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Thread: DIY Two-Stroke Engine

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    DIY Two-Stroke Engine

    For the last few years I have toyed with the idea of building a homemade two-stroke engine for UL use. What keyed my interest was reading about homemade model aircraft engines and reading and watching a re-enactment of the Wright brother's first flight with a replica engine (not a two-stroke engine).

    Has anyone made a 2-stroke engine from scratch? One may need to cast aluminium, may need a lathe and milling machine with boring head and hone or perhaps the boring and honing of the cylinder and bearing journals could be farmed out. A commercial carburetor and piston could be used. Two-stroke engines seem simple enough that home construction may be possible, if not practical.

    Brock

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    Re: DIY Two-Stroke Engine

    I think there are some homemade engines in the Flying and Glider reprints that EAA sells, though that's old tech and I'm not sure if any are cratch built. And they're probably pretty heavy too. And they might be four strokes!

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    Re: DIY Two-Stroke Engine

    Building an engine is entirely feasible, but you really have to want to. Considering most two strokes are pretty cheap in airplane terms, doing it to save money makes little sense.

    A chap I once knew, Ron Webster, built several radials using nothing more than a WW1 vintage lathe and , he says, "the ability to file very very flat" !

    I always thought a two stroke direct drive would be a good thing to try to build and just to whet your appetite, have a look at this:

    http://www.flightglobal.com/pdfarchive/view/1939/1939%20-%201393.html
    "Aeronautical engineering is highly educated guessing, worked out to five decimal places. Fred Lindsley, Airspeed."

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    Re: DIY Two-Stroke Engine

    Thanks for the responses

    I have been meaning to get copies of the Flying and Glider Manuals. I'll definitely have to get them now that I learn that they have engine construction in them.

    Direct drive is what I had in mind as well. I agree with the article that a simple piston ported engine should work fine for the narrow rpm range needed to spin a propeller. Fuel efficiency isn't a great concern at low power and I was planing a fairly low compression to avoid kickback of the propeller when hand cranking and to minimize harmonic vibrations.

    Although I have heard that direct drive two-stroke engines are inefficient and don't work well at low rpm I have as of yet not heard information as to why that is true. I believe their is a direct relationship for rpm to power and their is no engineering sweet spot at a certain rpm. The reputation has largely to do with the fact that engine designed for higher rpm simple don't perform very well at low rpms. An engine designed for higher rpm losses about 50% of its power when forced to run at a 2/3 lower rpm. The loss is even greater if the engine has been designed to run with a tuned pipe. The result is that the exhaust ports are open too long reducing the effective stroke of the engine and some fuel-air charge may escape. A fair complaint is that at about 4000rpm the propeller is noisy which may be a problem for some but my flying field is in the country so this isn't really a problem for me. I may have to wear earplugs.

    If a 500cc dirt bike produces about 60hp, without a pipe may halve that value to 30hp, reducing RPM to around 4000 would result in about 20hp/cylinder. A single cylinder engine doesn't provide enough power for most application but an inline double or triple might and there is also the possibility on an opposed engine with 2 or 4 cylinders. Also the lower power makes air cooling possible with simple cooling fins.

    Personally a purpose designed (spin a propeller) direct drive multicylinder two-stroke engine appeals to me. Is their something I'm missing or is the market just too small to justify their existence?

    Brock

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    Re: DIY Two-Stroke Engine

    Quote Originally Posted by durabol View Post
    Thanks for the responses

    I have been meaning to get copies of the Flying and Glider Manuals. I'll definitely have to get them now that I learn that they have engine construction in them.

    Direct drive is what I had in mind as well. I agree with the article that a simple piston ported engine should work fine for the narrow rpm range needed to spin a propeller. Fuel efficiency isn't a great concern at low power and I was planing a fairly low compression to avoid kickback of the propeller when hand cranking and to minimize harmonic vibrations.

