Can energy spent by mechanically sucking or blowing air be compensated by lower drag?

[Big time]

If a mechanical device (consuming energy) is used to blow or suck air to reduce the vehicle aerodynamic drag
Can this energy consumed by the mechanical device be overcompensated by a lesser amount of energy required to move the car at the same speed?
Thus net energy savings

Is there a thermodynamic law or something preventing this from happening?

Of course it all depends on vehicle speed.
At lower speeds when aerodynamic drag isn't important these mechanical air blowing/sucking devices should be turned off.

[freebeard]

I have some [untested] theories about this. Maybe you could start here:

"Topic: Interesting proof of exhaust thrust from a piston engine. (Read 3269 times)"
Interesting proof of exhaust thrust from a piston engine.

It's probably critical how you do it.

http://en.wikipedia.org/wiki/Coandă_effect

Images for coanda effect air nozzles

[Hersbird]

Chrysler's turbine car may be able to use this effect. A turbine coupled to a generator with electric drive would be pretty sweet. That's howsome trains and submarines work.

[user removed]

I always wanted to take a bicycle, put an aero shell on it and stick one of those model airplane ducted fans on it for propulsion. It would be neat to see what a 50cc 4 cycle engine could do, since it is unregulated in Va other than top speed.

regards
Mech

[sheepdog 44]

Past 50 mpg, the energy used to push a car through the air gets progressively smaller. Your saving fractionally small amounts of energy when your mpg starts to skyrocket. So the ratio of propulsive energy saved, to the energy spent actively blowing or sucking in air correcting aerodynamic drag becomes a losing prospect. For high mpg, passive aerodynamics is best.

Though if you want to go really fast it might work. Aerodynamic drag at high speeds becomes like a brick wall. It may be easier to redirect that horsepower to better aero than to waste it overcoming drag.

[freebeard]

Development of Pneumatic Aerodynamic Devices to Improve the Performance, Economics, and Safety of Heavy Vehicles

Abstract:

Under contract to the DOE Office of Heavy Vehicle Technologies, the Georgia Tech Research Institute (GTRI) is developing and evaluating pneumatic (blown) aerodynamic devices to improve the performance, economics, stability and safety of operation of Heavy Vehicles. The objective of this program is to apply the pneumatic aerodynamic aircraft technology previously developed and flight-tested by GTRI personnel to the design of an efficient blown tractor-trailer configuration. Recent experimental results obtained by GTRI using blowing have shown drag reductions of 35% on a streamlined automobile wind-tunnel model. Also measured were lift or down-load increases of 100-150% and the ability to control aerodynamic moments about all 3 axes without any moving control surfaces. Similar drag reductions yielded by blowing on bluff afterbody trailers in current US trucking fleet operations are anticipated to reduce yearly fuel consumption by more than 1.2 billion gallons, while even further reduction is possible using pneumatic lift to reduce tire rolling resistance. Conversely, increased drag and down force generated instantaneously by blowing can greatly increase braking characteristics and control in wet/icy weather due to effective ''weight'' increases on the tires. Safety is also enhanced by controlling side loads and moments caused on these Heavy Vehicles by winds, gusts and other vehicles passing. This may also help to eliminate the jack-knifing problem if caused by extreme wind side loads on the trailer. Lastly, reduction of the turbulent wake behind the trailer can reduce splash and spray patterns and rough air being experienced by following vehicles. To be presented by GTRI in this paper will be results developed during the early portion of this effort, including a preliminary systems study, CFD prediction of the blown flowfields, and design of the baseline conventional tractor-trailer model and the pneumatic wind-tunnel model.

Long story short you can increase or decrease drag, lift and yawing moments.

One of the reasons I like the air-cooled VW/Porshe is you get 1500 cubic feet per minute of engine cooling air at the lower rear for free.

[ChazInMT]

I say simply, it depends on your starting point. If you're starting with a brick, then maybe you can get beyond a break even point with an active device. If your starting point is a modern car, then it isn't real likely.

This brings up a point which illustrates a lack of ability by some people to contain their leap of logic tendancies. They think that if a brick gets 50% better mpg by putting a kamm on it, they should be able to get 50% better mileage putting a kamm on an already reasonably well designed car.

Ummmm....uh...hmm.

[aardvarcus]

Exactly. If you are forced by design to use a un-aerodynamic shape (like a large 53’ long box, 8.5’ wide) and you are forced by design that you cannot do standard aerodynamic treatment (no extra length for a permanent kamm-back on a max length semi trailer) then just maybe you could improve fuel economy by having active blowers enough to offset the power to run them. Weight becomes an issue though, how much would all the active blowers, high efficiency motors, controllers, etcetera weigh? Also where do these blowers go, inside the trailer volume? Tractor trailers operators need to be able to haul a full load, which is typically either limited by volume or by weight. It would be a hard sell if the trailer weighted 2-3k lbs more, which meant 2-3k lbs less cargo or was 2’-3’ shorter which meant 2’-3’ less cargo. At that point, assuming full loads you have to consider that it might take more trips to move the same amount of stuff, so are you really saving any fuel overall?

[aerohead]

The drag reduction due to blown or suctioned wing slots in regards to STOL aircraft has been well understood,however the energy tradeoff always shadows the process.
J.J.Cornish III,chief engineer for Lockheed,and member of the American Institute of Aeronautics and Astronautics believed that suction slots could be employed for road vehicle drag reduction.
Dozens and dozens of researchers have attempted and failed to pull it off.So far,the energy required for the pump exceeds the power saved from the drag reduction,for a net energy loss.Every time.

[kach22i]

See post 42
http://ecomodder.com/forum/showthrea...y-26356-5.html