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altitude matters

coma24

Active Member
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May 20, 2024
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147
Location
Northern NJ
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Air GT, Honda S2K AP1
None of the discussions about range and miles per kwh appear to take into account the density of the air. Simply put, doing 70mph in calm winds near sea level is going to require more power to push the air out of the way than it would to drive at 70mph at 5300ft above sea level.

I say this because the efficiency numbers you're going to see from people driving in Denver/Boulder in the summer SHOULD be considerably higher than people driving at sea level under otherwise similar circumstances.

Pilots of aircraft deal with the implications of low vs high altitude flight on a daily basis...it's absolutely a thing. It's also why if you look at the airspeed indicator on an airplane flying at 35,000ft, it might read, say, "280", but that actual speed through the air is more like 430.

Another way to look at it is that, linear increases in rolling friction aside, you should be able to drive considerably faster at 5300ft (even more so on a warm day) with the same power output from the power plant as compared to the same setup at sea level.
 
TL;DR: sea level to 1600m (5250ft) would have a ~17% decrease in air density and thus drag force. Not sure how much of the vehicle energy loss is due to that drag force, but I assume it’s a large portion.

Disclaimer: I did this all on mobile, so I could have messed up a number. https://www.omnicalculator.com/physics/air-density Was used for density calculations and https://www.engineeringtoolbox.com/air-altitude-pressure-d_462.html was used for pressure numbers if someone wants to check my work.

I’d assume people don’t mention it too often because it’s not too crazy of a difference (which is subjective) and it can get muddied by humidity which kinda counterintuitively decreases air density, but not really by an appreciable amount.

The drag force equation is Fd = 1/2 × ρ × u² × A × Cd, so it’s linearly dependent on air density. Playing with some numbers, sea level pressure (100kPa) at 24°C (75°F) and 50% humidity has a density of ~1.17kg/m3. Air at 1600m (5250ft) has pressure around 83kPa, so the same temperature and humidity lead to a density of ~0.97kg/m3.

That’s a 17% decrease in density (not accounting for the 1600m air most likely being drier than the sea level air especially if we’re talking about Denver, but using 20% humidity for the 1600m air and 60% for the sea level only changes the gap to 15% roughly)
 
I should note that I, subjectively, don’t see it as a massive change since it really only starts coming into play at higher speeds, so a good portion of what I would think is the average drivers experience is at lower speeds where the air drag is just less impactful. If we’re talking strictly 70mph highway driving, yah, it’s going to matter more.
 
You're on the right track. In terms of concrete numbers, 75mph at 5300ft at 77 degs F, equates to 67mph at sea level. It would be similar to having a 8mph tailwind at sea level in terms of efficiency.
 
Unless there's an inversion, the temperature will fall at roughly 3° per 1,000 feet. So that will mitigate the decrease in air density.
Still, your point that altitude matters is a good one. One of the nice things about electric motors is that output does not vary with air density, unlike ICEs.
 
Unless there's an inversion, the temperature will fall at roughly 3° per 1,000 feet. So that will mitigate the decrease in air density.
Still, your point that altitude matters is a good one. One of the nice things about electric motors is that output does not vary with air density, unlike ICEs.
Ahh yes, the mythical standard atmosphere. And yet, I lost count of how many times I've been flying at 10,000ft with it still being 10-15C outside. Even with the 'expected' temp drop, there is still a range-altering difference in the resistance encountered by the car between 5300ft and sea level at freeway speeds, given that efficiency is known to start tanking at the 75mph mark. This means you can 'get away' with driving a tad faster at altitude and enjoy effectively higher miles/kw/h figures, not the least of which is because you're effectively going faster for a given amount of kw/h being drawn.
 
