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Rockerblock I.C.--Something a little different-

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  • #31
    The whole of it looks compact and muscular ... very nice visually.

    I think Brian may have taken upon himself the implementation of a fixed crank in this model.

    Norman

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    • #32
      Originally posted by Paul Alciatore View Post
      Taking your points backwards:

      "...drastically altering the compression stroke piston speed in comparison to the power stroke piston speed...". What is the potential advantage here? And what would you want? Faster compression and slower power stroke or the opposite? And WHY?????
      The question "why" is the exact reason, just would be fun to play with and find out, a simple crank main holder that's rectangular yet with a hole closer to one end and can be flipped on both sides to experiment once the thing is up running and tuned - is there an advantage with the burn rate of the fuel used?
      it's just a hobby engine - the more versatile the better to have fun with.

      "...you no longer have to put the piston skirts through the radical loading that's accompanied by the crank angles...". Just what are the piston skirts? How are they "radically loaded in other designs? And just how does this design decrease this radical loading?
      this engine just has a rocker attached to the "lower end" con-rod, what this means is all's it has to do is go through a light arc of travel deviant from the linear piston,,, the loads do not get off-set to one side of the piston or the other,,,

      with a conventional crank design this is not so - when the crank reaches the 90 degree mark the rod is at a great angle, there is great side loading of the piston in the compression stroke and far greater yet in the combustion stroke,,, you can offset a little of this simply by going a longer connecting rod - but you can't go to long - it not only makes an engine structurally weak - it adds to the complete sizing and weight as the crankcase has to accommodate it all.

      Brians design loads piston skirting less than conventional without having to compromise on engine size and/or strength...

      Finally, I took AK's remark differently. I thought he meant that parts that were oscillating in opposite directions helped the balance. So they weren't just nice to look at, but functional. Hence, the two connecting links in this design could be in almost perfect balance both left-right and up-down.
      the two connecting rods can never be in perfect balance due to one being allot higher than the other - hence my comment about "block-rock"
      But - they are at least trying to cancel each other out as far as the reciprocation --- so yeah that's a good thing and also why I stated that all's he's really going to have to try to counter with the crank weights is the piston itself - again unlike conventional which has to counter piston and rod,
      but in doing so he has to move his cranks weights opposite of a normal conventional, that's what the rocker effect does.

      I guess my main question is just how is this design an improvement over the standard piston-link-crankshaft design? You got some splaining to do, Lucy.
      Like Bri just said - it's a hobby engine - not out to use it in formula 1 - but it has some unique advantages that are worth mentioning, also has some disadvantages too like typically more moving parts.

      it's going to be a cool hobby engine though cuz some are so standard they are not even worth the effort in my opinion - this thing is not standard at all...
      Like where you put the tank Bri.

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      • #33
        Originally posted by bob_s View Post
        Brian:

        Suggest that you rotate the exhaust pipe /muffler to vertical.
        Not so, Bob. I used to do that.--But--I run a small amount of two cycle oil with the fuel to lubricate the Viton o-ring on the piston. Unlike two cycle engines, these engines never run hot enough to burn the oil, so there is always a nasty mist of carbon suspended in miniscule oil droplets coming out of that exhaust. When it points straight up, the engine is filthy in no time at all. I much prefer to have it shooting off to one side.---Brian
        Brian Rupnow

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        • #34
          It seems to me the engine would be more rigid if the ends came all the way across the width and the sides were inside the ends.
          Then the assembly bolts would be in tension rather than shear and the ends of the side plates would brace against any push/pull forces. Then again, maybe it doesn't matter. Just a suggestion.

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          • #35
            Originally posted by brian Rupnow View Post
            I almost got caught. Look at the creative carving I had to do on the bottom of that water jacket to clear the revolving crankshaft.
            I'm just rambling below. Not saying anything is wrong with your design, it should work fine. Sometimes my mind just wanders and I let it go thinking what if....

            I look at the above picture and all sorts of thoughts come to mind about piston speed and dwell times at the top and bottom of the bore.

            With the upper piston and rocker arm near the same plane the rod angle will always cause a bit of down force on the piston. Raising the c/l of the piston will change these forces. Changing the the relation of the connecting rod to the rocker pivot, by using a straight rocker, will have an affect. Now the rod end on the rocker moves from a high to low point. Lengthening the piston rod and moving the upper rocker point to above the rocker pivot one could have the piston rod end move in an arc, low to high and back to low. Low at each end of the stroke and high mid stroke. Moving through this arc would keep the side forces on the piston minimized

            Having rocker to crank connecting rod at such an angle definitely has some strong effect on piston speed throughout its stroke. Not sure where to take this idea other than lower the crank c/l to be somewhat even with the rocker connection point. As it is its like a crank shaper drive, slow cutting and fast return. Depending on which direction you turn this engine your intake/power timing is going to be different than your compression/exhaust timing. I THINK. I don't know if your cad system has motion but it would be interesting to check.

