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A few stupid/imp questions

  1. Dec 25, 2006 #1
    OK I have a few stupid questions, some of which I need to confirm answers too.

    1. How does a car increase it's speed? When a driver accelerates, more fuel is injected so the explosion size increases, and the piston moves with more force downwards at a greater speed. Is that true?
    So the intake of gasoline by the cylinder depends on how hard you press on the acceleration?
    So a car's speed depends on the movement of the piston, which in turn depends on how much fuel is injected?

    2. If what I said above is true, then how long does it take the piston to move from the bottom of the cylinder to the top when the car is at its lowest speed? How long does it take to move from the top to the bottom when the car is at its lowest speed?
    How long does the piston take to move from the bottom to the top when the car is at its highest speed? From the top to the bottom?

    3. What is the best way to reduce friction between two pieces of wood so that the friction between the two pieces of wood be at its lowest?

    4. Is there anyway to block out the magnetic field of a magnet? What is it?

    Thanks a lot.
    And sorry mods if I posted this in the wrong section, I wasn't sure which section to post this in.
     
  2. jcsd
  3. Dec 25, 2006 #2
    I guess this is the wrong section :p
     
  4. Dec 25, 2006 #3

    FredGarvin

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    TRUE.

    TRUE.

    It depends on how much fuel and air are injected. The speed also depends on the transmission/drive train as well.

    That depends entirely on the engine being considered. What you need to know in particular is the "stroke" or the length of piston travel for each and every engine out there. They are not the same.

    It depends on what you mean by "best." If you mean the absolute lowest friction coefficient possible, then the best would be a very thin layer of air between the two pieces that provides just enough space as to not let the two pieces touch. However, that's not very realistic in practice. So the answer is most likely some kind of grease or similar fluid. What that fluid/grease would be would also depend on what one is doing.


    Encase it in very thick lead. This is what is used for magnetic shielding in things like speakers and TVs.
     
  5. Dec 25, 2006 #4

    russ_watters

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    Basically, yes. They try to keep a good mixture of fuel and air, though...
    Ever been in a car with a tachometer? It tells you how fast the drive shaft is spinning. Most cars idle at 500-700rpm. That's a tenth of a second for a cylinder to go up and down. The red line can vary, usually from about 5,000-10,000 rpm Please remember that there is a clutch and gears, though, so you don't always have a direct ratio (or the same ratio) between speed and rpm.
    Sand them.
     
  6. Dec 25, 2006 #5

    Astronuc

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    Other than adding a lubricant, one can smooth the surface or reduce the surface 'roughness' and make the surface as 'flat' as possible. Sanding with fine grit, and polishing, is the method.


    Magnetic shielding materials -

    Mu-metal - http://www.mushield.com/
    http://farside.ph.utexas.edu/teaching/jk1/lectures/node52.html

    GIRON - http://www.lessemf.com/mag-shld.html

    http://www.magnetic-shield.com/

    http://www.amuneal.com/pages/magshield-intro.php
     
  7. Dec 25, 2006 #6
    Thanks a lot guys that really helped!
    Yes, that's what I meant making the coefficient of friction as low as possible. I guess, the best method is sanding them and then polishing them. What about PTFE, FEP, PFA, acetal, is it possible to use them? But I doubt if I can get them from anywhere, so I think polishing will do the work hope fully.

    And I need to know about a rough estimate of the time it takes for the piston to up and down. Maybe an average of sedan cars without not really really awesome specs (ferrari's, merc's etc.). Let's say an average car, suppose, err.... a Honda civic.

    The magnetic shield is going to cost a lot! :p. Thanks a lot for the links.
    P.S. All that stuff on those sites are confusing.:P
     
    Last edited: Dec 25, 2006
  8. Dec 26, 2006 #7

    russ_watters

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    The numbers I gave you were revolutions per minute. Flip them over and you get time. Ie, 600 rpm is 10 revolutions per second or one revolution every tenth of a second. Every revolution of the crankshaft takes each piston all the way up and down. A typical sedan that redlines at 6,000 rpm then, has its pistons go up and down 100 times a second at its max.
     
  9. Dec 26, 2006 #8
    How can something mechanical moves so fast?
    How is the piston powered?
    It's hard to imagine something going so fast.
     
  10. Dec 26, 2006 #9

    FredGarvin

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    They do move quite quickly. The motions of a piston are very well studied and are part of a basic engineering dynamics class. The motion creates some pretty large accelerations which in turn create some large forces. So if you ever wonder why a connecting rod or crank shaft is so beefy, that's part of the reason why.
     
  11. Dec 26, 2006 #10

    Astronuc

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    'Soft iron' is also used for magnetic shielding.
    http://tpub.com/neets/book1/chapter1/1j.htm

    Mechanically. The explosion in the combustion chamber increases pressure/force on the piston. The piston transfers the force mechanically via the rod to the crankshaft, which in turn rotates and transmits force to the other pistons. The crankshaft is also couple to a transmission which transfers the mechanical force to the axles either directly as in front-wheel-drive (or with certain rear mounted engines like VW's) or indirectly through a drive train in a rear-wheel-drive system.
     
  12. Dec 26, 2006 #11

    DaveC426913

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    I believe the fuel amount is not increased, nor is the size of the explosion.

    The only thing that increases is the speed at which the explosions happen.
     
  13. Dec 26, 2006 #12
    The amount of fuel and air drawn into a gasoline engine is increased to increase the power output. Technically, they aren't even considered explosions.
     
  14. Dec 26, 2006 #13

    DaveC426913

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    Yes, but unless I am wrong, the extra air/fuel goes into faster cycling of strokes, not more powerful strokes. (I may be wrong on this. ICEs may be more sophisticated than I thought.)
     
  15. Dec 26, 2006 #14

    russ_watters

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    Force must accompany (cause) acceleration. The rpms cannot increase unless the force on the piston increases.

    If you push down on the gas pedal, you start to accelerate - if you push down the brake at the same time to counter that, you keep the same speed, but are absorbing the acceleration force with a friction force. Your rpms and haven't changed, but your brakes are exerting a force and absorbing energy - extra energy that was put in via extra fual and air in each firing.

    Now you know why cars run so far below their theoretical thermodynamic efficiency...
     
    Last edited: Dec 26, 2006
  16. Dec 27, 2006 #15

    DaveC426913

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    Frequency of application of the force can accomplish the same thing.

    eg. The guy on the office chair that throws a beanbag every second will accelerate and move faster than the guy on the office chair throwing a beanbag every ten seconds.
     
  17. Dec 27, 2006 #16

    russ_watters

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    I just said you aren't changing the frequency if you hold down the brake while holding down the gas pedal. Changing the frequency itself is an acceleration. Certainly, you can change the force simply by changing gears, but you can also accelerate without changing gears (and while the clutch is down, the engine is free to accelerate if there is a force to accelerate it).
     
    Last edited: Dec 27, 2006
  18. Dec 27, 2006 #17
    I still don't understand.
    A piston moves down after the explosion, and the piston turns the crankshaft. How does the piston move back up?
     
  19. Dec 27, 2006 #18

    FredGarvin

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  20. Dec 27, 2006 #19
    stored energy in the flywheel for a single cyl motor
    returns the piston to begin a new cycle
     
  21. Dec 27, 2006 #20
    Thanks a lot!
     
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