consumption are not a smooth process. (Some versions of the Tesla S even have active suspension, which really makes things interesting.) I expect you'd need some pretty serious finite element modeling, and engine modeling, to really understand the linkage.

masklinn 13y ago

> Sporty behavior not on a highway is still going to get better consumption than extremely high speed driving, because wind resistance rises as the square of the velocity. It's a heavy car, and one built for highway, ramping up and slowing down all the time is extremely expensive. > With a 70L tank at 250km/h you may well exhaust the tank in under an hour. No. That would be a fuel consumption of above 28L/100, that's twice the worst case I've seen from any user of the car. Hell, you can run long-haul trucks with a better efficiency than that: http://newsroom.scania.com/en-group/2011/05/27/scania-fuel-efficiency-duel-mission-accomplished/ http://newsroom.scania.com/en-group/2011/05/27/scania-fuel-e... > These are worst case numbers though They're closer to lunacy than worst-case numbers (and the autobahn is not free-speed throughout). Although we'd need an actual 535d with an actual driver to know for sure.

NickM 13y ago
Calling my numbers "lunacy" will, unfortunately, not change the laws of physics. If you drive twice as fast, air drag multiplies by four, and the energy to overcome that air drag has to come from somewhere. At highway speeds air drag is the only appreciable force at work. Tire drag makes a little bit of difference too, but that will be more than canceled out by the fact that ICE efficiency drops at higher RPMs. Your "long-haul truck" citation is irrelevant to this discussion because those numbers are for drastically slower speeds than the ones you're talking about.
- masklinn 13y ago
  > Calling my numbers "lunacy" will, unfortunately, not change the laws of physics. If you drive twice as fast, air drag multiplies by four, and the energy to overcome that air drag has to come from somewhere. At highway speeds air drag is the only appreciable force at work. As you will. Not that this changes a thing, the Model S's efficiency will also drop further, thus further lowering its range. The issue remains, even driving at inane speeds, the Tesla will have a significantly lower range than the 535d.