            On edit; I THINK this is going to affect your cam timing also...

            I'm pondering what might be the affect of a rocker arm that was not a 1:1 ratio. What if the piston had a 1" stroke and the crank had an effective 2" stroke and the rocker arm was a 2:1 ratio.

            lg
            no neat sig line
            Last edited by larry_g; 04-07-2017, 08:53 PM.

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            • #36
              Originally posted by Toolguy View Post
              It seems to me the engine would be more rigid if the ends came all the way across the width and the sides were inside the ends.
              Then the assembly bolts would be in tension rather than shear and the ends of the side plates would brace against any push/pull forces. Then again, maybe it doesn't matter. Just a suggestion.
              That might be a factor of simply wanting to leave things exposed - Bri's not going to be pulling people out of ditches with this engine, it's a hobby engine and most of the reason it's being built is to one; make it run, and two; have fun watching it run... the more moving parts exposed flailing about the better...

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              • #37
                A.K.Boomer--I've been trying to wrap my head around your concept of reversing the crankshaft counterweights. I have three books here that deal with balancing of small engines, and they are all rather vague. In a vertical cylinder engine, if you remove the piston rings and assume no friction whatsoever, then when the piston is at mid stroke, the counterweights on the crankshaft should, theoretically balance the piston and connecting rod perfectly so that the piston doesn't fall any farther under the influence of gravity, and so that the weight of the offset in the crankshaft doesn't cause the piston to raise back up in the cylinder due to gravity acting on the offset weight of the crankshaft throw.--When you have a horizontal cylinder, then a lot of that reasoning goes out the window, because gravity isn't really acting to move the rod and piston one way or the other, although it still acts on the offset weight of the throw on the crankshaft. Indeed, the biggest job of balancing that the crankshaft counterweights have is to balance the con-rod journal and the portion of the "throws" that support it on the far side of the crankshaft centerline. In a perfect world, if you set the crankshaft alone on a pair of knife edges, it should be perfectly balanced with no tendency to roll one way or the other because of a "heavy" side. If there is a heavy side to the crankshaft, or due to the weight/drag of the piston in the cylinder, the twin flywheels should compensate for that. The fact that the engine is intended to be a "low speed" engine also does a lot to nullify any great "out of balance" issue. The bottom line is that I don't really know. I'm just running my mouth here. I know that the counterbalances make the crankshaft "look better" to my eye anyways. I think in the end the only way I'm going to know is to build it and run it, and then perhaps I'll know. I always bolt my engines down before I fire them, so even if it is out of balance, it won't run off the end of my reference table and fall on my foot.
                Brian Rupnow

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                • #38
                  I ended up having to angle both the exhaust and the carburetor out 12 degrees from centerline to clear the gas tank. I could have squeaked by with leaving them "in line" but it would have been severely crowded, and since I have to make the yellow "elbows" anyways, I might as well do this and buy a bit of room.
                  Brian Rupnow

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                  • #39
                    Im going off of typical RPM engines and am just doing some guestimation thinking with this one,

                    maybe you could leave the crank full circle with plug cut outs like a two stroke, except use four cut outs instead of two - two on one end and two on the other - try two with a solid steel insert and empty on the other end and then reverse - see what works best, don't know about the aesthetic appeal though and im with you I like the looks of the conventional counterweights on a crank like the one in your pic,

                    there's an idea though, just the crank throw alone might be enough offset to counter the piston (if my original thought of having to go 180 degree's from conventional hunch is right) then you would really have an old style looking crank as many did not even counter balance "back in the day" might work good and look appealing too.

                    Bri --- looks BETTER with the angles of the intake and exhaust - and like you did it intentionally as an old hot-rodder,,, don't want to suck up any of that exhaust on the next intake stroke, so get it the hell out of there and go for the cold air intake bennies along with the added oxygen
                    Last edited by A.K. Boomer; 04-07-2017, 10:15 PM.

