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Automotive Inertia within the engine

  1. Mar 16, 2012 #1
    With all the weight either recipricating or rotational flying around I suppose a basic conversation should be brought up when analyzing engine operation and trying to reduce power losses, rpm limiting effects.
  2. jcsd
  3. Mar 17, 2012 #2


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    That was a fairly vague statement. There has been a vast amount of work done though the years by many people on engine balancing, shaking force reduction, etc., so just what do yoou have in mind to add to the conversation?
  4. Mar 17, 2012 #3
    Yes, it was a little vague, more to just get the idea rolling around in the brain in the mechanical portion of the engine. I have a curious mind of what has been seen during testing in regards to the interia involving all our moving parts inside. Having torsional, upward and downward as well as a combination of the two, the action going on in an engine has to be great even with the smaller moving parts.

    I realize we have to deal with the overall dimensions and weight of the moving part as well as the speed our part we are watching is moving, but, what I am wanting to look at is what are these parts actually going through and enduring as far as forces?

    Crankshafts are resisting torsional, pulling and pushing
    Con-rods are resisting stretching & compression
    Lifters are resisting compression
    Pushrods are resisting compression and so on

    For everything to work according to the plan, bearing and other clearances need to be employed as well.
  5. Mar 17, 2012 #4


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    Yup, that is why engine design is such a time consuming, difficult affair.

    Con rods also see bending.
    Cam shaft sees bending and torsional loading.
    Wrist pins see bending.
    There are all sorts of springs that see fatigue loadings here and there.

    So, are you familiar with all of these things, are you new to them all, or where do you stand? Your name indicates that you are a racing team or some such, so that would suggest that you might already know all about these matters. But that raise the question, why do you bring it up here?
  6. Mar 18, 2012 #5
    I am aware of each as far as stresses from operating, I would just like a little more indepth insight on these involving the velocities, mass and the calculations so I can relate this or that to what I may modify and be able to do the math. I have not taken a course in physics which I regret, being limited on funds to buy a physics book and study myself, I chose the forum to start me off to dig into.

    If we have a piston that weighs 453.592 grams/1lb in a bore diameter of 4.000" and a con-rod at 5.7" with a srtoke of 3.48" operating at 4500rpms for example, I am sure cylinder block layout will effect the motions of the rotating assembly. You have the rings producing friction from creating the seal and scraping so theres another aspect that is needed to be viewed. In short, I think what I am hoping to learn not only involves interia, but Friction horsepower comprehension as well.
  7. Mar 18, 2012 #6


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    Most of what you are asking about is covered in the junior level mechanical engineering course called Theory of Machines. A textbook for that course will answer many of your questions, provided you can do the math (quite a lot of calculus). There is a good book, Mechanics of Machines by Doughty available rather inexpensively from Lulu. There are also many others by Shigley, Paul, and various other authors.
  8. Mar 19, 2012 #7
    Thank you, I have Charles Fayette Taylor's Internal Combustion Engine in Theory & Practice which I was skimming through the chapter of this topic and it seems pretty dense with the calculus. I am not worried about that its just getting the textbooks in my hands to learn.

    Are there any good threads here to read about mechanical inertia aspect of the ICE that you, or anyone has seen?
  9. Mar 19, 2012 #8
    The topic is far too large to address in a single thread, and infact even when designing and engineering an engine all the bits are never analysed as a full system. Each calculation is actually very simple, but the complexity arises from quantity.

    So to make it simple you have to break it down and address one component, or system on components.

    So do you have a topic you'd like to address first? The piston inertia loading is probably a decent topic to start with.

    What i'm thinking:
    Single cylinder.
    Equations for motion, and the assumptions made.
    Effect of changing mass.
    Effect of RPM
    Effect of Rod Length/Throw ratio.
    How this affects mean piston speed.
    Side forces.

    There will be more topics, but that seems like a fairly comprehensive start.

    I'll still have to start reading up on this as I've not done anything like this in quite a long time. But have done something similar before. It may take a while to get a comprehensive post up, due to work commitments etc.
  10. Mar 19, 2012 #9
    My thoughts exactly. The size of an equation in calc is what probably pushes people away. I was thinking about starting in the bottom end of the engine since its where we transfer our torque to the drivetrain.

    I appreciate your generosity Chris!
  11. Apr 3, 2012 #10
    After reading some I see that most vibrations we are dealing with are created by the accelerations and decelerations of the rotating assembly, particularly the connecting rod and piston assembly weight?
  12. Apr 3, 2012 #11


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    Well, sort of yes and no. Have you noticed that the pistons do not rotate at all? The con rods sort of wobble side to side, but do not execute full revolutions? So when you speak of the acceleration/deceleration of the rotating assembly, properly speaking that should include only the crankshaft. The vibrations are due to the acceleration/deceleration of all of these components.
  13. Apr 4, 2012 #12
    Forgive me, the connecting rod and piston assembly reciprocates but is attached to the rotating assembly which is the crankshaft itself.

    Since the crankpin is revolving around our main bearing center the connecting rod does make a full circle, however if you look at the movement from the piston point of view, I think the motion is more oval due to crankpin location and rod length causing different speeds before and after both BDC & TDC.
  14. May 5, 2012 #13
    Some where along the line here, we have to incorporate ring tension & friction correct?
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