Opposed two cylinder engine question

Discussion in 'General Engine Discussion' started by Jake2465, Mar 18, 2018.

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  1. Mar 18, 2018 #1

    Jake2465

    Jake2465

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    Hi everyone,

    I would like to start a summer project where I build a two cylinder engine that is horizontally opposed and uses off the shelf components to help make the build easier. Things such as:

    Two 6hp 2 stroke chainsaw engine cylinders with their matching pistons and rods.

    A diaphragm carb and a reed block to go with it.

    Ball bearings for the crankshaft.

    I believe most everything else will need to be fabricated. The idea would be to buy a couple pieces of aluminum billet and machine out a simple crank case for the cylinders to be mounted to. The crankshaft would allow both pistons to be on the same cycle during operation so I can get some nice scavenge action in the case and hopefully get the carb to come alive and provide fuel/oil to the case and cylinders.

    My question is if there are already plans out there for this kind of engine? Something tells me that it would be unlikely that this has not been done before and plans are not in existence for it.
     
  2. Mar 18, 2018 #2

    Brian Rupnow

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  3. Mar 18, 2018 #3

    lohring

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    How about an alternate firing 180 degree twin. It would need a balance shaft but could achieve nearly perfect balance with it. Otherwise there are ready made inline twins. All these 26 to 30 cc cylinders can be modified to give over 6 hp each with suitable tuned pipes. This is a parts diagram for the single cylinder, water cooled Zenoah that is raced in model boats. This is a single cylinder billet crankcase.

    Lohring Miller

    Crankshaft Assembly (2).jpg

    ScottsZenoahMods%Cylinder.jpg
     
  4. Mar 18, 2018 #4

    Jake2465

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    With that kind of engine, would the carb still be mounted to the crank case so oil can lubricate the bottom end?
     
  5. Mar 18, 2018 #5

    MachineTom

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    The prior photos are a for a piston ported engine. The are no reed valves, The fuel charge enters the crankcase as the piston nears the top of the stroke, then is compressed as the piston moves down, shortly before BDC the piston uncovers the charge ports, and the fuel charge enters the cylinder as the burnt charge goes out the exhaust.

    In a reed valve engine, the fuel charge enters the crankcase directly through the reed valves as the piston is moving up, then as the piston moves down it is compressed,until the charge ports are uncovered, and the charge enters the cylinder.

    Both arrangements lube the bottom end of the engine. To work a multi cylinder 2 cycle engine must have either all pistons move up and down together, or the crankcase has to be divided with seals between each cylinder, and crankshaft bearing, to seal the charges for each cylinder. And a carb for each cylinder. The prior photos show the seal in the center of the crankshaft, so it can be alternate firing.

    Those old seals were the death of many a motor, as the cranks had to be pressed apart, then back together. Many manufactures just sold complete crankshaft assemblies which cost big bucks.
     
    Last edited: Mar 18, 2018
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  6. Mar 19, 2018 #6

    Jake2465

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    All of what you said makes a lot of sense. For simplicity sake, I thought that I would just build a horizontally opposed twin cylinder engine that has the rod journals 180 out of phase to each other. This way a single carburetor could be used. The only problem is that since the crank is rotating in one direction, I believe the fuel charge will tend to favor one cylinder over the other unless the carb is offset to one side the case. I don't know if that would help.

    I can definitely see the benefit of letting each cylinder have it's own carb as they can be individually tuned.

    The only way I know where rods for two stoke engines can be added without pressing cranks together or having dedicated oil passages would be if special rods are made where they have needles that would need to be nested in the rod journals and then have the caps bolted over them. I don't know if anyone has done that before. Slots would have to be cut in the rods so the needles could get lubrication just like the standard weed whacker rods have. I feel like that would be good in theory but likely a real pain to get it to operate correctly without the needles brinelling the inner surfaces of the rod journals... Come to think of it, don't outboard two stroke boat motors have this kind of thing? Needle bearings for their rods?

