Air Density (pressure) and Range

Given the same temperature,flat terrain, and constant speed. What difference would air density (pressure) make on the range of an electric vehicle?

Are there any charts that show this impact at constant temperature and speed. I know there are range charts for specific cities in the US and elsewhere, but they take into account the monthly temperatures for those areas, which I think would have a greater impact on range(at least at legal speeds). Also, are the EPA range calculations based on a sea level air pressure?

I doubt it matters much. If it does matter, then humidity should be at least as much a factor.

To use anecdotal evidence we collected over the years from Prius drivers, altitude makes a pretty big difference if you drive fast. It means I get 45mpg to 48mpg going 80+mph on the highway at 7000ft to 10000ft above sea level whereas those on the coasts would see 40mpg to 43mpg at those speeds.

At city speeds, it matters less because the air is not the dominant factor

The range chart has all these factors for range.

Whether a plane, a bullet, or an electric car, they all move through air and are affected by its density. I've spent a lot of time discussing it, too.

Put this in a Google search:

site:mynissanleaf.com "density altitude" tonywilliams

The main reason that high altitude affects fuel economy so much is that there is less O2 available for combustion. This is well known, as fuel delivery systems have to be adjusted for it.

Fun fact for the day: higher humidity results in lower air density. We've known this for hundreds of years but still seems counter intuitive to most folks.

LeftieBiker said:

The main reason that high altitude affects fuel economy so much is that there is less O2 available for combustion. This is well known, as fuel delivery systems have to be adjusted for it.

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This false. Most modern cars have fuel injection systems that adjust automatically for air density and mileage is little affected by that factor. Older cars with carburetors had to be adjusted for altitude to have the proper fuel/air mix.

The reason those of us who live at high altitude get much better gas mileage (and EV mileage efficiency) than those at sea level is because lower air density = lower drag, i.e. fewer air molecules to push out of the way. It's that simple. Air density is also something that pilots have to learn to account for since it greatly affects the length of a takeoff run (in that case, lower air density decreases the lift at a given ground speed so the takeoff run is longer).

I've often watched the gas mileage in my ICE cars drop sharply when I venture down to sea level.

1.225 kg/m^3 at sea level.
~0.9 kg/m^3 in Alma, Colorado (~3,100 meters)

Drag is directly proportional to air density, so a ~25% reduction in air density corresponds to ~25% reduction in drag force, all else being equal.
=Smidge=

Orchard said:

Given the same temperature,flat terrain, and constant speed. What difference would air density (pressure) make on the range of an electric vehicle?

Are there any charts that show this impact at constant temperature and speed. I know there are range charts for specific cities in the US and elsewhere, but they take into account the monthly temperatures for those areas, which I think would have a greater impact on range(at least at legal speeds). Also, are the EPA range calculations based on a sea level air pressure?

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This isn't quite what you want but it gives a general idea of how air density decreases with altitude at a given temperature:
Air density, temperature, and range.

I believe that EPA range numbers are done at (or corrected for) sea level, but I've never looked into it.

dgpcolorado said:

Orchard said:

Given the same temperature,flat terrain, and constant speed. What difference would air density (pressure) make on the range of an electric vehicle?

Are there any charts that show this impact at constant temperature and speed. I know there are range charts for specific cities in the US and elsewhere, but they take into account the monthly temperatures for those areas, which I think would have a greater impact on range(at least at legal speeds). Also, are the EPA range calculations based on a sea level air pressure?

Click to expand...

This isn't quite what you want but it gives a general idea of how air density decreases with altitude at a given temperature:
Air density, temperature, and range.

I believe that EPA range numbers are done at (or corrected for) sea level, but I've never looked into it.

Click to expand...

I'd be very surprised if all EPA mileage testing wasn't corrected to ISA (International Standard Atmosphere) conditions at Sea Level, ie. 1013.2 mb/29.92" Hg, 15C/59 deg. F etc. See http://en.wikipedia.org/wiki/International_Standard_Atmosphere" onclick="window.open(this.href);return false;

This false. Most modern cars have fuel injection systems that adjust automatically for air density and mileage is little affected by that factor. Older cars with carburetors had to be adjusted for altitude to have the proper fuel/air mix.

