h a l f b a k e r yRecalculations place it at 0.4999.
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I was mucking about with a bathroom fan I was supposed to be installing the other day, and noticed something that I found amusing. If I blocked the inlet or the exit while the thing was running, the little "squirrel-cage" centrifugal fan would actually speed up frantically. Then, when I allowed the air
to flow through it again, it would slow down with a happy sigh. In trying to justify my time-wasting cruelty-to-electrons, I came up with a way to use this effect to assist automobile brakes.
To start (or should I say "to stop"?) stick a centrifugal fan or two in the car, driven off the wheels or the drive shaft. Use a belt or gears, as you please.
Arrange the ducting to suck air from under the car, and to blow air upward. That part is similar to _Brake_assist_ by [Twizz] (see link).
The key here is that the fans are spinning at all times, but the airflow is blocked off by a flapper similar to a throttle in a carburetor or to a crazy man's hand. The flapper is linked to the brake system, and opens when brakes are applied. When the brakes aren't engaged, the fan spins freely in its vortex.
On braking, the flap opens, air moves, and the fan instantly begins working, with no spin-up time and no clutches (the weaknesses in [Twizz]'s idea). The wheel or drive shaft the fan is driven by is slowed as the fan begins to perform work, and the car is pulled and pushed downward.
This system delivers a braking assist and an aerodynamic assist simultaneously. But, as an adjustable aerodynamic method, it won't be allowed on Formula One cars.
It may not be allowed on any cars for several reasons: I don't know how much energy is consumed by a blocked-off fan, but it is something, at least at this state of the art. Next, a centrifugal fan that is pumping air doesn't consume all that much horsepower--one big enough to slow the car may be bigger than the car.
Plus, I wouldn't want to be in a car with a giant fan spinning away, no matter how good the housing. Squirrels might like it, though.
Brake assist by [Twizz]
Brake_20assist
Use braking energy to provide downforce [baconbrain, Sep 01 2008, last modified Feb 20 2011]
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Any suggestions for a better title will be appreciated. |
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How about "Blocked Fanny Braker"? |
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Duly appreciated. And "eww". |
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Title: How about "modulated fanniness" |
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Thanks for the reference. It is certainly true as you observe that a fan will consume far less energy when flow is prevented.
The reason I went for clutches is that the gearing required to run the fans fast enough would itself consume a significant amount of energy even without the fans. Each wheel already has a clutch (referred to as a brake because one side is fixed). I had calculated that for a 1500kg vehicle, the fans would be 100mm diameter with a rotating mass of 250g each. Using currnetly available materials, these could be accelerated at 1000m/s/s to create significant airflow within 30milliseconds of applying the brakes. The greatest time delay would likely come from the air space (ducting) between the fan and the ground. This would require careful design.
Bun for the positive attitude and application of observed behaviour! |
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maybe it could cool the disks..... |
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[Twizz], thanks for the input. You did much more math than I am capable of. Glad to see that you are still around. |
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A clutch with one side fixed (aka a brake) is much simpler mechanically than one which must transmit rotation. Still, the idea of using a clutch to connect a wheel to a turbine is interesting. The turbine would have some inherent anti-lock braking behavior (since it would only provide friction while it was actually turning), and while a turbine wouldn't be good as an "only" brake, the friction in a normal turbine would impart heat mainly to the air flowing through it, thus cooling itself. Directing the airflow over a conventional brake could cool that as well. |
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This is operating on the same principle as the fan in a rowing machine - the higher you set the resistance, the more you open up the inlet on the side - hence more power absorbed. Some trucks also make use of this idea by engaging the cooling fan when applying the engine brake. |
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On the downside, there is going to be significant parasitic draw from this system, even with the intake throttled. The rotating mass downstream of the gearbox is also increased, diluting acceleration. This means you'll see worse fuel economy and acceleration. |
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Unfortunately, you'll also find that this technique can onnly be used as a retarder, not as a brake - the power absorbed by a fan varies with the cube of speed. This means that if you can absorb 100hp at 60mph (probably 5-10hp parasitic when not activated), you can only absorb 100 * 0.5 * 0.5 * 0.5 = 12.5hp at 30mph. |
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