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  1. #41
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    Apparently you haven't seen the inside of that engine...those carbs do not feed directly into the cylinders. That's a piston-valved engine. There are basically three ways to valve the airflow on a two stroke. The simplest and most common--especially with your weed wacker and MX bikes--is reed valves. The crankcase vacuum pulls thin flexible valves (reeds) open to allow the air in, and when the vacuum goes away, the reeds close. The little model Cox engines use these. Another way (which I believe was ruled out in MX racing because of its "unfair advantage" in the 70's) is the rotary valve. Basically at the end of the crank is a rotary valve that opens and closes to time the air coming in with the vacuum in the chamber. This is the most common system with RC airplane engines.

    The third way, and the method used on the engine you see here, is I think called piston porting. The skirt of the piston acts as the valve. When the piston is down, the port to the carb is closed--the intake does NOT go directly into the cylinder. (You can tell by how far away the spark plug is from the top of the intake track.) When the piston is up (and there's vacuum in the crankcase), the piston skirt clears the port to the crank, and air/fuel goes in. These are cheap, because there are fewer parts than the other two methods. Sometimes an engine can look like this type, but in fact have reeds, and not the piston valve timing--they use the air velocity of the crank to help open and close the reeds.

    I am not aware of any 2-stroke that has the carb intake going directly to the cylinder. You really need to get a book (or a half dozen) and read up on 2-stroke design if you want to build one. The $100 you spend on good books will save you many hours of frustration and failure trying to reinvent the wheel. There is an excellent article series in the strictly IC magazine. You really should invest before you start making chips.

  2. #42
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    I'd try a single cylinder first and then branch out. Put a "RFQ" in for a 2T barrel and cylinder complete with nikasil coating/cast iron liner and transfer ports and check the replies. The picture you posted is a piston ported design btw. Read the book that's been recommended a dozen times already. You'll get the hang of it.....Or just build a class 47 Deltic engine as a beginners project! :idea: As a side thought I just looked on ebay and priced an 80CC 2T... Crankshaft £65 Piston and barrel £109 21MM Carb £39 not much else needed...just a pair of cases (easily milled) and a spark really....I couldn't find any bags of sparks though!! :toot:
    Keith

  3. #43
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    PoWaKiD42, As explained above, if you want a V-4 naturally aspirated engine, the individual cylinders require seperated crankcases. The pictured example is a crankcase induction type. It could be reed valved.

    There is a certain ratio of crankcase to swept cylinder volume. Too much volume will produce low crankcase pressure which will produce low velocity for the fresh charge moving from the crankcase thru the transfers. This results in under filled cylinder. Too little cranckase volume will not provide sufficient fresh charge either because there just isn't enough fresh charge there.

    Just to be a pain in the A**, IT'S ALL IN BLAIR'S BOOK.

    This is not for beginners but, there are direct filling 2-stroke V-4 engines with oil filled crankcases. The air is supplied by positive displacement superchargers and usually are fuel injected. I believe I mentioned one earlier called the DeltaHawk aircraft engine. It is both supercharged and turbocharged and also happens to be a diesel. Runs on jet fuel instead of AVgas.

    Adobe, 1979 was a very good year. We still had high octane (113) leaded race gas. That let us run higher compression, tighter squish bands on pistons without pre-ignition etc. Ah, the good old days!!! They've been going downhill ever since. lol
    DZASTR

  4. #44
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    Smile elegant 2 stroke engine

    super 2 strokes are not all taht elegant...............

    the best way to build a really cool 2 cycle engine is to buy an engine say from ebay at a fair price...............

    this is what i did,

    i bought a C Z 250 the 72` model and machined a brand new motor from raw mat. and of course added all the exotic metals mag ect.
    the crank case has steel inserts in case where the crank bearings fit.

    i use a belt drive unit much like a snowmobile unit to drive my dirt bikes. no shifting ....... this makes for a light crank case.
    you can use any engine to do this with.......
    i have much more info on this subject...........
    i will wait for a responce !




    Quote Originally Posted by RICHARD ZASTROW View Post
    PoWaKiD42, You can make the ignition, fuel input, exhaust system as simple or complicated as your knowledge wiil permit. High output 2-cycle engines are "elegant designs". Far more complicated than apparent.

    You could use the programable electronic ignition, add electronic fuel injection and if you like, adjustable exhaust port timing. Add to that adjustable length tuned exhaust pipes to vary the powerband up and down the rpm range.

    Personally I'd keep it simple on the first one. Reed valve intake, magneto ignition with timing adjustment, tuneable carburetor like Mikuni and an exaust system with a wide powerband rather than high power narrow powerband.

