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IndustryArena Forum > Hobby Projects > I.C. Engines > 2-stroke ported OPOC engine (125cc x 2)
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  1. #41
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    In most piston ported designs the piston skirts are not "spring-loaded". They are a close clearance sliding fit. The skirts act as a "guillotine" valve.

    I have played around with a cam/crank design myself (back in the 1960's). Gave up. Then the 2 parallel rods, opposed piston, Scotch crank w/slider block. Gave up. Ford came out with a cam/crank truck engine design. Never showed up in production.

    The I.C. engine @30% efficiency leaves a lot of room for improvement, have at it.

    Dick Z
    DZASTR

  2. #42
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    Quote Originally Posted by RICHARD ZASTROW View Post
    In most piston ported designs the piston skirts are not "spring-loaded". They are a close clearance sliding fit.
    i did try telling my friend and associate that! i will now kick him into submission with what you've said

    The skirts act as a "guillotine" valve.

    I have played around with a cam/crank design myself (back in the 1960's). Gave up. Then the 2 parallel rods, opposed piston, Scotch crank w/slider block. Gave up. Ford came out with a cam/crank truck engine design. Never showed up in production.

    The I.C. engine @30% efficiency leaves a lot of room for improvement, have at it.

    Dick Z
    thanks richard. revetec's design manages a dramatic improvement: they got around 0.2L/kW/h and that was with the independent testing station refusing to allow it to run at the leaner mixtures and higher compression ratios. so it can be done.

  3. #43
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    well, the pistons arrived: uuu, good call there. they're simple standard 70mm "big bore" scooter 2-stroke kits, and their use has saved me quite a lot of time, effort and messing about - so thank you!

    the next phase is to create a pair of piston underside chambers, one each side, that can take the piston rods through to the cams, and also will be happy to have a carburetor slapped on the side of each. as i can actually do CAD/CAM stuff, just in a rather weird fashion by using the python programming language and a software package called pyopenscad, i think what i will do is make the chamber out of stacked 15mm plates. laser-cutting should do the trick, with the possible exception of the piston rod holes (for accuracy) because they'll need a slipper bearing.

    once i get round to it i'll post the DXF files, for anyone who's curious, to play with.

    l.

  4. #44
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    Hope the pistons work for you, but remember they are for an engine with around 9:1 or 10:1 C/R. And yours will be around twice that. So maybe the piston head thickness and wrist pin bosses may fail. But should be good for a start.

    Please keep posting your progress!

  5. #45
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    Quote Originally Posted by packrat View Post
    Hope the pistons work for you, but remember they are for an engine with around 9:1 or 10:1 C/R. And yours will be around twice that. So maybe the piston head thickness and wrist pin bosses may fail. But should be good for a start.

    Please keep posting your progress!
    will do. i'm expecting some spectacular youtube videos as well as some successful ones. but yes, first i'm going to keep the C/R at the same, just arrange the cams to give a longer dwell time at TDC. also, the exhaust outlet on the cylinder is far too big (for a bourke cycle), and is far too low down the cylinder wall (ditto) - but.... we'll just have to see what happens. should be exciting, whatever it is!

    am currently looking on ebay for "magneto rotor ignition kits"... y'know, i feel like someone should be telling me i'm cheating, or that it shouldn't be this easy to get all these parts off-the-shelf, but i'd tell them i'm not into self-punishment and want to get to the bit where it goes "bang". (preferably a lot, rather than just once...)

  6. #46
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    You might be better starting with battery/coil ignition rather than magneto?

  7. #47
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    Quote Originally Posted by UUU View Post
    You might be better starting with battery/coil ignition rather than magneto?
    yes - i didn't want a dynamo (aka magneto) but i'd heard about these magnetic-triggered relays, to which the coils are connected (and normally a magneto on the other side). apparently they're quite common on motor mowers

    so i was going to do:

    * battery ->
    * magnet-fired ignition relay ->
    * ignition coil ->
    * spark plug

    the alternative is a home-made distributor, but i'd kinda prefer to use off-the-shelf parts (they're cheap and quick). with a static ignition relay suspended above one of the gears i can tape three magnets to it (there's three points on the tri-lobe cams, duh). ok, per side. two pistons.

    so that's what i _was_ going to do: can you think of a better/simpler one? i thought about optical triggers but that means i have to make them

    l.

  8. #48
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    I had a four cyclinder motorbike where one coil fed two cylinders. They were wired in series, so the electricty flowed from the metal of the engine through one plug, through the coil, then back to the other plug.

    Advantage was half the number of coils required - ( and half the number of triggers this was a fourstroke, with the trigger on the crank rather than the cam and fired once every revolution for each pair of cylinders - but this avantage wouldn't apply to your two-stroke).