    Although I have heard that direct drive two-stroke engines are inefficient and don't work well at low rpm I have as of yet not heard information as to why that is true. I believe their is a direct relationship for rpm to power and their is no engineering sweet spot at a certain rpm. The reputation has largely to do with the fact that engine designed for higher rpm simple don't perform very well at low rpms. An engine designed for higher rpm losses about 50% of its power when forced to run at a 2/3 lower rpm. The loss is even greater if the engine has been designed to run with a tuned pipe. The result is that the exhaust ports are open too long reducing the effective stroke of the engine and some fuel-air charge may escape. A fair complaint is that at about 4000rpm the propeller is noisy which may be a problem for some but my flying field is in the country so this isn't really a problem for me. I may have to wear earplugs.

    If a 500cc dirt bike produces about 60hp, without a pipe may halve that value to 30hp, reducing RPM to around 4000 would result in about 20hp/cylinder. A single cylinder engine doesn't provide enough power for most application but an inline double or triple might and there is also the possibility on an opposed engine with 2 or 4 cylinders. Also the lower power makes air cooling possible with simple cooling fins.

    Personally a purpose designed (spin a propeller) direct drive multicylinder two-stroke engine appeals to me. Is their something I'm missing or is the market just too small to justify their existence?

    Brock
    I can' tell how often I heard that myth myself: "two strokes need to rev to make any power" - it's nonsense. There is no earthly reason a two-stroke running at, say, 2500 rpm cannot be efficient, or at least as efficient as two-strokes get. Slowing it down does only good things; scavenging becomes far more effective, heat is less of a problem.

    A simple 3 or 4 cylinder inline would make a great little engine of maybe 60-80 hp or thereabouts.
    "Aeronautical engineering is highly educated guessing, worked out to five decimal places. Fred Lindsley, Airspeed."

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    Re: DIY Two-Stroke Engine

    There's no inherent reason why a 2-stroke has to rev high to get power... except that's true for any engine. To get more power out of an engine you have to get more air and fuel through it. There are three ways to do it: make it bigger, compress the mixtutre (super- or turbocharge it), or turn it faster. Only the third comes "free", i.e. it doesn't add weight or complexity.

    Note that a tuned pipe is a means of supercharging. It can work on slower turning engines, too, but the pipe gets a lot longer (and heavier).

    Can you build a slow turning 2-stroke? Sure, but it'll be heavier than a faster turning engine of the same power... and you gain more performance by reducing weight than by any other means.

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    Re: DIY Two-Stroke Engine

    Describing a tuned pipe as a means of supercharging is pretty misleading, its purpose is to help the engine's volumetric efficiency by the use of a "tuned" megaphone (diffuser section).
    The 2-stroke is usually revved higher because it can, and very easily (no 4-stroke poppet valves)
    Transfer port and exhaust port shape & timing is very, very critical. A good analogy of this timing problem is imagine yourself driving your car, and getting up to speed on a high speed Interstate highway, and "smoothly" enter into the traffic. So, port area and length of the port and the engine displacement and port & piston timing is all critical.
    Don't do piston port intake, use a reed block.
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    Re: DIY Two-Stroke Engine

    An aviation engine for lower rpm direct drive should have a longer stroke, I think. Model airplane engines usually have a stroke about the same as bore.

    I don't know what the stroke is with snowmobile and motorcyles, what is used?

    Chotia built a 2-stroke for his ultralight that looked like a giant model engine.

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    Re: DIY Two-Stroke Engine

    The Wilksch WAM 120 is an example of a 2-stroke engine that makes great power at low RPM. Granted, it's a diesel...but it's an example nonetheless.




    Wilksch Airmotive Ltd

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    Re: DIY Two-Stroke Engine

    Quote Originally Posted by Dana View Post
    There's no inherent reason why a 2-stroke has to rev high to get power... except that's true for any engine.
    ...which was the point.

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    Re: DIY Two-Stroke Engine

    Quote Originally Posted by jumpinjan View Post
    Describing a tuned pipe as a means of supercharging is pretty misleading, its purpose is to help the engine's volumetric efficiency by the use of a "tuned" megaphone (diffuser section).
    Well, it is, sort of... you're forcing more fuel/air mixture into the cylinder than would otherwise be possible, though it's not (as far as I know) near the pressures possible with conventional turbo- or supercharging.

