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How was the answer found?

  1. Oct 22, 2009 #1
    Basically using the engine size, the power of the starter motor was found to be be 17.98w. The cars engine is a 1.4L. What's the equation used to find the power of this starter motor?

    Basically I need to find how they got this so I can move onto my next part of the coursework.

    Would be grateful for any assistance.

    Thanks,
     
  2. jcsd
  3. Oct 22, 2009 #2

    berkeman

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    What are your thoughts on this? Hint -- think initially in terms of torque, rather than power.
     
  4. Oct 22, 2009 #3
    Am I supposed to be looking at the torque equation?

    torque = r x F??
     
  5. Oct 22, 2009 #4

    berkeman

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    Partly. What resists the turning of the crankshaft of the motor while you are trying to start it? You need to overcome that resistance to turning, correct?
     
  6. Oct 22, 2009 #5
    The engine oil. Its cold and not up to temperature.

    It seems like the atmospheric temperature is meant to be -20 for this assignment. Yes, I have to overcome that resistance. How do I overcome it though if I don't have a value for it?

    Sorry for being so confused!
     
  7. Oct 22, 2009 #6

    berkeman

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    No, that's a small part of it. Think about pull-starting a lawn mower, or kick-starting a dirtbike. What causes all that resistance that you feel?

    Are you pretty familiar with how internal combustion piston engines work? There's a pretty good intro article at HowStuffWorks.com:

    http://auto.howstuffworks.com/engine1.htm

    The animation of the 4-stroke engine cycle is pretty helpful.
     
  8. Oct 22, 2009 #7
    I'm pretty familiar with them but can't think of what this resistance could be. I'm thinking its crank resistance through the crankshaft. Is that correct? I've never pull started a mower (all electric here) or kick started a dirtbike :(

    I haven't got two deep down with cars either :(
     
  9. Oct 22, 2009 #8

    berkeman

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    From the engine web page that I linked to, what are the 4 parts of the 4-stroke engine cycle? If the engine is being cranked trying to start, what is missing so far from those 4 parts of the cycle? But what is still present?
     
  10. Oct 22, 2009 #9
    Only intake and combustion will be present when the engine is trying to start. Correct?
     
  11. Oct 22, 2009 #10

    berkeman

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    No, combustion is the thing that's not happening yet. What are the other parts of the cycle?

    (I have to bail for a few hours. Will try to check back later tonight.)
     
  12. Oct 22, 2009 #11
    Sorry, I mean intake and compression (its late here :P )
     
  13. Oct 23, 2009 #12

    berkeman

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    No worries. There's also the exhaust stroke, where the (unburnt during starting) mixture is blown out the exhaust valve. So on intake the intake valve is open, and on exhaust the exhaust valve is open. What valves are open on compression? What does that mean about the resistance you might feel on the crankshaft during these three parts of the 4-stroke starting cycle?
     
  14. Oct 23, 2009 #13
    On compression they are closed. Is this the resistance??

    Thanks,
     
  15. Oct 23, 2009 #14

    berkeman

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    Yes, good. The resistance to turning the crankshaft during starting is from the compression of the cylinders. Now you need to calculate what that resistance is, and convert it into the starting torque required. That will be how you size the starter electric motor.
     
  16. Oct 24, 2009 #15

    vk6kro

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    That 17.98 watts seems suspiciously low.

    Maybe 17.98 KW? That's about 24 HP which could be reasonable, but maybe a bit high.

    Perhaps 1798 watts. 12 V at 150 amps is in the right area. Maybe that decimal point was just not meant to be there.
     
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