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Old 11-15-2009, 02:34 AM   #11 (permalink)
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On a race car you'd only need it on the straightaways at higher speeds. Under braking, the extra drag would be useful. The car would also use a sort of regenerative braking...simply pump up the air tank with a wheel driven air pump.

On a car driven at highway speeds for long periods of time, you'd have to see if it was better to burn fuel to make X amount of horsepower to go a target speed or better to reduce the amount of horsepower required by reducing drag and then burn fuel to (somehow) make your compressed air contraption work.

Still...I don't think it would be that difficult to duct the inside of the rear wheel wells to the back of the trunk. That should be a straightforward HIGH pressure to LOW pressure job.

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Old 11-15-2009, 10:45 AM   #12 (permalink)
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Renault are working on something similar: The Synthetic Jet.
Green Car Congress: Renault Altica: 44MPG Diesel Concept with Active Airflow Management

It's not strictly compressed air but it is doing the same job. It's also been talked about here without much resolution on the matter of compressor efficiency.
Didn't the P-51 mustang have this technology? It enabled it to go further on a tank of gas. I think there would be a smaller percentage improvement on a car using this but it would be an improvement all the same.


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Old 11-15-2009, 12:56 PM   #13 (permalink)
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Perhaps this is already answered somewhere, and I missed it, but why go to the trouble of compressing air when you already have a large volume of hot gas under pressure, flowing out the exhaust? Just duct it to the appropriate places, making sure that it doesn't get into the cabin...
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Old 11-15-2009, 01:05 PM   #14 (permalink)
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moving engine or at least the radiator the the back, could accomplish two tings at once... it would delete a grill at the front allowing for better aero and allow for better controlled cooling if air would be ducted (in part) trought the radiator that would exit in the wake of the car
aer·o·dy·nam·ics: the science of passing gass

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Old 11-15-2009, 05:20 PM   #15 (permalink)
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Thanks for the brainstorming guys.

It's too bad you can't just connect a Shop-Vac to your cars AC outlet.

( It wouldn't produce nearly the amount of power needed though !!! ( right ? )
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Old 11-15-2009, 05:22 PM   #16 (permalink)
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Old 11-15-2009, 05:31 PM   #17 (permalink)
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I have no clue.
I'm wondering the same thing.
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Old 11-15-2009, 06:01 PM   #18 (permalink)
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Rising Fuel Prices Renew Interest in Fuel-Saving Technologies for Heavy Trucks | Georgia Tech Research Institute

The tests showed that the techniques could provide drag coefficient reductions of up to 31 percent, which translates to a fuel efficiency increase of 11 to 12 percent. When the energy required by the air compressor installed on the truck to provide the compressed air for these prototype tests was subtracted from those savings, the tests showed that the low-drag techniques could produce an overall fuel efficiency increase of 8 to 9 percent.
This may have been posted already.

It also looks like it is sucking air in from the side.

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Old 11-15-2009, 07:59 PM   #19 (permalink)
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If half of the HP required to drive 55 is to push the car through the air, it stands to reason that there is a lot of HP available to redirect to the problem of form drag. Specifically, reduction in the form of pressure redistribution.

For example, If 50% of your drag at 55 is pressure drag, then a car that takes 20hp to go 55 would have 5hp to burn to break even. Using any less would be an improvement. 5hp to run a high flow turbine is a lot. I'll bet in the above story, they are using about a centrifugal compressor, not a piston compressor. If you run it off the exhaust, then you even use some waste heat and have even more hp to play with.

I was toying with the idea of a hole thru my van a while back and this makes that idea seem less crazy that it did at the time. Pressurize the air in that duct and you have something.
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Old 11-15-2009, 08:18 PM   #20 (permalink)
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From a DIY standpoint, the biggest stumbling block of this approach is figuring out how best to apply it to a particular vehicle.

With boat tails, Kammbacks, wheel skirts etc., we can fairly easily see whether the air likes them or not by tuft testing.

But there's really no easy way for the backyard aerodynamicist to visualize the best position/angle/flow rate of a compressed air device without a wind tunnel. We can't tape tufts onto air behind the vehicle!

You could guess, and then do coastdown testing, but it's very time consuming and getting good data in on-road testing is very hard.

Until one of us comes up with a way to smoke test our cars...

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