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Geez, I didn't mean that fuel injection systems have to be *manually* adjusted. I meant that all fuel delivery systems have to have a way to compensate for the lower O2 in the air. If you really have 25% less drag, then that's great, but there is still less O2 in your air. This too isn't a controversial fact. I also didn't write that higher humidity means more drag. It means *different* drag.

Thanks guys for the links. Very interesting topic. What got me thinking about this is when people talk about their "range reduction with elevation". Sometimes it is confusing to me at first glance, when what they really saying (I think) is: the range was reduced whilst driving their EV from a lower elevation to a higher elevation. This does not mean at the higher elevations the EV's theoretical range from there on is less, when in fact it should be slightly higher.

So basically someone living in Denver will have a slightly (about 5%) higher range in their LEAF than someone driving a LEAF on the coast given the same temperature, humidity, and terrain.

Orchard said:

Thanks guys for the links. Very interesting topic. What got me thinking about this is when people talk about their "range reduction with elevation". Sometimes it is confusing to me at first glance, when what they really saying (I think) is: the range was reduced whilst driving their EV from a lower elevation to a higher elevation. This does not mean at the higher elevations the EV's theoretical range from there on is less, when in fact it should be slightly higher.

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Yes, the main elevation issue usually discussed is driving up and down hills. One uses more energy to get up a hill than is gained going back down a hill, even with a bit of an assist from regen braking. That reduces range versus driving the same speed and distance on a level road. And it matters a lot whether one blasts up a steep hill at high speed (high power usage) versus climbing a hill slowly, which is more efficient [empirical observation].

This is a reason why anecdotal reports of "range" don't have much meaning for those of us who drive hills all the time (and there are a number of us here at MNL). It also makes "Guess-o-meter" numbers completely ludicrous.

So basically someone living in Denver will have a slightly (about 5%) higher range in their LEAF than someone driving a LEAF on the coast given the same temperature, humidity, and terrain.

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The altitude improvement to range is generally quite a bit higher than 5% in Denver, but it depends on speed. At freeway speeds the range increase is considerable. At 25 mph it would be negligible. This is because drag increases at (approximately) the square of velocity. At freeway speeds drag is the dominant force slowing the car down, requiring a lot of energy from the battery to hold constant velocity. Below about 35 mph other forces, such as the rolling resistance of the tires and friction in gears and bearings, become more significant.

So, the range improvement at higher elevations will vary with speed, but it can be considerable. And it doesn't seem to be widely appreciated by many who live at low elevations. (It is the secret of those of us who live in Colorado, Utah, and high mountain parts of California.)

If you are interested in the effects of drag, you can play with it yourself if you can find a stretch of level (not just flat: level) road with a high speed limit. You can measure how much power it takes to hold a constant velocity using the console energy meter, at various speeds from 25 to 65 (or 75, if that's possible). You should get a curve, as opposed to a straight line.

Air temperature and humidity, barometric pressure, and elevation all matter, to any vehicle. Air is ~6% denser at 30F than it is at 60F. And considering a typical car pushes several tons of air aside for each mile traveled, the mass of that air matters a lot.

NeilBlanchard said:

Air temperature and humidity and elevation all matter, to any vehicle. Air is ~6% denser at 30F than it is at 60F. And considering a typical car pushes several tons of air aside for each mile traveled, the mass of that air matters a lot.

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And this is one of the factors that reduces range in winter. A lot of attention is placed on the reduced capacity of a cold battery (in the LEAF, with no TMS), and the use of the cabin heater. But more dense air, as well as cold tires and gear lube, are also significant factors in the winter reduction in range, even on dry roads with no wind.

Smidge204 said:

Drag is directly proportional to air density, so a ~25% reduction in air density corresponds to ~25% reduction in drag force, all else being equal.
=Smidge=

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The aero-drag element is. There are other lossy drag factors. Probably a half or so is aero-drag at 40mph, so reduce that by 25% and you get a, say, 12.5% improvement overall.

Go faster and the aerodrag becomes more significant. Say 2/3rds is aero-drag at 60mph, so that 25% density drop becomes 16% improvement.

Not sure if EPA do correct for ISA. I would have expected the test track has to conform within certain bounded requirements, and if it meets those requirements then they do the test.