Given that I have long forgotten my physics I wonder if a dirty car makes for more resistance. Oh heck, I just prefer to drive and not worry about it
LISTEN....I NEED TO KEEP MYSELF BUSY WHILE I WAIT FOR THIS #$#$ CAR TO BE DELIVERED. I've never seen 2 weeks pass so slowly. lol
 
LISTEN....I NEED TO KEEP MYSELF BUSY WHILE I WAIT FOR THIS #$#$ CAR TO BE DELIVERED. I've never seen 2 weeks pass so slowly. lol
Patience and calculate as needed - it you will be rewarded with adventure and comfort. Its worth the wait. And most of us had the same pain at some point.
 
The curious thing about range discussion ... the wife has a mid-sized sedan that gets 30 mpg all day on pump gas.
Nobody cares. You can drive at the speed limits and piss everyone off, and get 36 or 40 mpg.
You can drive like French Connection and get 29 mpg. Nobody cares,

But an EV ? !

our cars are efficient: a little bit of a percent is a lot.

skill matters

{not only is the car excellent fun to drive ,,, just how efficient a driver car you be ? LOVE THIS!}
 
The reason for the efficiency obsession is because of the lack of fast charging options while enroute compared to gas.
 
The reason for the efficiency obsession is because of the lack of fast charging options while enroute compared to gas.
In reality the main limitation deals with the off the beaten path roads on long road trips. On the interstates it makes little difference. You can easily travel 6 hrs on a charge which is too long for a single stretch and there are charging stations located every 1-2 hrs. If you get in the middle of NV off the main roads - well good luck. Thats the main thing I miss about my Audi. But not enough to care about it.
 
In reality the main limitation deals with the off the beaten path roads on long road trips. On the interstates it makes little difference. You can easily travel 6 hrs on a charge which is too long for a single stretch and there are charging stations located every 1-2 hrs. If you get in the middle of NV off the main roads - well good luck. Thats the main thing I miss about my Audi. But not enough to care about it.

That's great feedback. I'm planning my first trip (for this weekend) in the hopes that the car will be here by then. There are zero fast charging options in the town we're staying or nearby area, so I'm spec'd out some options along the interstate before we enter the area. Thankfully, there are two stations (EA and EV Go) roughly 80 miles from where we'll be staying, so I can arrive in the area having charged to 100% minus those 80 miles, giving me plenty to get there and drive around locally during the weekend, then 80 miles back to that DCFC before heading home. There are plenty of Level 2's in the area in a pinch, but that would be a poor start to the road trip experience if we have to rely on them.

I'm being conservative because I have no confidence in predicting a specific kw/h for my driving style with the specific car (touring, 20") yet. I'll certainly be watching it as we head out, though to start building a data set for various driving conditions, speeds, weather and styles.
 
None of the discussions about range and miles per kwh appear to take into account the density of the air. Simply put, doing 70mph in calm winds near sea level is going to require more power to push the air out of the way than it would to drive at 70mph at 5300ft above sea level.

I say this because the efficiency numbers you're going to see from people driving in Denver/Boulder in the summer SHOULD be considerably higher than people driving at sea level under otherwise similar circumstances.

Pilots of aircraft deal with the implications of low vs high altitude flight on a daily basis...it's absolutely a thing. It's also why if you look at the airspeed indicator on an airplane flying at 35,000ft, it might read, say, "280", but that actual speed through the air is more like 430.

Another way to look at it is that, linear increases in rolling friction aside, you should be able to drive considerably faster at 5300ft (even more so on a warm day) with the same power output from the power plant as compared to the same setup at sea level.
There is about a 20 percent difference between density of air at sea level and at 5300 ft. Drag is linearly proportional to density ( as opposed to velocity where it is the square of the velocity) so theoretically you could see a 20 percent difference in efficiency I guess.
 
I suddenly want to learn how to fly. Thanks, all, for inspiring yet another money-sucking hobby found on the internet.

Finally, something I can help with! Drive to the airport in style, fly somewhere fun, rent a Lucid, enjoy the weekend, fly to home airport, then drive home in style.

Or, just set fire to the money.
 
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