    To repeat myself, try to get a copy of Blairs book (or other competent authority). Re-inventing the wheel is a waste of time and effort. All the port heights, tuned pipe geometry etc. is in there. Also, pay close attention to combustion chamber design (squish band size, clearance to piston dome etc.)

    Basic engine volume ratios (crankcase volume / swept volume etc.).

    Many important details involved. If you already have that knowledge ignore everything I said.

    BTW, that Sportdevices CDI looks good.

    Good Luck,

  5. #45
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    Quote Originally Posted by RICHARD ZASTROW View Post
    [In 1979], we still had high octane (113) leaded race gas.
    I used to buy 110 octane leaded at a corner gas station in the early 80's! Those were the days!

    I admire PoWaKiD42's ambition. He obviously is not afraid to spend a lot of time on something unknown. It would sure as heck be a huge dissapointment if he goes through all of this effort and his multi-cylinder 2-stroke fails to fire up! A more 'experienced' individual would likely take on a bit less challenging design to learn on - like a single cylinder or at most a twin.

    Good luck! And please keep us posted on your progress.

    Scott
    Consistency is a good thing....unless you're consistently an idiot.

  6. #46
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    I like the general design as you have it, but it won't work as a two stroke at all ! Not without going valves and forced induction. Then you'll have a potential grenade ! Maybe add some design work and make it a 4 stroker. The ignition isn't too hard using something like the "ch ignition electronics" & the system used on some of the big model radials like what are sold and displayed @ "Rc Showcase" What it all really comes down to is "The Journey" so keep on designing and enjoy it !
    Don
    IH v-3 early model owner

  7. #47
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    I just skiped through these. If you want a good cheap lesson in 2-stroke. buy a used weed eater and disassemble it and put the calipers to it. a lot of guys build their own rc engines. steel sleeves in alu. or steel billet block.

  8. #48
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    I used to race modify and build components for 2-strokes, and it really is sort of a black art. My first advice on scratch-building one would be, simply, don't. That being said here are a few observations.

    1. Cooling is actually easier than you'd think. The VAST majority of the heat is concentrated in the head. You usuallly get the most cooling fins present on the barrel in most designs only as that is where there is the most surface area. Its not because thats where the heat starts from. They run without a gasket between the head and barrel solely so the heat transfers down to the barrel and then gets dissipated from there. If the head is cooled directly, the barrel tends not to heat up at all.
    Liquid cooling a head is way easier than liquid cooling the block. Block cooling isn't usually necessary or even desirable.

    2. The power and efficiency is really made or lost in the head. The precise characteristics of the very subtle spark plug placement and profiles of the cavity there takes even the biggest manufacturers years to get right, even in production engines. You can't afford the lab to model it. Really.

    3. As pointed out before, half the action happens below the piston in the crankcase, whther it's reed, piston, or rotary valved. Primary compression is sort of important. You need to get the intake charge to start down there.
    Nice design work, but it is all for naught if you don't learn even the most fundamental theory of what you are attempting to design.

  9. #49
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    superrotary, FYI & no bad intended, when I referred to an "elegant design" it was intended as Websters dictionary describes it, "cleverly apt and simple". I suppose I confused it by saying "far more complicated than apparent". What I was trying to say is there is/are many simple yet not readily obvious details that are in those designs.
    DZASTR

  10. #50
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    no big

    Quote Originally Posted by RICHARD ZASTROW View Post
    superrotary, FYI & no bad intended, when I referred to an "elegant design" it was intended as Websters dictionary describes it, "cleverly apt and simple". I suppose I confused it by saying "far more complicated than apparent". What I was trying to say is there is/are many simple yet not readily obvious details that are in those designs.
    thank you dastard

  11. #51
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    Quote Originally Posted by PoWaKiD42 View Post
    Yes, i understand that... my one cylinder engine has that type of intake... crankcase intake... but i saw that some two stroke engines have intake directly in the cylinder... because it is a v4 i canot separate 4 crankcases... i can only separate 2, thats the case... the lubrification is done by the bypasses on the cylinder, that takes the fuel with oil to the carter, lubricating the engine.

    I atached a pic of a inline 2 engine with intake directly in the cylinder...

    Thanks

    Nuno
    The engine you have in your post doesn't have cylinder intakes. The piston is acting as a reed valve, exactly as on old 50cc motorcycles. The fuel is taken into the crankcase.
    There is no problem with running a two stroke V4, but the cylinders can't share crankcases because there will never be a pressure that force the fuel up to the head.

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