Engine Physics?

Discussion in 'Engine Topic' started by Dante2904, Jul 4, 2010.

  1. Dante2904

    Dante2904 Member

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    I am currently trying to find all of the formulas that are needed to run an engine. I know this can go deep in integral calculus, but I do not want to go that far, nor do I need to. I just have to rig an animation, which will require formulas that are roughly close. I also have to build a program that can estimate max RPMs and calculate the torque/hp of any classic car build. I am fairly certain the torque and hp calculations will not be too difficult provided I get the right weights on all of the components and figure out how to find the compression, but max rpm?

    My biggest question is about the blocks themselves. What is the real difference between a 350ci SBC and a 302ci SBC if, literally, all the parts can cross-over, and the block dimensions are the same? Common sense seems to tell me that a 350ci engine has more volume... but where, if the outer dimensions are the same? Is my data wrong?

    If the camshaft has a high lift, I would assume the valves open further - but what is actually happening with your torque/hp - is it just effecting your compression?

    What does a rocker arm's "rocker ratio" affect and what is it the ratio of?

    Sorry to ask this, but I have googled a lot this morning and found out a lot of really useful information, but I can't get any good hits on these questions.

    I think I have a really strong grasp on the bottom components now, but the valve train, intake, and carburetor mechanics are really much more complex and seem to deal with many more variables.

    Thanks for any advice
     
  2. wiseryder

    wiseryder Veteran Member Lifetime Gold Member

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    I gotta ask , Why?
     
  3. 74RAT

    74RAT Veteran Member

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    i'll pick on a couple of parts of your questions.

    on paper or by the old 1900's to 1920's or 30's textbooks,, big cams aren't supposed to work. study valve overlap and you'll see what i mean.

    back in the day,, it was thought that valve overlap would cause an intake manifold explosion with both valves open at the same time using a non-textbook high overlap cam during that timeline era. cams with closing and opening points exposing exhaust to intake at the overlap period.

    valve overlap is described as the area in crank degrees between the exhaust stroke and intake stroke where todays cams hold both valves open at the same time for a brief moment in time,,,, measured in crank degrees to describe it. this is where this happens using todays cams with higher overlap. lobe separation angle is the only cam "degree of duration angle" that is actually measured and spec'd/listed as a measurement DIRECTLY OFF OF THE CAM ITSELF in cam degrees. all other duration degrees are measured in crank degrees.

    you are fighting time with valve events vs atmospheric air pressure and inlet restrictions to get air in the cylinders. faster engine speeds reduce the time to get air in the cylinders. more cam duration "measured at the crank" gives the cylinder more time to get air/fuel mixture into the cylinders,, at the expense of worse low rpm power and torque due to mixture quality at low rpm.

    old school engineers thought that it was not possible to extend the valve periods past the 180* marks. then iirc it was isky that did his own "brazing experiments" welding up and grinding up his own cam lobe designs and here we are today.

    the 302 chevy has a 3.000 stroke with the same bore size as the 350. shorter stroke gives you the different cubic volume with the same block design.
    bore x bore x stroke x .7854 x # of cylinders = cubic inches.
    hope it helps.
    andy
     
    Last edited: Jul 4, 2010
  4. 74RAT

    74RAT Veteran Member

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    rocker ratio effects rate of lift velocity per degree of crank/cam rotation at the valve when changing ratio's.

    seat to seat timing doesn't change,, but max lift will be slightly different.

    from when the valve starts moving,, the lift rates increase or decrease with ratio change per degree of crank rotation/cam rotation with a change of rocker ratio.

    kind of like comparing an agressive roller lobe to a flat tappet lobe. with the same seat to seat specs. makes the lobes act more or less agressive with a rocker ratio change.

    the fastest piston speeds are listed as being between 75*-80* after tdc on the intake stroke. the more open you have the intake valve at that crank degree,, the more air fuel mix you can get into the cylinder.

    that's the theoretical point in piston position where the intake velocity is the highest and can produce the most draw or intake manifold depression on the intake runners. if we could only be at max lift at that point in crank degrees?????? dang.

    an agressive cam lobe or increase in rocker ratio helps with that,, being that most max lift points on most cams are between 99* and 116* after tdc on the intake stroke. the cam kind of "lags behind" on it's valve events from what the engine could really use. but mechanical valve train speed/velocity limitations prevent that in todays world without solenoid operated valves. it may be comming though.
    andy
     
  5. 74RAT

    74RAT Veteran Member

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    those are general statements with ALOT of variables involved^^^.
    andy
     
  6. Dante2904

    Dante2904 Member

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    Simply put, I am building a very complex animation that allows users to swap out different components such as blocks, cams, carbs, cranks, pistons, heads, rocker arms... etc... and get a dyno-like feedback. This will not be completely accurate, it only has to seem almost accurate with the dyno torque and hp curve.

