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Trivia Question

Discussion in 'The Garage' started by MudFlap, Mar 2, 2004.

  1. MudFlap

    MudFlap 1/2 ton status

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    With an engine turning at a constant RPM. Does a piston move faster in the top half of the cylinder, the bottom half of the cylinder, or does it move at the same speed top and bottom?

    For instance if it is a 4" stroke, the top half of the stroke would start when the piston is 2" from the bottom of the stroke, then go to TDC, then go down 2". And the bottom half is when the piston is 2" from the bottom of the stroke and then move to the very bottom and then back up to 2" from the bottom.

    Trey
     
  2. Resurrection_Joe

    Resurrection_Joe 1 ton status

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    I dont know much on engines besides basic stuff, but if it did move at different speeds becasue of combustion, wouldn't that cause a kinf of impact on the bearings?

    I guess constant speed but whooooraaaa

    Trivia - Where did the Chevy bowtie originate

    Answer - moor letoh hcnerf a ni repapllaw morf
     
  3. MudFlap

    MudFlap 1/2 ton status

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    As I said in the post, the engine is at a constant RPM, so ingore any combustion issues. Assume that the crank is turning at exactly the same speed the whole time. And no the answer is not the same speed at the top and the bottom.
     
  4. 55Willy

    55Willy 3/4 ton status

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    it moves faster in the top half. /forums/images/graemlins/woot.gif
     
  5. MudFlap

    MudFlap 1/2 ton status

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    Now explain why.
     
  6. 55Willy

    55Willy 3/4 ton status

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    As the crank turns the piston travels down, as it comes to the bottom of the cylinder the crank is swinging the rod away from the cylinder and the pistons sits there longer as the cranks come back around to force the piston back up. the pistn sits at bottomdead center londer then it does at op dead cneter do to the change from reciprecating motion to circluar motion (like how a some sawzall functions)

    does it make sence? I just two bic pens on my desk to show thepiston and rod and the crank movements and it was obviouse to me then.
     
  7. heavy4x4

    heavy4x4 1/2 ton status

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    Let's break this down.
    Very top and very bottom of stroke, the piston has no velocity and no acceleration.

    Now here comes the thinking part (forgive me...getting kinda late) The angular velocity (omega) of the crank is constant, as is the angular acceleration (alpha) - this is all under the impression that you're running at a constant RPM. So, the crank turning at a constant angular velocity means that on the radius of the crank (where the connecting rod is attached), the velocity of that point is (omega)*radius directed in a direction tangent to the circle formed by the rotating crank. Now, the crank is traveling a certain amount of radians per second. But, at the outer/side points of the cranks circle, it will travel a greater delta y (change in height) than towards the top and bottom during a certain delta t (change in time).

    My answer is: none of the choices you gave. The piston moves fastest at the middle of its stroke.

    /forums/images/graemlins/eek.gif Now shame on you for making me think this late...especially if I'm wrong. /forums/images/graemlins/grin.gif
     
  8. BlueBlazer

    BlueBlazer 1/2 ton status

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    I concur /forums/images/graemlins/grin.gif
     
  9. u2slow

    u2slow 1/2 ton status

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    [ QUOTE ]
    The piston moves fastest at the middle of its stroke.

    [/ QUOTE ]

    Yup.

    Think of it this way:

    At exactly mid-stroke, the direction of motion of the crank throw is the same as the piston. Near the top and bottom, the crank throw is moving almost perpendicular to that of the piston.

    Does that help? Clear as mud??? /forums/images/graemlins/ears.gif
     
  10. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    At a constant rpm the piston must change direction from down to up. Logic says it must stop before it changes direction so there would be a deceleration at the extremes of the stroke and then an acceleration to max velocity again.

    Think of a Yo-Yo...

    Rene
     
  11. 4xcrazy

    4xcrazy 3/4 ton status

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    it just moves really fast all of the time,,,,unless it's turned off. /forums/images/graemlins/thumb.gif
     

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