TEMP vs DRAG Estimator (winter / summer)

[aerohead]

Hello all.
zjrog was lamenting mpg loss at another forum.At minus 19-degrees his Jeep is getting slammed at the pump.
I put this chart together as a quick reference guide to allow anyone to anticipate how drag might be influenced by changing air temps.
I've attempted to make it self-explanatory.Ambient temps are listed at the bottom.Locate your current temp,go straight up until you intersect the curve,and read your new air density directly to the left.(You can use linear interpolation to arrive at values that fall between values listed)
Divide this value of rho by the SAE standard 0.002377 to get your multiplier and use this with your vehicles published or estimated Drag-Factor( CdA ) to get your "virtual" CdA at the different temp.
I've included an example using my T-100 @ - 20-degrees F.As HWY MPG varies as 50% of a drag change,the 17.795% drag increase would reduce HWY MPG almost 9%.
Factor in thicker viscosities of all the lubes,and increased rolling resistance of colder tires,and it's a triple-whammy.
For summer operation,you will notice that on a 100-degree day,you can pick up 3.5% mpg without doing a thing.
I apologize for the US Standard units,and necessary conversions for SI units.

[FastPlastic]

Holy cow!! Cd for your truck go's from .44 to .518 at -20F? It's no wonder it seems like something is holding you back when it gets cold.

[aerohead]

Originally Posted by FastPlastic

Holy cow!! Cd for your truck go's from .44 to .518 at -20F? It's no wonder it seems like something is holding you back when it gets cold.

Yeah,living in Texas,I've seen single-digit weather and I've camped in a tent at -30 F,but I don't remember trying to drive in those temps.
I'd originally done the table to +20-F,then zjrog said he was driving at -19,and it blew my mind when I saw the numbers at those kind of temps.What a punch!

[Frank Lee]

Not only that, the tires get square and the bearings get mired in molasses. And if you like breathing, you then can't see out the windows. God's Country!

[Thymeclock]

Originally Posted by Frank Lee

Not only that, the tires get square and the bearings get mired in molasses. And if you like breathing, you then can't see out the windows. God's Country!

And this is assuming there is no wind blowing against you.

Leave the car home - hitch up the dog sled. Mush!

[Fubeca]

Thanks for posting this - this is really interesting. I'd never thought of the significance of the increased density of the air. I've always blamed it on winter fuel, heat loss and viscosity changes.

[Weather Spotter]

I had heard that the density changed, just had never seen any hard numbers. those look about right from what I have seen.

Thanks!

[Thymeclock]

Originally Posted by Fubeca

Thanks for posting this - this is really interesting. I'd never thought of the significance of the increased density of the air. I've always blamed it on winter fuel, heat loss and viscosity changes.

I've always blamed it on cold start operation. Any vehicle will consume more fuel before it reaches operating temperature.

But all of the above factors apply. Winter's a b**ch. It's hard on cars and it's hard on people.

[3-Wheeler]

Originally Posted by aerohead

...the 17.795% drag increase would reduce HWY MPG almost 9%...

Hi Aerohead,

Apparently the ISO paper must have some other variables besides the absolute temperature that affects your calculated 18% increase in drag.

If the air drag increase was strictly based on temperature (air density), then the variation between 70°F and -20°F would be:

= (460°R + 70°F) / (460°R + (-20°F))
= (530) / (440)
= 20.5%

Jim.

[aerohead]

Originally Posted by 3-Wheeler

Hi Aerohead,

Apparently the ISO paper must have some other variables besides the absolute temperature that affects your calculated 18% increase in drag.

If the air drag increase was strictly based on temperature (air density), then the variation between 70°F and -20°F would be:

= (460°R + 70°F) / (460°R + (-20°F))
= (530) / (440)
= 20.5%

Jim.

Jim,many of the densities were tabulated in my fluids text.For the others I found the specific weight at given temps( at standard pressure) and divided by the gravitational constant to get rho.Everything is calculated at Standard barometric pressure.
Some of the tabular data I ignored,as it dealt with elevations we'd never encounter in a vehicle.- 12.3 degrees is not encountered until 20,000 feet. - 20 degrees is up where commercial aircraft are operating.
The data I have must reflect temps/densities impacted by meteorological effects.
I'm hamstrung here at the copy center asI don't have all my notes with me.If I'm off base I apologize and will dig back in when I get home and rectify.
I'm also going to post a table on temp/elevation and baro. pressure/elevation.If you can shoot some holes in those let me know,as things are tough enough as it is let alone going down some dead-end path.
I appreciate the heads-up.