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                    • #40
                      Has anybody used this type of con-rod connection at the crankshaft? I'm pressed for room where my rod attaches to the one piece crankshaft, and this would get me out of trouble. I think the actual hole for the rod journal would be the last step in the fabricating operation so everything would run true. I haven't used this before, but there's a first time for everything.--Brian
                      Brian Rupnow

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                      • #41
                        Originally posted by brian Rupnow View Post
                        A.K.Boomer--I've been trying to wrap my head around your concept of reversing the crankshaft counterweights. I have three books here that deal with balancing of small engines, and they are all rather vague. In a vertical cylinder engine, if you remove the piston rings and assume no friction whatsoever, then when the piston is at mid stroke, the counterweights on the crankshaft should, theoretically balance the piston and connecting rod perfectly so that the piston doesn't fall any farther under the influence of gravity, and so that the weight of the offset in the crankshaft doesn't cause the piston to raise back up in the cylinder due to gravity acting on the offset weight of the crankshaft throw.--When you have a horizontal cylinder, then a lot of that reasoning goes out the window, because gravity isn't really acting to move the rod and piston one way or the other, although it still acts on the offset weight of the throw on the crankshaft.

                        Been thinking about what you wrote here, although not dynamic I think its your best starting point by far,
                        piston should not even have any oil on it, and rods and all pivots a very light lube but should be enough to show what's off-set weight wise, even if you kinda have to see the coasting effect after light movement in each direction.

                        I would not be concerned that you have a horizontal beast, just stand it on end vertical for the balancing,,,

                        there's one slight balancing flaw that you will never be able to correct unless you threw in strategically placed balancer shafts, and that is the fact that the piston is at a higher horizontal running level than the crank, so again you will get "block-rock" as a secondary out of balance,,, don't think its going to ruin your day...

                        Now maybe it's easier to see the crank balance being reversed on this engine Bri, stand it on end like your talking and use gravity as your guide, you will see how the rods cancel each other but put the piston at half stroke and the crank at 90 degree's and you should see how conventional throws on the crank will actually "help" the piston coast through it's stroke, not good im thinking... ----------?

                        Edit; your first diagram is the best to see this, stand engine on its head, both the piston and the cranks counterweight will end up at the lowest point... the imbalance does not get neutralized - it gets compounded.
                        Last edited by A.K. Boomer; 04-08-2017, 11:52 AM.

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                        • #42
                          Originally posted by brian Rupnow View Post
                          Has anybody used this type of con-rod connection at the crankshaft? I'm pressed for room where my rod attaches to the one piece crankshaft, and this would get me out of trouble. I think the actual hole for the rod journal would be the last step in the fabricating operation so everything would run true. I haven't used this before, but there's a first time for everything.--Brian
                          I can't see reason why not to use it on a very lowly stressed display model engine. F1 or supercharged dragster engine and my opinion would be different.

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                          • #43
                            Dang, this thing is like a big chocolate cake!! All the pieces look so good, I don't know where to start. The beauty is in the simplicity. I shouldn't get so excited about my own work. Every time I do this I swear to myself that I'm going to take it easy, there's no rush, I'll just work on it in my spare time. Then it takes over my life and making another part is all I can think about. Good wife is away at her job in the local library, and won't be home until 3:00, so I think I'll make gears today. Gears are always fun. A bit intimidating, but fun, nonetheless.
                            Brian Rupnow

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                            • #44
                              Re balance
                              I tried to do a diagram of the first harmonic only, showing balance weights at the position of Brian's drawing,
                              and also per Boomers suggestion.
                              I think in one position there will be forces on the block to oscillate more horizontally and less rotationally
                              ( eg slide back and forward on a table)
                              And in the other position there will more of a couple on the block to oscillate more rotationally and less horizontally.
                              https://app.box.com/s/pi28mmudjct9co8o3yyifglo17qo0xvj
                              I don;t know which one is better, maybe it does not matter.
                              Minimizing for first harmonics would need a calculation using all the weights, their centres, and the dimensions.

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                              • #45
                                It's looking cool Bri like Norm said very "robust"

                                Full agreement Wombat this things going to have issues either way, What i was calling "block rock" you calling rotational so were on the same page with that... I think it the lesser of the worry due to the short distance between crank and piston.

                                Static balance is one thing - dynamic is an entirely other,

                                even the best engines are never perfectly balanced - it's all a compromise, take crank counterweights, they are actually full on oscillating - not exactly what you want to counter piston reciprocation - but they do match up for positive benefits twice per every revolution and they also counter the big end of the connecting rod some,

                                also with you as far as possibly not being much of an issue regardless as I don't think it's going to be running at crazy RPM's

                                Bri's built enough of these little guys to know if some have danced across the table when running or not, maybe it's never even been an issue for him ---- ?

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