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Thread: Mechanical Aptitude Test

  1. #21
    Quote Originally Posted by ashsimmonds View Post
    It's not gravity though, turn the piston upside down and it will still "suck" air in when it creates a low pressure zone in the cylinder... in fact that's what "sucking" is! (you should know... )
    But if it weren't for gravity, the atmosphere would've f*cked off into space ages ago

  2. #22
    Senior Member AshSimmonds's Avatar
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    Quote Originally Posted by VMax View Post
    But if it weren't for gravity, the atmosphere would've f*cked off into space ages ago
    What's that got to do with how the air got in the cylinder?

  3. #23
    Quote Originally Posted by ashsimmonds View Post
    What's that got to do with how the air got in the cylinder?
    Gravity is the reason the atmosphere is still here, and thus the root cause of atmospheric pressure

    Edit - "exists" -> "is still here"

  4. #24
    Senior Member AshSimmonds's Avatar
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    Quote Originally Posted by VMax View Post
    Gravity is the reason the atmosphere is still here, and thus the root cause of atmospheric pressure
    My Mum is the reason I exist, and thus the root cause of why I purchased a refrigerator.

  5. #25
    Well OK... what causes the atmosphere to exert 14.7psi of pressure if not gravity?

  6. #26
    Senior Member AshSimmonds's Avatar
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    Quote Originally Posted by VMax View Post
    Well OK... what causes the atmosphere to exert 14.7psi of pressure if not gravity?
    Yeah that's fine, but does that mean the act of sucking doesn't actually exist?

  7. #27
    Quote Originally Posted by ashsimmonds View Post
    Yeah that's fine, but does that mean the act of sucking doesn't actually exist?
    I think National Bingo Night is proof that it does.
    It exists in the same way as (say) centrifugal force, IMO

  8. #28
    Senior Member flamestone's Avatar
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    Quote Originally Posted by VMax View Post
    86%. Maybe I should've been a mechanic instead of a geek.
    Also, pulleys can fuck directly off
    86% also. So yeah, maybe us computer geeks should get out in the garage more often

  9. #29
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    Quote Originally Posted by VMax View Post
    Well, if the absolute pressure is 1.7, then I'd say 0.7 boost. Cause, you know... "boost". Like, how much above ambient. If it was a specific "pressure" thing with no mention of the word boost, then yes, 1.7. English is great that way.

    Anyway... the fact remains... if you want to call a vacuum 0psi/bar/atm then the question is unanswerable, no air. If you want to call 14.7-odd psi "0 atmo" (for some reason I don't quite grasp), then that 14.7psi is what's making it happen. In neither case is the piston causing air to enter the cylinder, and under no circumstances I can conceive does it do so, it simply allows it to happen.
    Ok, so what about piston speed/area/swept volume and valve area... your argument (and the test) would appear to have no facility for this...

  10. #30
    Quote Originally Posted by ferrarifixer View Post
    Ok, so what about piston speed/area/swept volume and valve area... your argument (and the test) would appear to have no facility for this...
    They don't make any difference to whether air goes in or not (assuming that they are all non-zero and the valve is actually open, of course).

  11. #31
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    no, but the question is "what causes"..

    and if piston speed is such that it creates a depression greater than atmospheric pressure, then its SUCTION that is causing the ingress of air...

    so the answer is..

    it depends on piston speed...

    but in reality, its suction at anything from idle speed and above..

    the 4 stroke otto cycle..

    SUCK, squeeze, bang, blow...

    can we call it blow?

  12. #32
    Senior Member AshSimmonds's Avatar
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    Quote Originally Posted by ferrarifixer View Post
    but in reality, its suction at anything from idle speed and above..
    Well that's what we came to a dozen posts ago - is there such a thing as "sucking"? Doesn't matter, I call it sucking, others call it filling a hole due to pressure.

    I just remember back in about year 4 we were asked to write an assignment explaining how we can get liquid from a glass into our mouths without using the word "suck"... Completely baffled at the time!

  13. #33
    Quote Originally Posted by ferrarifixer View Post
    and if piston speed is such that it creates a depression greater than atmospheric pressure
    Huh? That would be every speed higher than zero, then?

  14. #34
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    Quote Originally Posted by VMax View Post
    Huh? That would be every speed higher than zero, then?
    no, turning an engine by hand creates no suction unless the throttle is closed, the throttle is not mentioned here.

  15. #35
    Quote Originally Posted by ferrarifixer View Post
    no, turning an engine by hand creates no suction
    OK... so what happens to the volume, pressure and number of molecules inside the chamber when you do that?

  16. #36
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    Quote Originally Posted by VMax View Post
    OK... so what happens to the volume, pressure and number of molecules inside the chamber when you do that?
    you're now trying to take the piss

    my point remains.. the correct answer is "it depends on piston speed". The test is flawed.

  17. #37
    Senior Member AshSimmonds's Avatar
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    Quote Originally Posted by ferrarifixer View Post
    my point remains.. the correct answer is "it depends on piston speed". The test is flawed.
    It's the same answer at any speed.

  18. #38
    Quote Originally Posted by ferrarifixer View Post
    you're now trying to take the piss

    my point remains.. the correct answer is "it depends on piston speed". The test is flawed.
    Yes, it depends on the piston having some speed which isn't zero. Beyond that... no.

    They're called "gas laws", not "gas crazyarse theories" or "gas vague suggestions".

    PS damn you ash

  19. #39
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    Quote Originally Posted by VMax View Post
    Yes, it depends on the piston having some speed which isn't zero. Beyond that... no.

    They're called "gas laws", not "gas crazyarse theories" or "gas vague suggestions".

    PS damn you ash
    I'm not arguing the gas laws..

    I'm arguing the question wording...

    the "cause" of the air entering... is variable depending on the pressure/depression... at a certain piston speed, the swept volume "could" be greater than the flow capacity of the inlet port/valve.. creating a depression... which then becomes the primary cause for air entering the cylinder...

    It's the questions that are wrong.. not the answers!


  20. #40
    Quote Originally Posted by ferrarifixer View Post
    I'm not arguing the gas laws..

    I'm arguing the question wording...

    the "cause" of the air entering... is variable depending on the pressure/depression... at a certain piston speed, the swept volume "could" be greater than the flow capacity of the inlet port/valve.. creating a depression... which then becomes the primary cause for air entering the cylinder...
    I can see an argument for saying the piston sucks it vs pressure pushes it. But I think it's fair to say the atmospheric pressure one is the MORE correct answer, piston is correct but less so, and gravity is technically correct but silly

    However! The piston speed doesn't matter, nor does the swept volume. All those mean is that there may come a point where pressure has no time to equalise. It doesn't change what's doing the pushing or pulling, and both answers are no more or less valid on one side of that point than they are on the other.

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