Project: Geo Metro boat tail prototype - 15% MPG improvement @ 90 kph / 56 mph

[NeilBlanchard]

Hi Greg & welcome to EM,

We are all over this -- check the list of ecodriving techniques.

100+ Hypermiling / ecodriving tips & tactics for better mpg - EcoModder.com

This boattail is just a prototype -- I'm sure the final version will be made out of Coroplast...

[MetroMPG]

Originally Posted by Nerys

in my metro slowing down from say 45mph to 35mph WOULD NOT net me 15% improvement in mpg. 2 or 3% maybe.

Actually, the MPG change going from 45 to 35 mph in a Metro is pretty darn close to 15%! (Crusing in top gear, that is.)

[3-Wheeler]

Hi Guys,

So if Metro had a fuel mileage efficiency gain of 15% with the boat tail, then according to what Aerohead has mentioned...

Metro would also have had a CdA reduction of 15% * 2 = 30%

So if Metro were to do a coast down test, his distance should improve by roughly 30% since he now has that much less aero drag (ignoring Crr for a moment).

Again I can't vouch for what Aerohead is saying, but if true, then above is accurate.

Not to belabor the point, but Metro would certainly want to do his mileage test on an almost calm day to make deciphering the data easier.

Fortunately he measured his fuel consumption going both directions on the road for A..B..A, so any change in air drag (wind speed and resultant fuel consumption) would show up.

Jim.

[3-Wheeler]

Originally Posted by MetroMPG

...Actually, the MPG change going from 45 to 35 mph in a Metro is pretty darn close to 15%!...

Hi Metro,

Thanks for pointing that out!!

Actually this should hold true for any speed above, let's say 10 or 15 mph or so, when the air pressure across the vehicle is dominant over the effect of Crr or rolling drag.

As the air velocity increases, it swamps out the Crr rather quickly since Crr is close to linear and air drag goes up as the square of speed, so the 15% improvement should hold true for most driving speeds.

Jim.

[Nerys]

then why dont i see a 15% improvement at 45 versus 55? i tried it for a tank and it resulted in less than 1mpg difference. that is not even 2%

maybe i will have to try it again for a few tanks and see what happens

[Daox]

Originally Posted by 3-Wheeler

So if Metro had a fuel mileage efficiency gain of 15% with the boat tail, then according to what Aerohead has mentioned...

Metro would also have had a CdA reduction of 15% * 2 = 30%

So if Metro were to do a coast down test, his distance should improve by roughly 30% since he now has that much less aero drag (ignoring Crr for a moment).

Again I can't vouch for what Aerohead is saying, but if true, then above is accurate.

Not to belabor the point, but Metro would certainly want to do his mileage test on an almost calm day to make deciphering the data easier.

Fortunately he measured his fuel consumption going both directions on the road for A..B..A, so any change in air drag (wind speed and resultant fuel consumption) would show up.

Jim.

Darin is very careful about selecting days to test when wind is as small of a variable as possible. If its windy, he cancels the test and waits for another day.

What Aerohead said is true, but much more of a general rule. I assume you are talking about the rule that says something to the effect of 'A 20% reduction in Cd gives a 10% improvement in FE'. That is not going to the case at 55 mph and at 20 mph. Its just a general rule.

[PaleMelanesian]

My daily commute records show about a 3% drop in mpg with a 10 mph increase in headwind. Likewise a 3% increase with a 10mph tailwind. That's all at approximately the same speed. 3 years of data, and trends start to show up.

[3-Wheeler]

Originally Posted by Daox

...What Aerohead said is true, but much more of a general rule. I assume you are talking about the rule that says something to the effect of 'A 20% reduction in Cd gives a 10% improvement in FE'. That is not going to the case at 55 mph and at 20 mph. Its just a general rule...

Hi Daox,

Yes, you are right. That's exactly what I was trying to say.

As I do coast down tests and discover just how much the air drag and rolling drag contribute to the overall vehicle drag, maybe a glimmer of light is appearing for me, on what Aerohead is alluding to.

At lower speeds, since the air drag is almost the same proportion of overall drag as Crr is, then each contributes about 50% to the overall.

As the speed increases, let's say towards 100 mph, then 90% of the overall vehicle drag is mostly air drag, and Crr is only 10%.

So if Metro lived in Germany and could drive the Autobahn at 100 mph, his decrease in CdA because of the boat tail would give him a resounding 30% decrease in drag and:

90%/100% * 30% CdA decrease = 27% decrease in fuel consumption at that speed

However, at lower speeds, let's say 20 mph where the CdA contribution and Crr contribution are each 50%, then his fuel consumption would be:

50%/100% * 30% CdA decrease = 15% decrease in fuel consumption.

If this is not the case, then there are other factors involves that dictate overall fuel consumption on the vehicle such as tuning of the engine, gearing of the vehicle and so on.

If Aerohead has some info on this, it would be nice to elaborate.

Jim.

[3-Wheeler]

Originally Posted by Nerys

then why dont i see a 15% improvement at 45 versus 55? i tried it for a tank and it resulted in less than 1mpg difference. that is not even 2%

maybe i will have to try it again for a few tanks and see what happens

Hi Nerys,

You're on to something here.

My gas mileage is not 15% better at 45 than 55 either. And that is because at these lower speeds, the Crr contribution to the overall drag of the vehicle is probably close to let's say 30% or so.

So if you decrease your speed from 55 to 45 the decrease in aero resistance should be:

(45)^2 / (55)^2 = 0.67 of 70%(air drag) = 47%(air drag)

So now your air drag went down from a 70% contribution to 47%, and the resultant Crr went from 30% contribution to 53%. That's not to say that the Crr increased, but rather it's contribution to the overall drag is now a higher portion since the aero drag portion dropped such a large amount.

These numbers are obviously not exact, but try to make a point.

And the point is that are our overall speed decreases, and we add a boat tail or whatever, that the Crr component will start to show it's contribution and the overall reduction in fuel consumption will become less due to the boat tail.

To go to an extreme, if we could make a perfect body shape like an airplane wing, then the large contributor to overall drag at lower speeds would simply be the rolling resistance of whatever we are driving. Lighter would be better in this case.

I think this is what Aerohead has been mentioning in different threads.

EDIT:
Nerys, if I had to guess, your low decrease in fuel consumption has more to do with how your particular engine is tuned, engine size, the gearing, the rpm's your spinning and the actual load on the engine.

I would say that engine is size is such that it is basically loafing at speeds of 45 and/or 55 mph and that is why there is a such a small difference in fuel consumption.

The air drag really is dropping quite a bit at the lower speeds, but the design of the vehicle is limiting the gains that you will see by your careful driving.

This is all just a hunch on my part, but does seem very likely from what you are describing.

EDIT2:
I see that you are sometimes driving a Metro, so your mileage should increase more than 2% under the conditions mentioned.

The larger engine sizes limit the fuel mileage gain, since a larger engine has larger cylinders, more internal surface area to heat loss, and thus lower overall gas mileage. Larger vehicles also have a higher Crr so the aero gains will be a smaller overall contribution compared to engine efficiency and Crr.

Jim.

[Nerys]

ahh so Cr is coefficient of rolling resistance or something like that?