Minivan Kardboard Kammback boosts MPG +3.7% (6.6%, counting roof rack delete)

[MetroMPG]

While Trebuchet03 shows us how to do things by the book with CFD (computational fluid dynamics) to optimize a Kammback shape on his Jetta sedan, I employed the other kind of CFD on my brother's minivan:

Cardboard Fabrication with Ducttape

This is a variation of Craig Vetter's CAD process: Cardboard Aided Design

This was an A-B-A on road test under perhaps the most ideal conditions I've ever encountered on my favourite test road. No wind, no traffic, perfectly functioning cruise control , tight distribution in the results. I'll post the full details later, but here's the skinny:

Vehicle is a 2005 Pontiac Montana, long wheelbase version, 3.4L V6, automatic...





(More photos below in post #20)

This roughly slapped together Kammback, with fairly conservative angles (10 degree plan & roof taper), improved fuel economy by 3.7% in AAAA BB AA results.

If you add in the initial roof rack removal I did to get to the clean roof, the total fuel economy improvement was +6.6%. That's significant, and hopefully may motivate some of you van/trucklet drivers.

Absolutely nothing new here of course. Much attention has gone to studying and reducing the size of the wake behind trucks, since their cargo-carrying capability dictates a boxy profile with no taper at all. Researchers have known for decades that:

Aerodynamic drag can be significantly reduced with trailer add-ons that reduce the wake and increase the base pressure. - Source: SAE 2000-01-2209

Edit: added the following, Nov 23...

Details of the test...

- Test speed: 88.5 km/h / 55 mph
- Cruise control set once, cancelled with the brake pedal between runs; "resumed" for subsequent runs

- Bidirectional averaged runs on a straight and nearly level test route; runs were abandoned & re-run where I was overtaken or caught up to another vehicle (aero interference)

- Wind conditions were perfect: none!




A runs, average of all: 33.72 mpg (US)
A runs, standard deviation (of average of bi-dir pairs): 0.35 mpg

B runs, average: 34.975 mpg (US)
B runs, standard deviation (of average of bi-dir pairs): 0.18 mpg

Improvement of B over A: 1.255 mpg / 3.7%

Roof racks on, Kammback off (one bi-dir run): 32.8 mpg (US)
Improvement of B over roof racks: 2.175 mpg (US) / 6.6%

[trebuchet03]

Originally Posted by MetroMPG

Cardboard Fabrication with Ducttape

This is a variation of Craig Vetter's CAD process: Cardboard Aided Design

I approve

Is the rear hatch still functional?

[93Cobra#2771]

I don't see how it could raise ALL the way, but looks like it might go most of the way.

Fun project!

[SVOboy]

Darin's amazing!

[Christ]

Originally Posted by 93Cobra#2771

I don't see how it could raise ALL the way, but looks like it might go most of the way.

Fun project!

Unless, of course, the kammback were attached to the hatch, and sealed to the body with weatherstripping...

Legal implications for the taillights? Most states have a visibility provision in the vehicle code... outlining exactly how much (area) of your taillight must be visible from a given angle... mostly rearward, but I believe they might have something to say about your blocking light from the side/rear-angle view.

All in all, great work though.. and far less intelligence necessary to pull this off, making it a great mod in regard to effect vs. work/thought.

We all like the thoughtless mods... like we don't have enough stress in our lives to add physical permutations of fluid dynamics and airflow based on a given scale to our schedules... roffle.

All in all, a big .. can't wait for updates.

[groar]

Incredible result
The minivan is so long compare to the Kammback that I would have bet the difference was going to be non-measurable.

It could be interesting to correlate the gain with the rear area reduction.

It could also be interesting to see efficiency difference when the lower part of the Kammback is cut-out at the light level.

(I will have to search again for an after-market cruise control available in France...)

Denis.

[dichotomous]

6% gas milage increase would be less 2mpg in that thing, thats hardly a deal to have to drive around with that on the back. I'm sorry but I dont feel 2mpg is enough of a savings to deal with that thing hanging off the back. you'd get better than 2mpg savings just driving ecostyle

[MetroMPG]

At $3 gas, 6750 highway miles/year*, 23 mpg = $58 savings. Less as gas drops, more as it increases obviously.

Another note: the speed of testing was just 89 km/h / 55 mph. Actual savings of course increase with speed.

(* that's 45% of EPA's average annual distance of 15k miles total city+hwy)

[groar]

Originally Posted by dichotomous

6% gas milage increase would be less 2mpg in that thing, thats hardly a deal to have to drive around with that on the back. I'm sorry but I dont feel 2mpg is enough of a savings to deal with that thing hanging off the back. you'd get better than 2mpg savings just driving ecostyle

Which ecomod is better than ecodriving ?

This 6% is granted whoever drives, ecodriver or not.
An ecodriver will have a 6% effort-less bonus, not negligible

15000 miles at 35mpg is 428 gallons
15000 miles at 33mpg is 455 gallons
Result : 455 - 428 = 27 gallons free and 250 kg CO2 not sent to the atmosphere (see my sig). My son (and I if I'm always alive) will thank these efforts in 50 years.

And this cardboard ecomod is only a test. Done in a more weather proof way, as metrompg and others know how to do it, it could be stealth. My white coroplast front grill block on my light grey car doesn't attire a lot of looks.

Denis.

[MetroMPG]

Originally Posted by trebuchet03

Is the rear hatch still functional?

It was effectively taped shut for the test, but I don't see why it couldn't be designed to open all the way if the lower sides were attached differently.

Originally Posted by Christ

Legal implications for the taillights?

Oh, yeah, that goes without saying.

This was just a quick 'n' dirty test. A permanent version (my brother has no interest in one) would need proper lighting - lights brought out to the end of the addition is the obvious solution.