    [/quote]The 2-stroke is usually revved higher because it can, and very easily (no 4-stroke poppet valves)[/quote]

    True... but common 2-stroke aircraft engines don't rev any higher than modern 4-stroke car engines. Racing bike engines, of course, are a different matter.

    Don't do piston port intake, use a reed block.
    One wonders why more 2-strokes don't use rotary valves like model airplane engines, which give more precise control of intake timing.

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    Re: DIY Two-Stroke Engine

    Love a good 2 stroke, Bridgestone in the 60's and early 70's made motorbikes that were rotary valved and they were excellent compared to the competition. The big three Japanese makers made them agree to stick with tyres or they would cancel the OE tyre contracts. Rotary valving is very good, and deserves its place in the sun. A tangent on this can be a diesel two stroke like the Wilsch or you can get wild and do a rotating cylinder with integral drive reduction. Rotary cylinders work great as a 4 stroke. Look at the pics below This is a RCV model four stroke, with integral reduction drive that also does the timing. It is a very clever low part design that can be made ultra compact with ideal cooling. The parts count is excellent and much more two stroke like but with the ability to run high compression and better economy. It is ideal for a big single and other configurations can be multiple cylinders. With the piston in the same plane as the prop drive I expect the harmonics to be excellent specially with the rotating cylinder/prop drive mass spinning at 1/2 engine speed. Such a engine would run a two stroke mix or could be Diesel or have a pressurised oil system. The design would have very large bearing surfaces and very little wear. I have never seen such a design anywhere else but it is pretty wild outside the square. I now its not a 2 stroke but rotary valve got me thinking.
    Attached Thumbnails Attached Thumbnails DIY Two-Stroke Engine-rcv-engine.jpg  
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    Re: DIY Two-Stroke Engine

    I don't know if a direct drive engine will be that much heavier when you take into account the weight of the drive reduction system. I have calculated the weight of an 80x80mm bore and stroke 2 cylinder opposed engine and it was a bit under 40lbs which should give about 1hp/lbs. I used 10mm cylinder and crankcase wall thickness and a 1.25" dia crank.

    I have got some idea of port-time-area from the freeware computer program called "BiMotion". I'm not sure how good the data is for lowish speed engines but I guess it is a start. I have also worked up a spreadsheet for similar information.

    I don't think a reed valve system is needed for this engine since it is only going to operate at a fairly narrow rpm range and the port timing isn't critical. Piston ported valves offer similar performance to other induction types but only over a narrow rpm range which is what I have planned for the engine. I plan to build an engine with a restrictive exhaust to ensure no fuel escapes. I have heard that piston ported engines can spit some fuel out of the carb at idle but this doesn't seem like a major problem. Rotary valves via crank shaft induction (disk or drum valves as well) is an interesting idea but I don't think I need the critical timing they provide.

    I was planning on using the largest two-stroke piston (not a diesel piston) I could find and using the largest stoke that was reasonable, something like 90x105mm

    I have seen some charts that indicate over 15psi boost from a tuned pipe so it can definitely act to charge the engine effectively. Although a tuned pipe isn't very appropriate for the engine I have envisioned.

    Regarding the rotating cylinder engine pictures: I couldn't tell from the pictures but is that the configuration where the piston spins in the cylinder and acts like the crankshaft (kind of) to which the propeller can be attached. Am I right that this engine uses 90degree gears that make it look like a normal model engine with the cylinder in-plane with the propeller?

    Brock

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    Re: DIY Two-Stroke Engine

    The rotating cylinder motor-

    The piston has a crank that turns a bevel geared cylinder at half engine speed. The cylinder also has the ports, head and prop drive as one solid piece. As the cylinder rotates the ports are opened and closed. The cylinder as it spins acts as a flywheel as well.


    Very few and solid moving parts.

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    Re: DIY Two-Stroke Engine

    One of the guys at our RC field has a rotating cylinder engine (.60 size). It did not impress me, it sounded like a bunch of gear noise to me and had less than normal performance.
    The owner did not seem very enthusiastic either.

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