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Old 11-19-2008, 09:10 PM   This thread is in the EcoModder Project Library | #1 (permalink)
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Minivan Kardboard Kammback boosts MPG +3.7% (6.6%, counting roof rack delete)

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%

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