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Thread: 40cc boxer gas engine

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  1. 40cc boxer gas engine 
    #1
    Join Date
    06-27-2012
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    Albury NSW Australia
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    Hello Mr Lee, I've folowed your column for 30+ years and been a modeler for 48 years. Thank you for your generosity of expert knowledge that you have given modelers during your illustrious career. You're a LEGEND.
    I'm in the process of designing a horizontally opposed twin 40cc gas engine based around two 20cc cylinders and piston assemblies, used in single pot gas engines, that I can buy from China. These engines will be simultaneous firing, rear pull start cooling fan/flywheel and specifically designed with front clutch/pinion assemblies for Gasser conversion of .90 size or 700 size electric RC Helis. I intend CNC machining crankcases and crankshafts to suit these head/piston assemblies. Induction will be via crankcase. I'm chosing this style of engine because the current gasser conversions use Zenoah engines which have serious vibration repercussions in our helicopters.
    My questions are ; What purpose do the flywheel counterbalance weights perform on one of these Boxer style engines ? If the opposing rod/piston assemblies go close to balancing each other out then why do similar Boxer engines, used in RC aircraft, have crankshaft counterbalance weights ? Is it to provide the reciprocating momentum without the associated torque stresses of the existing flywheel at the end of the crank ? Or because they don't have such a flywheel ? Can I design my engine without crankshaft counterweights ?
    kind regards, Andy (from Oz)
     

  2.  
    #2
    You will find a V-8 uses crankshaft counterbalancing between the sections where the con-rods attach and bearings.

    You have to balance near each con rod or the shaft will wobble and the engine fails fast.
     

  3.  
    #3
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    06-27-2012
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    Thanks. Sure but a V8 isn't nearly as naturally balanced as a Boxer. There isn't a piston/rod assembly moving close to 180 degrees opposing each piston/rod's travel.
     

  4.  
    #4
    Still needs to be balanced at each con rod...

    go ahead and try without. You'll have a very short engine life.
     

  5.  
    #5
    Join Date
    08-19-2002
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    Wichita, ks
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    My $0.02; What fhhuber seems to be saying to me is that you may have a smooth-running engine externally, but the forces on the crankshaft internally will cause deflections outside of a useful range. You need the balancing to cancel out forces and moments internally so that it doesn't run out of square internally.

    -Eric
     

  6.  
    #6
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    Can somebody explain the 'out of square' forces and how the crankshaft counterbalance weights reduce 'wobble' when there are opposing forces in a Boxer ? Any engineers here ? Do all Boxer engines have crank counterweights ? Does Clarence reply here ?
     

  7.  
    #7
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    08-19-2002
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    Wichita, ks
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    Haven't seen any replies from him in a long time, but I haven't exactly been watching either. As for my earlier explanation, I can see why it wasn't clear. What I meant by "out of square" was that the internal forces would cause deflection in the crankshaft, causing the crankshaft pin to be something other than parallel to the major axis of the crankshaft or perpendicular to the piston's direction of travel. This would create uneven loading on the connecting rod as well as the crankshaft pin and create extremely uneven wear, high pressure points, rapid buildup of heat and very poor lubricity at the contact patch.

    http://d5otzd52uv6zz.cloudfront.net/...723450-800.jpg Subaru Boxer crankshaft, each crank pin is in double-shear to avoid a bending moment on the crankshaft due to rotational forces and is counterbalanced to reduce the unbalanced "centrifugal force" loads generated by the connecting rod onto the crankshaft's rotation axis (not to mention preventing odd side-loading on the crank case bearings).

    The engine would likely seem to run smoothly without counterbalances, but would be unbalanced internally, and while it would probably work at low speeds if everything was light enough, I don't see how it would last very long at moderate or high speeds.

    I don't have the history of boxer engine design though; and there's always a chance it's never been tried and it may work just fine forever. It just intuitively makes sense that you'd want internal balancing, and if you follow the loading, it makes sense.

    If it's not too much effort to build it go for it! I'm sure you'll see exceptional throttle response if it works!

    -Eric
     

  8.  
    #8
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    Thanks for taking the trouble to explain it Eric but I'm a bit thick and I still can't visualize what you're telling me. Maybe it'd take a vector diagram. So I can't actually see how a crank counterweight could reduce or square up forces imposed by the rod. In fact the inertial resistance of the counterweight would increase the off-centre moment (force ?) imposed by the conrod.
    I'm not sure what your qualifications are but we probably need an engineer with internal combustion design expertise for this.
    You haven't fallen into the natural human nature knee jerk reaction of justifying the status quo have you ? Like "it must be correct because it's done that way" so I'll try to explain it.
     

  9.  
    #9
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    I don't have the internal combustion design experience, but I am a mechanical engineer.

    I have trouble believing anything that anyone says; I always have. That's why I am what I am. It makes sense in my head but describing with words and text is difficult. My intuition may also be incorrect, but it makes sense to me.

    You can always build it and see what happens! Innovation comes from people taking risks on things that "won't possibly work," and we get things like the Rotary engine. Never know, you might make some sort of breakthrough that will revolutionize horizontally-opposed engine technology.

    I wonder what a DA150 crankshaft looks like? I don't have time to look it up (running late to work now LOL), but I'm curious how those are counterweighted...

    Good luck!

    -Eric
     

  10.  
    #10
    Under enough applied force metal can perform somewhat like a rubber band. It bends and twists and stretches.

    The further apart 2 forces are applied to the same piece of metal the more likely you are to see the bending, twisting and stretching.

    Apply these deformations rapidly and repeatedly to metal and you will cause brittle fracture. This is the mechanism of "metal fatigue"
    Bend a piece of music wire back and forth a few times and it snaps.

    The counterbalances are more to reduce the distance between the forces on the crankshaft than to make the engine feel smooth. You can make it feel smooth by putting a balancing flywheel on out at the far end of the crankshaft.
     

  11.  
    #11
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    Someone else please give me an explanation.
    My understanding of physics doesn't make sense of "counterbalances are more to reduce the distance between the forces on the crankshaft"
    Thanks but No more please fhhuber.
     

  12.  
    #12
    Join Date
    08-19-2002
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    Wichita, ks
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    I don't think we're going to make much more progress here unfortunately. You might want to check back occasionally just in case someone does reply though. There's just not a whole lot of activity on these forums when it gets this in-depth as of recently. It's been a while since there have been loads of members who have built engines from scratch. We'll get back there eventually but for now, it's going to be hard to get a bunch of people on the subject who are qualified to discuss the topic.

    I would suggest trying to find a full-scale engine forum, maybe even a subaru forum where there are some engine builders who can either confirm or disagree with what you've heard here. And there's always the option of "see what happens."

    I know what we've said isn't what you wanted to hear, but it's likely you have two very well-educated opinions/estimations on the subject.

    Good luck!

    -Eric
     

  13.  
    #13
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    Wichita, ks
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    Let's keep it civil guys...

    Do you have a link to the other thread you started? I'm curious...

    You can P/M me if you'd rather.

    -Eric
     

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