    Disadvantage was you got a spark at every TDC, even between the exhaust and inlet stroke.

    Applying the same logic to your design you could fire the spark at both ends of the cylinder pair at the same time, even though only one end is on compression. And therefore need fewer coils.

  9. #49
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    allo uuu,

    ok, hmmm, what do you think: the air-fuel mixture is going to be rushing in just as the exhaust is being cleared, all ports are open, and the spark fires. i'm a bit concerned about that. i don't mind having 2 sets of magnet-controlled ignition relays: there are 2 gears, one on each side, so they can be mounted either side, that's not a problem.

    eventually this will be converted to diesel so it then becomes moot, but i have to find glow plugs about the same size as the sparks, with a 14mm thread diameter, to do that. and mess about with the cylinder head (probably weld and then grind!) to increase the compression ratio. ahh what fun...

    [update:] http://www.dlenginesaustralia.com/dl...g-adaptor.html

    ah ha! i have a cunning plan! ha. that, also, would solve the problem of the (standard car) diesel glow plugs sticking into the cylinder and bashing the top of the piston head. a shorter, smaller R/C glow plug is of course designed for a much smaller cylinder.

  10. #50
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    The easiest ignition I've found is the GM HEI module. Here is a good link for adapting them
    General Motors HEI Ignition Module For Points
    In this type of use you only use points as a switch, they carry no real current like points and coil do.

    I have a couple of Pan Head Harleys that I used them on and ended the point problem .

  11. #51
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    I see your point with the spark at the wrong time.

    To increase the compression ration, could you make up a circular disc/plug to fit in the top of the cylinder, attached to the bottom of the head? With a hole for the glow plug to stick into, this would reduce its projection into the space and ease your clearance problems. It would give you the opportunity to experiment with different ratios, by having a selection of disc/plugs, much like some model engines have a counter-piston.

  12. #52
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    How about externally adjustable compression ratio? www.psipowerinc.com/heads.htm

    I realize these are two-stroke snowmobile heads. I just intended to bring forth the idea.

    FYI, a 340cc two-stroke, naturally aspirated snowmobile engine in race trim on gasoline fuel produces~115 hp (86kw). Thats 338hhp(253kw)/litre

    Dick Z
    DZASTR

  13. #53
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    lkcl FYI DeltaHawk Diesel Engines

    Dick Z
    DZASTR

  14. #54
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    Crank case

    ok, well just an update: i've done the DXF drawings for the crank case that these big-bore kits will bolt onto. i used python (!) and a module called sdxf.py and i've created the crank case as 4 "slices" out of 12mm thick 100x75mm mild steel which the bolts will go through and sandwich onto the 70cc cylinders.

    pictures of the 4 DXF drawings are here: http://lkcl.net/engine/ - i will put the python program up online as well at some point, including the one that generates the trilobes.

    in theory i could actually make the entire cylinder this way, simply by making more slices, then putting in a steel liner to hold them all stably together.

    oh, one other thing: i found some 14mm brass adapters that can take the tiny RC-model glow-plugs. i figured that as this needs to be diesel, to start from there straight away. the reason for using tiny glow-plugs is to ensure that they don't protrude into the cylinder, as the piston is going to be right up against the top.

  15. #55
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    @IKCL, you don't really mean it literally when you say, "as the piston is going to be right up against the top." You need a small space there for combustion to take place and to keep from having the pistons making physical contact with the heads.

    The GM ignition modules will work with optical sensors or hall sensors also, not just points. Forgot to mention that in the last post.

  16. #56
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    Quote Originally Posted by RICHARD ZASTROW View Post
    FYI, a 340cc two-stroke, naturally aspirated snowmobile engine in race trim on gasoline fuel produces~115 hp (86kw). Thats 338hhp(253kw)/litre

    Dick Z
    Before MotoGP switched to four strokes the 500cc naturally aspirated, two stroke, V4 engines were similarly producing almost 200 hp, or 400hp/L.

  17. #57
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    Some people may have realized that some newer cars are said to be using the “Atkinson cycle,” especially on hybrids. The key characteristic of the Atkinson cycle is finishing all four “strokes” of an engine cycle with one turn of the crankshaft, which is more cost-effective. The Information on the Atkinson cycle was made clear. The goal of the modern Atkinson cycle is to allow the pressure in the combustion chamber at the end of the power stroke to be equal to atmospheric pressure; when this occurs, all the available energy has been obtained from the combustion process. For any given portion of air, the greater expansion ratio allows more energy to be converted from heat to useful mechanical energy meaning the engine is more efficient.

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