    I am not a mechanic be any means. I can put a car together from the ground up and fix what is wrong with mine, most of the time, but that is not enough to consider myself a mechanic in my opinion.

    What most people do not realize is that just doing a 3D animation means that you have to understand much more than what parts rotate inside and outside the block. You have to understand how the entire system works in order to get it reasonably believable.
    _______________________________________________

    Thanks 74RAT, that is very informative. I don't fully understand everything you said, but it gives me a better idea on how to find the information and actually comprehend what you said.
     
  7. ZS10

    ZS10 Moderator Staff Member Lifetime Gold Member

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    A 3d animation will be interesting, but there are common programs like Desktop Dyno and Engine Analyzer that will give rough numbers after entering component specs.
     
  8. Dante2904

    Dante2904 Member

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    I have been reading a lot of information over the past few hours and I am having trouble understanding the compression ratios.

    As I see it, right now, the compression ratio is just the air volume in the cylinder at BDC and TDC. The compression is calculated by the bore and the stroke of the cylinder (I am having a lot of trouble locating the stroke and bore of all the engine blocks I have chosen), the head's cylinder combustion chamber volume, the piston's relief pocket volume (I have these numbers, but the concept is quite hard to visualize between flat, dome, and reverse dome pistons)

    We get the BDC by calculating the square of the cylinder bore, multiplied by pi/4, multiplied by the piston stroke (is this the rod stroke or the piston stroke - I ask because pistons, without rods, I have looked at, have a stroke equal to most rods, and I am pretty sure you can swap out 5.700 inch rods with 6.000 inch rods provided you have the right crankshaft), then adding the clearance volume and dividing by that same clearance volume. In retrospect, it seems changing rod lengths will not effect the cylinder chamber in any way, it will only effect the crank rotation??

    I realize that the gaskets and piston rings also have a significance, but I hope that since this does not have to be perfect, those elements can be omitted.

    Now that I can get the CR, what is it's purpose? I know it determines the optimal octane needed for the engine. The CR also seems to be needed to determine the CFM needed for your carb...

    So far, I know that the compression ratio is a key equation for engine operation - these are the type of functions and equations I was originally inquiring about.

    With what I know I can make the bottom components of the engine and get the mathematical results for those, but I am only SLOWLY beginning to get to the heads and valve train.

    What I mean with that is, I can pick and choose different cranks, blocks, rods, and pistons correctly (I believe), but I do not understand how to determine heads, cams, valves, or rocker arms yet. I actually have the compression ratios almost capable of being automatically calculated, but I need to understand what is needed in dealing with heads.

    Thanks for the information earlier 74RAT and I know it will be very useful soon enough, but I underestimated the math involved up to that point and have to take a while to learn that.

    Thanks for any more advice.
     
  9. li0nhart123

    li0nhart123 Veteran Member

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    changing the length of your rods is not going to increase or decrease the stroke....the stroke is determined by a line drawn through the center of a circle created between points 180 degrees apart as the center of the rod journal rotates.

    changing the crank to alter the circle crated by the rod journal will.
     
    Last edited: Jul 5, 2010
  10. Dante2904

    Dante2904 Member

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    Thanks, I think you just introduced something else I need to look into - rod to stroke ratio...

    Looking that up now, but just thinking about it, if you have a 3.480 stroke crank with 2.1 inch rod journals with matching Flat Top Pistons (4 inch bore, -5.4 cc volume) and 5.7 rods, then replaced them with 6.0 rods, would the piston not punch the head combustion chamber? I think I am seeing stroke and length as the same variable here.

    From what you defined stroke as, I am seeing the stroke as a line through the midpoint of a cylinder then extended to the center of the rotating rod journal (where rod connects on crank), therefore making the stroke a constantly changing variable.
     
    Last edited: Jul 5, 2010

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