    So, perhaps I should think about a two cylinder engine with each cylinder having it's own carb and forgetting about putting the carb on the crank case so I can ditch the reeds and save maintenance costs.
     
  7. Mar 19, 2018 #7

    petertha

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    That is quite a common platform for large scale RC, but you are right, 2-stroke twin plans don't seem to be very common. You could reverse engineer something like this.
    https://hobbyking.com/en_us/5-5hp-53cc-twin-cylinder-gas-engine.html?___store=en_us


    2-strokes seem simpler on the surface but replicating the cylinder gas ports that are commercially cast & cutting the intake/exhaust windows profile in the liner isn't exactly easy. Same for the crankshaft which also accommodates induction flow. So if you are buying the cylinder jugs, liners, matching head, maybe crank, maybe ignition system... put it this way - you wont be making it cheaper than $270 haha

    There are plans for 4-stroke opposed twins. Brian showed one, here is another.
    http://www.cad-jung-shop.de/epages/62479729.sf/de_DE/?ObjectPath=/Shops/62479729/Products/00-Z0012-0
     
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  8. Mar 19, 2018 #8

    Jake2465

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    Petertha, yes you are more than likely right about the cost. I think for me it is more for the experience than the immediate practicality. One thing I am noticing here is that the crankcases are pretty tight looking; the castings almost form fit the rotating crankshaft assembly. I would imagine that it would need to be that way so the pistons have more authority in changing pressures within the crankcase.

    I have this book from Gordon Blair; "Design and Simulation of Two-Stroke Engines". I have yet to read most of it as I have not yet gone through college level physics. Anyway, when I do get to that point, I am sure that book will be a wealth of information in helping me answer so many of the questions I have.

    Yes, I was thinking about buying off the shelf components and I would in turn machine out the crank case to suit whatever I was able to get a hold of. I may be able to machine out a multi-cylinder crankshaft, but if one can be had for a reasonable price, then it may be good to just go with that. Although, the throws on the off the shelf crank would probably limit me on what cylinders I would buy as it would have likely been made for a specific set.

    Like you said, I could definitely just shell out a few hundred and be done with it, but I don't think I would be learning very much in doing that. So, I guess I believe that having an applied approach to the theoretical stuff is important. What would be interesting is if everything in the engine was assumed to be done correctly, and in the end it won't fire off and run. Then what? Well, then I see that as a great opportunity to learn and problem solve to develop a fix :cool:.
     
  9. Mar 19, 2018 #9

    Jake2465

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    Are those plans only in German? Ugh! That must mean that the dimensions are in metric.
     
  10. Mar 19, 2018 #10

    lohring

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    If you're serious about engine simulation, look at EngMod 2T. It costs $400, but is worth every penny for serious students of engine design. Neels was a student of Gordon Blair and the simulation started from the ideas in the book. The simulation has been tweeked with help from some of the best modern two stroke tuners. It has been used to develop race winning engines in karts, motorcycles, and snowmobiles.

    Lohring Miller
     
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  11. Mar 19, 2018 #11

    Cogsy

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    You mentioned you were planning on studying college level physics didn't you? If so, you will get very familiar with the SI (metric) system as it is the language of science...
     
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  12. Mar 19, 2018 #12

    Jake2465

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    Yes :), that was meant to be more of a joke than anything else. However, I do machine parts and think in the imperial system when I see things. And, chances are I will never change. So, for calculations dealing with orders of magnitude and unit conversions within the metric system, I comply with SI :cool:. The used models and formulas are generally already setup to handle SI units, so I just use them and have no issue.

    With all that being said, I am personally more familiar with what, lets say, 150,000 BTU's feels like coming out of a heater, or what a 1/4 mile looks like down the road, or what .050" gap looks like in the jaws of my caliper.
     
  13. Mar 19, 2018 #13

    Jake2465

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    Thanks for the info :). I will definitely check that out.
     
  14. Mar 19, 2018 #14

    petertha

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    Like you said, I could definitely just shell out a few hundred and be done with it, but I don't think I would be learning very much in doing that.

