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Automotive Crankshaft axis offset from cylinder axis?

  1. Aug 21, 2013 #1

    Baluncore

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    I have always thought of connecting rods as straight and the crankshaft axis being line bored in the same plane as the cylinder axis. Is there an advantage in offsetting the crankshaft slightly to compensate for the difference between the compression and power stroke pressures ?
    Is the answer different between diesel or petrol engines, or for two or four stroke ?
     
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  3. Aug 21, 2013 #2

    SteamKing

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    It's not clear what you are describing. How would such an offset accomplish this compensation?

    In most engines, the connecting rods articulate w.r.t. the piston. This is to allow for the movement of the bottom of the con. rod as the crank throws rotate about the crankshaft centerline. The pin in the piston which connects to the rod is called a gudgeon pin in the UK or a wrist pin in the US.
     
  4. Aug 21, 2013 #3

    Baluncore

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    I quite understand the mechanics of IC engines. I enjoy rebuilding big diesel engines.
    The changing angle between the cylinder axis and the connecting rod axis, (as defined by the big end journal axis and the gudgeon pin axis), effects the mechanical advantage of piston pressure to crank torque throughout the crank's rotation.
    The compression stroke occurs on one side of the crank rotation, the power stroke on the other. By offsetting the crankshaft to one side slightly the compression could be achieved with a different advantage to the power stroke. This offset becomes more significant with a shorter connecting rod or bigger throw. I know that the piston skirt clearance would be asymmetric. But the payoff would be that the power stroke could be “Chebyshev optimised” to push straighter than the compression stroke. Even if an offset gave only a 0.1% advantage then it would be very worthwhile economically.
     
  5. Aug 21, 2013 #4

    SteamKing

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    Your proposal might work for an inline engine, but what about a V-type engine? It seems like a 0.1% advantage would lead to a much more costly engine to fabricate. Still, to me, the concept is not clear. I am sure that after 100+ years of diesel engine construction, if this concept were feasible it would have been tried already.
     
  6. Aug 21, 2013 #5

    Baluncore

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    The position of the bores would be effectively moved sideways slightly. Or the line of the crankshaft could be moved relative to the two banks of cylinders. All things are relative. There is no reason why it could not be applied to a radial engine.

    Moving a bank of cylinders sideways slightly is not expensive. It is just a different number in an NC program.

    If not yet tried it would be a missed opportunity.

    Any added complexity is avoided unless it is the best return on investment at the time. Many changes have happened over time already. Maybe it is time to consider this.
     
  7. Aug 21, 2013 #6

    Averagesupernova

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    I have thought of this a number of times. Even to the point of having two pistons attached solid end to end similar to a horizontal opposed engine except they would be attached solid and obviously not arranged to cancel vibration. The wrist pin would be in the middle and the connecting rod coming out at a fairly sharp angle. Advantages? I really can't say there are any off the top of my head. Baluncore has pointed out some advantages. If a person wanted to remove any side load from the piston you could have a dual crankshaft setup to cancel the side load that the rod puts on the piston. Dual crankshaft setups have been done. One thing I have learned over time is that lots and lots and lots of engine designs have been conceived AND tried. Just do some creative google searches and you will find them.
    -
    I have also thought of a similar setup with a double ended solid piston with the crank in between the ends. Sounds impossible, but with a double eccentric I think it can be done. Why do something like this? No idea, just something my mind stumbled across.

    I like this way of thinking. We can't blindly go about just trying stuff and expect awesome results. That is not how engineering is done. But sometimes it isn't a bad thing to just say 'what if?' and experiment with it.
    -
    Edit: Pistons will have the bore for the wrist pin offset from center, so some of what you describe is already occurring. The reason I believe is to quiet the engine. What I have described is in my opinion extremely offset.
     
    Last edited: Aug 21, 2013
  8. Aug 21, 2013 #7

    SteamKing

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    I still remain unconvinced. To be sure, it would bolster your point if a sketch could be prepared which describes your proposal. Often times, one picture can replace pages of verbal description and much hand waving.
     
  9. Aug 21, 2013 #8

    jim mcnamara

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    Hmm. Seems like Mr Google knows this:

    http://www.fev.com/fileadmin/fev-resources/Publications/Powertrain_Mechanics/CrankshaftOffsetAndIt_sImpactonPistonAndPistonRingFrictionBehavior.pdf [Broken]

    Piston offset alters friction, favorably if done correctly. It may be that the current ROI does not justify doing this.

    There are several dozen hits on this topic - cross-plane engine design one the posters here might want to look at.
     
    Last edited by a moderator: May 6, 2017
  10. Jun 7, 2014 #9

    jim hardy

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    Last edited: Jun 7, 2014
  11. Jul 7, 2014 #10
    the Toyota NZ engines are the most common application for an offset crank/bore axis in current automobiles. It helps both power and economy on the atkins and normal 4 stroke engines in the Prius and the Yaris.

