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EVEZY code d55d6 *** Try my car cost calculator
'19 i3 120Ah / '20 Kona 64kWh / '21 e208
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Discussion Starter · #1 ·
Some things we can note from the graph is:
  • A nice tailwind of 10 m/s (36 km/h, 22 mph) decreases consumption of a typical Tesla Model 3 by 6% at highway speeds
  • A headwind of 10 m/s increases the consumption a Model 3 by 19%
  • Direct cross winds of 10 m/s can increase the consumption by 8%
    3.thumb.png.d1ab72b90b75462d0f11de4cccec51f1.png


 

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Acc to the link, the highway speeds are from 62 to 75 mph. Midpoint is 68.5 so I'll use that as the "actual" speed.

22 mph tail wind reduces speed-into-wind to 68.5 - 22 = 46.5 mph. So I expect drag to reduce to (46.5/68.5)**2 = 0.46 of what it was, so roughly halved. Power saved was about 1.5 kW.

22 mph headwind increases speed-into-wind to 90.5 mph so drag should increase to 1.745 of what it was. Extra power needed was about 3 kW.

Acc to ev-database.uk Tesla 3 LR 73.5 kWh does 255 miles highway mild weather, so at 68.5 mph 255 miles takes 3.723 hrs, so to drain 73.5 kWh in that time the consumption must be about 19.75 kW continuous.

If the only load on the car is due to wind resistance, I'd expect the tailwind to reduce the load to half what it was, so I'd hope for a saving of 10kW. We see actual 1.5 kW saving.

If the only load on the car is due to wind resistance, I'd expect the headwind to increase the load to 1.75 times what it was, so I'd expect an increase of 0.75 from existing load, so extra 15 kW needed. We see actual 3 kW extra load.

These extra loads are about a fifth of what the aerodynamic-drag-only theory predicts. So I think this is saying that the fixed loads due to tyre rolling resistance, viscous drag in gears/bearings, inefficiency in converting the 73.5 kWh to mechanical power at 68 mph, heating loads, air-conditioning loads, anything else you can think of that's consuming power at a steady rate, these contribute about 4/5 of the car's loads, and the wind-drag is contributing around 1/5.

Does this sound about right? Or did I get my estimates badly wrong somewhere? Would be nice to have an actual figure of instantaneous Model 3 power consumption at 68 mph in windless "average" day on the flat!
 

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EVEZY code d55d6 *** Try my car cost calculator
'19 i3 120Ah / '20 Kona 64kWh / '21 e208
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Discussion Starter · #5 ·
Could be worth posting that as a comment beneath the ABRP blog post - Jason from ABRP May be able to offer some further insight.

 

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I was under the impression that whilst the air resistance increased as a square of the speed, the power needed to overcome that resistance increased as a cube of the speed. Or is my memory of school physics of 50 years ago lacking?
 

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EVEZY code d55d6 *** Try my car cost calculator
'19 i3 120Ah / '20 Kona 64kWh / '21 e208
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Discussion Starter · #7 ·
Wind is often accompanied by rain. I wonder how/if they were able to separate the effect of wind from the effect of rain.
 

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I was under the impression that whilst the air resistance increased as a square of the speed, the power needed to overcome that resistance increased as a cube of the speed. Or is my memory of school physics of 50 years ago lacking?

This is correct. The wind resistance, or aerodynamic drag, is given by D = 0.5 . ρ . Cd . A . v², where D is the drag (in N), ρ is the density of air (in kg/m³), Cd is the drag coefficient, A is the projected frontal area (in m²) and v is the velocity (in m/s). The power needed to overcome drag is given by P = 0.5 . ρ . Cd . A . v³, where P is the power (in W).

The reason for this is that the power needed to overcome a force, F, is given by P = F . v, and in this case F = D.
 

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Wind is often accompanied by rain. I wonder how/if they were able to separate the effect of wind from the effect of rain.

That's an interesting one, as humid air is less dense than dry air, so drag actually decreases as humidity increases. This seems counter intuitive at first, but can be quickly proven to be true by looking at data for air density, ρ in the equations above, for different conditions. Anyone that's flown an aeroplane will be aware of the impact of both temperature and humidity on performance, as like drag, lift also varies with air density.
 

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I was under the impression that whilst the air resistance increased as a square of the speed, the power needed to overcome that resistance increased as a cube of the speed. Or is my memory of school physics of 50 years ago lacking?
The power goes as actual road speed (times drag), the drag as apparent wind speed (^2).
 

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EVEZY code d55d6 *** Try my car cost calculator
'19 i3 120Ah / '20 Kona 64kWh / '21 e208
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Discussion Starter · #11 ·
But…rolling resistance is obviously significantly increased in wet conditions, so I would hope ABRP have been able to ignore any data points collected by cars when the app was reporting rain in a given location.

Otherwise, the apparent impact of wind on consumption could be skewed by data reported by cars being driven in wet conditions.

Maybe they’ve taken the view that some cars would have been driving into a headwind in the rain, and some with a tailwind in the rain, and so they roughly cancel each other out.
 

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I was under the impression that whilst the air resistance increased as a square of the speed, the power needed to overcome that resistance increased as a cube of the speed. Or is my memory of school physics of 50 years ago lacking?
The power needed goes up as the cube, but as your speed is going up, the time you need to expend that power for goes down with the speed as you've done the trip more quickly. Actual energy expended = work done = force x distance; the distance is the same for the trip, but the force during that trip went up by speed squared acc to drag coeff theory.
 

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The thing is that if the truck brakes and you are ridiculously close, the impact speed differential is less, still not something I would try though. In theory if you were touching there would be no impact damage as the braking truck would also slow the car. Just abrasion damage.

It would make far more sense if manufacturers improved the aero shape, when you think a lot of EV's are slab fronted, it makes no sense for economy just a style thing. Take the Range Rover, or E-Tron, presents themselves as a brick to the airflow!
 
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