    I was never suggesting that you simply buy a commercial engine when the fun & spirit of this particular hobby is in scratch building, modifying or whatever towards your own creation. If you already have 2 sets of chainsaw cylinder assemblies, then the rest of the design will necessary revolve around that. I just provided a link to one particular engine platform that is quite similar to your goal in many respects. So it might provide learning's by checking out the layout, parts manual etc. Also, if you don't already have the cylinders, this particular engine might be one of the least expensive ways to access specific parts in comparable HP you were contemplating.

    Like I mentioned, re-configuring a common cylinder/liner/head assembly into several different engine configurations has been going on for a long time in RC. Its worth a looksee.
    https://www.zdz.eu/inpage/zdz-360b4-j/
    https://www.dle-engines.com/dleg0040.html

    Happy building!
     
    Last edited: Mar 19, 2018
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  15. Mar 19, 2018 #15

    Brian Rupnow

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    In my opinion, two cycle engines are much more difficult to get running properly than four cycle engines. That seems like a contradiction, because two cycle engines have fewer parts. I find four cycle engines with camshafts and valves to be very forgiving for new builders.---Brian
     
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  16. Mar 20, 2018 #16

    XD351

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    Another thing with multi cylinder two strokes that share a common crankcase - they don't like big cavernous crankcases as this lowers the crankcase compression ratio and there is more air fuel mix flying around in there that will migrate to any dead spots giving fuel separation at low rpm .
    Also ensuring the port opening events are precisely the same for each cylinder becomes even more critical as you can end up with one cylinder running fast while the other starves . Even in commercially made cylinders there may be some difference cylinder to cylinder especially with cylinders that come off an engine that was a single cylinder unit .
    If you look around on the net for some exploded views of some old two stroke outboard engines you will see some use a split plain bearing ( usually part of the reed block ) to separate the crankcase cavities and there is no rubber seal just a few grooves cut into the internal bearing surface to retain some oil for lubrication and to help seal the two crankcases from each other.
    I know the really old mercury engines used this ( thunderbolt engine ) but newer engines may have gone onto something better like multi peice roller bearing cranks with rubber labyrinth seals .
     
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  17. Mar 20, 2018 #17

    Jake2465

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    That's a good point. I don't have any components for this yet.

    Those ZDZ engines look well designed. That reminds me of some people that had mentioned that DLE four cylinder engines had crankshaft problems. I guess the cranks had a tendency to come apart after some use.
     
  18. Mar 20, 2018 #18

    lohring

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    Two strokes are mechanically simple but dynamically very complex. The flows through them is just being modeled by CFD programs. The simulation I mentioned is a one dimensional simulation with empirical models for scavenging and combustion. It is very helpful for modeling the pulses through the various volumes and their connecting passages in an engine. I wrote a series of articles on high performance two stroke design on the NAMBA web site. This is the first one. If you are interested I can send you links to the rest.

    Lohring Miller
     
  19. Mar 20, 2018 #19

    Jake2465

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    Lohring, with so many dynamic variables acting on these two stroke engines, would that mean that one could only get ever closer to a optimum running engine and not perfect? I would imagine that even changes in something like density altitude would knock the engine off of its ideal gas pressures and result in HP loss?

    I would be interested in knowing more about it. Also, I do have interest in aviation, but more for full scale use instead of models. Would this kind of technology be scalable to multi-liter engines?
     
  20. Mar 20, 2018 #20

    petertha

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    Lohring is an expert but I think I can say 'perfect' is a never ending, perpetually moving target. Today's record breaker will be tomorrows broken record.

    Ambient density variation makes its way into several tuning factors throughout intake, power & exhaust. There is interplay & tradeoffs, you don't get something for nothing. A big part of high performance 2-stroke power comes from tuned exhaust - sucking intake charge out past the exhaust flange, but then reverse packing this charge back into the cylinder via the reflective wave, essentially 'boost'. But its all an orchestra of balancing many factors: compression ratio, timing, gas ports, distribution... the more you learn about 2-S the more you realize how complex they actually are.
     
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