    I have always wondered why this is not more common, either in OE designs or as a modification to get a few extra HP in racing classes where most things are closely regulated, but there are no specs for this.

    A simpler retrofit on a lot of chevy v8s is to turn the stock pistons around, or swap banks. the pin is offset in the bore to make them quieter stock. it is offset the opposite way though to make the rod straighter on compression. Flipping them is said to gain 5-10Hp on a 400-450Hp engine. I'd be a little suspect of those numbers, but I bet it does something.

    I don't really see a downside to offsetting the bores, unless it affects piston noise, like pin offset.
     
    Last edited: Jul 7, 2014
  12. Feb 26, 2017 #11

    jim hardy

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    Hmmmmm ...........

    https://en.wikipedia.org/wiki/Desaxe

    Marine engines are often sold as pairs, one runs clockwise and the other counterclockwise.

    Does anyone know if this offset (desaxe) means the blocks are bored differently depending on rotation?

    thanks,

    old jim
     
  13. Feb 26, 2017 #12
    Judging by the wiki animation, It looks like the crank is exactly in between the bores, so it could operate in either direction because the engine is in fact symmetrical, however the gas flow would be reversed depending on rotation direction.

    Taking a marine Inline or common "V" engine for example, I would think they keep the bores aligned with the crank specifically to avoid having to make a left and right handed version of them depending on desired rotation.

    The way I see it, is the primary reason for offsetting the crank isn't so much for mechanical advantage (that's a bonus though), but to reduce the side load on the piston during the power stroke.. This is different from offsetting the wrist pin in the piston, that is usually done so that the piston deliberately is slightly cockeyed in the bore, and always cockeyed the same way, which prevents piston slap by keeping some load on it
     
  14. Feb 27, 2017 #13

    Baluncore

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    I would assume that if one engine is desaxe then the other hand must be the mirror image. Not only the camshaft needs to be changed to reverse an engine. Lubrication and cooling will also change.

    There are multiple effects, gyroscopic torque when turning or when pitching in a sea swell, roll on acceleration due to angular inertia, and propeller walk cancellation. A LH gearbox will be different from a RH gearbox due to thrust direction. If propeller walk was the major problem then only the gearbox design would need to be changed.

    Thanks for finding the link. That wiki page has been around since 2008. All I needed in 2013 was the correct technical term.

    The animation is of an engine that has a blower pump on the left followed by a power cylinder on the right. It is desaxe, but it confuses the issue when included there.
     
  15. Feb 27, 2017 #14
    yes, that animation does confuse things..
    According to that Wiki, Desaxe engines are in fact commonplace, with Ford having used them, and VW using them in the VR6's
    Yes, engine rotation does affect many things, and it's not always easy to just make them turn the other way.... Many ship diesels are 2 stroke, which would lend itself to that... Perhaps making the front and back ends of the block symmetrical would allow you to just turn the engine around and not need a new casting :)
     
  16. Feb 27, 2017 #15
    Cylinder offset is common. It is done to give greater mechanical advantage over the crankshaft during the points of combustion that have higher pressure.
     
  17. Feb 27, 2017 #16

    jim hardy

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    I found some Boating magazine articles from 1930's that advise against trying to reverse desaxe engines, but back then they just called it " offset cylinder bore"

    Here's Henry's 1932 V8 it shows the desaxe offset, 0.158 inch
    FordFlathead.jpg

    From what i've found Mopars are desaxe so you need the right block

    i cant find anybody who says for sure whether GM small block is bored offset or not. Many sites refer to wrist pin offset and say you have to put pistons in backward for a reverse Chevy engine. There's a gear camshaft setup to replace the timing chain. Reverse crankshaft has oil grooves at rear seal reversed , somebody said, and journal oil holes located differently.

    It's never simple is it ?


    Anyhow son has bought a boat project. Twin 350's that got under saltwater .
    My local junkyard has a 50 foot high mountain of reject smalllock cores from a race engine builder , if i can find casting number for reverse GM engine i'll pick through them.
    Might be that smallblocks aren't desaxe at all, just i havent found a source that seems authoritative.

    old jim
     
    Last edited: Feb 27, 2017
  18. Feb 27, 2017 #17

    jim hardy

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    Thanks jack that's what i needed !

    Here's how they handle the crankshaft to camshaft relation
    standard

    454Std.jpg
    reverse

    454Rev.jpg
    pictures are big block, smallblock is similar i'm sure.

    Thanks again for those drawings !

    old jim

    old jim
     
  19. Feb 28, 2017 #18
    Are we fixating only on V configuration engines?
     
  20. Feb 28, 2017 #19
    no, the logic behind it applies to both V and inline engines
     
  21. Feb 28, 2017 #20

    Baluncore

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    No. But some of us find it easier to think about rotating engines in the complex plane.
    The 'V' engine has cylinder banks like the sine and cosine coefficients of a Fourier series.
     
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