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Author Topic: Is it possible to create a pulse drive in cars?  (Read 1970 times)

Offline thedoc

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Is it possible to create a pulse drive in cars?
« on: 06/08/2015 10:50:02 »
John Noles asked the Naked Scientists:
   
For years I have wondered if it was possible to create a pulse drive for cars? Every time I hear the people radio's in their cars and all you hear is a boom boom from the bass. I keep saying they are running a pulse drive.

What do you think?
Edit: "pules" (to cry thinly) → "pulse" (throbbing)
« Last Edit: 06/08/2015 11:12:01 by evan_au »


 

Offline evan_au

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Re: Is it possible to create a pulse drive in cars?
« Reply #1 on: 06/08/2015 11:47:38 »
Human hearing is remarkably sensitive - a sound power of 1mW is clearly audible in quiet conditions.

To totally drown out all outside sounds, some extreme car audio systems may put out hundreds of Watts. The high frequencies are produced by small loudspeakers within the car, and these sounds are contained within the car. However, low-frequency loudspeakers must change the amount of air within the body of the car, and they do this by pumping it into external spaces - usually the car boot, which is often vented to the outside air.

When you hear a car coming down the street with the sound turned up loud, you mostly hear the sound of the bass "woofer", giving that pulsing/throbbing sound.

There are several problems with using sound to propel a vehicle:
  • You need a continuous thrust for periods of 5-10s or more to accelerate to traffic speeds in city traffic. However, the lowest notes from a woofer have a frequency of about 20Hz, ie they only last for around 0.05s, far too short to accelerate a car in traffic.
  • The sound pressure is alternating, so after accelerating the car for 0.05s, the sound pressure reverses, decelerating the car for 0.05s. The net effect is that the car goes nowhere.
  • The sound pressure level is far too low to accelerate the car noticeably. Even a woofer fed with 100W (and perhaps 50% efficiency) is nowhere near the 100kW or so that a car engine can produce while accelerating in traffic.
  • Rather than accelerating the car, most of the sound energy is dissipated inside the car (producing no thrust), or propagates away from the car in all directions as sound waves (producing almost no net thrust).
What you really need is a "DC" sound source - one which produces a continuous air pressure difference between the front and the rear of the vehicle. This is exactly what a jet engine does.

So, for the rest of us (who are not jet fighter pilots), the traditional gearbox and wheels is far better than a sound system at transferring chemical or electrical energy into kinetic energy of the vehicle as a whole.
 

Offline wolfekeeper

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Re: Is it possible to create a pulse drive in cars?
« Reply #2 on: 06/08/2015 14:52:04 »
I'm wondering if the poster means pulsejets. If so, yes, driving a car with pulsejets can be done, but it turns out that it's horribly inefficient (as are other jet cars, basically the exhaust goes too fast, you're accelerating the exhaust more than the car, unless the car is moving at near sonic speeds).
 

Offline syhprum

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Re: Is it possible to create a pulse drive in cars?
« Reply #3 on: 13/08/2015 21:27:46 »
I remember a car running at Brands Hatch in the early sixties with a pulse jet engine similar to those in the V1 flying bomb, one disadvantage (amongst many !) was that it would not operate at less than 100kph
 

Offline syhprum

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Re: Is it possible to create a pulse drive in cars?
« Reply #4 on: 15/08/2015 15:35:34 »
All though there much glib talk about putting 100 watts into speakers I am sure that speakers which are only about 1% efficient would soon go up in smoke if the voice coils tried to dissipate 99 watts.
the actual power in sound waves is very low I recall reading that a symphony orchestra puts out about 1 watt
 

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Re: Is it possible to create a pulse drive in cars?
« Reply #4 on: 15/08/2015 15:35:34 »

 

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