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IndustryArena Forum > Hobby Projects > I.C. Engines > 2-stroke ported OPOC engine (125cc x 2)
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  1. #1
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    2-stroke ported OPOC engine (125cc x 2)

    folks, hi,

    i've just joined this forum because i'd like to make an engine... from scratch, and whilst i am a software engineer, used to make my own lego model designs, did metalworking at O'Level at school and can weld (badly) i would greatly appreciate advice in keeping this as simple as possible, low-cost as possible and easy to do as possible.... and that includes the tools: i will be literally starting from scratch.

    here's where i'm keeping pictures and documentation on the design:
    Index of /engine

    the design is constructed from expired and invalid patents. it is a double-opposed piston arrangement, 2-stroke *ported* design - not one set of ports and a valve per cylinder but a *purely* ported design like the old steam engines. moving parts are kept to an absolute minimum.

    the compression ratio can go up to 30-to-1 (!) and i aim to use this not for power but for ultra-efficiency instead. the maximum RPM will be 2,000 (the original design used triple-sleeve bearings and could do over 15,000 RPM. back in the 1950s this was quite an achievement). i aim to make the cylinders each 125cc: 50mm bore and 50mm stroke, just because it sounds like reasonable round numbers (advice appreciated here).

    the only thing i have to watch out for is that the design uses "detonation" at TDC. i.e. the spark is fired at around 90 degreees *BEFORE* TDC, when the 30:1 compression is about 1/2 way along and has compressed the air-fuel mixture down to about a 15:1 ratio (which is enough to make it ignitable).

    by the time the burn gets to TDC it's so damn hot - over 1800F - that the air-fuel mixture goes into hydrogen-oxygen burn and it's pretty much all over, there and then, within a few degrees of TDC. this is *completely* different from a standard 2-stroke or 4-stroke, where the fuel is still burning when the exhaust valves open at BDC.

    so the only thing to watch out for is this rather hot flash at TDC, but even there i'm advised that it's over so quickly, and the decompression stroke is so immensely long (1:30, obviously) that the piston head hardly has time to even heat up, and the exhaust gas temperatures are (obviously, given the 1:30 decompression) down to something like 250F or less.

    also, if you look at the (rough!) image, you'll see 4 red blobs per piston: these indicate piston rings - one on each of a pair of piston heads. the set in the firing / exhaust chamber will need 3 rings; there is only one ring on the piston in the intake / mixing chamber. the pistons themselves can be incredibly flat, because there is *no* side-loading. at all.

    the only thing is that there must be an inner tube which seals (approximately: it does not have to deal with huge pressures) against the side-wall of the piston, to shut off all the ports in between TDC and BDC. a perfect seal is *not* essential. i figured that using a screw-threaded rod to join all three parts together, and lock the central tube down tight, would be sufficient. perhaps put grooves in each of the pistons for the central tube to fit into?

    anyway: advice on materials that are the simplest to use and work with, as well as some recommended tools, would be greatly appreciated. contrary to potential appearances, this isn't a particularly sophisticated design: it's just different. the only thing i will have to get made by someone else is the cams: they have to be hardened, they have to be absolutely precise dimensions, they need to be plated, and there's no way i can make them myself.

    thanks in advance.

    l.

  2. #2
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    You do know that just because an idea was granted a patent it doesn't mean it will work. Not trying to shoot you down but stop and think about what it is trying to do. This looks like an over-unity type idea. Could you link to an engine like this that ran?

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    Quote Originally Posted by packrat View Post
    You do know that just because an idea was granted a patent it doesn't mean it will work. Not trying to shoot you down but stop and think about what it is trying to do.
    i did. i have. for several months. i am good at 3 dimensional analysis and physics. it works. i have also read up on the chemistry, and it's consistent enough to be convincing as well (even though i got a D at A-Level Chemistry). also i have spoken to another team who have a variant on this design concept - their first prototype was done over 15 years ago, but their patent was denied by the USPTO because it contains too much prior art.

    This looks like an over-unity type idea.
    naah. bollocks to that (said with the greatest of respect and i really mean that ). its thermal efficiency is higher than that of a standard otto cycle engine but it is still constrained by the laws of thermodynamics (the precursor to which was of course that equation Texhaust / (Texhaust - Tin) where the temperatures are in Kelvin). i didn't mention any kind of vortices, red mercury, van de graaf generators, tesla or anything like that, did i! the combustion temperates are however at levels which would destroy an otto cycle engine... but this is not an otto cycle engine. it's down to chemistry, not fantasy.

    Could you link to an engine like this that ran?
    there are at least three variants, maybe four. they're all based around sine-wave (or near-sine-wave) piston motion: the bourke engine and the vaux are the most well-known. then there's a greek guy who has "inverted crank" which gets an even longer dwell time at TDC than sine wave although he did a single-cylinder thumper not an OPOC, and there's one other team that have this (unpatentable, due to significant prior art) cam arrangement but they used valves not ports. additionally there's another completely different type of engine which uses fuel detonation: i believe you'll find it by googling "wave disk engine" or such. it was in the news some time last year. they had absolutely mad temperatures which have some chemists/scientists slightly concerned: i believe they have the combustion gases up to around 3,000F which is hellishly hot, and not a temperature that i have the *slightest* bit of interest in approaching (aside from anything, the "wave disk" team are approaching temperatures at which the nitrogen in the air intake starts to burn as well, resulting in dramatically increased NOx).

    btw, i don't wish to be funny or anything: i am not here to discuss the ins and outs of "engine designs other than otto or rotary". i am here to *make* an experimental non-otto, non-rotary engine, and i am here to spend my own time and money proving without a shadow of doubt to myself that this particular non-otto non-rotary engine design actually works.

    that is my choice.

    so, with the greatest of respect - and i really mean that: if you would like to find out more about this or any other non-otto, non-rotary type of engine design and its variants please feel free to research it for yourself and i am happy to point you in the right direction as long as it does not take up significant amounts of my time nor result in protracted conversations (*especially* not on this thread), the end result of which is summarised as "i don't believe you". i am not here to "believe", nor am i here to be dissuaded by others' "belief", lack thereof, lack of willingness to experiment, lack of curiosity or other means, methods or reasons to procrastinate. i am here to *do*. to *experiment*. if the experiment fails, then i will *know* that it fails. and so will you, because i will tell you. if it succeeds, it will be amazing, and i will *know* that it succeeds. and i will tell you, and will publish the design so that you can try making one for yourself, and then *you* will know.

    so at the same time and completely separately from that which i specifially wish to avoid (spending time talking about designs rather than experimenting with designs), i reiterate: i would greatly appreciate the technical advice of people on this forum, who have enormous engineering expertise, in order to assist me to embark with a minimum outlay on this interesting deviation from well-established otto cycle engine design, in order to prove to myself - not to anyone else - whether it actually works.

  4. #4
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    I come out at 98cc with your bore and stroke - but that's not really the issue here.

    Most model IC engines seem to use aluminium for quite a lot of parts - but yours is rather larger. Perhaps you could think about cast iron for the cylinders, and for the piston rings, with aluminium pistons.

    You've discussed the sealing requirements of the rings - but you also have the gland to consider where your piston rod goes through.

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    uuu, thank you for responding here.

    Quote Originally Posted by UUU View Post
    I come out at 98cc with your bore and stroke - but that's not really the issue here.
    ah - now here's where my inexperience comes in: i need it to be about 125cc per cylinder. so... the stroke would need to be about 65mm not 50, to do that - is that correct?

    Most model IC engines seem to use aluminium for quite a lot of parts - but yours is rather larger. Perhaps you could think about cast iron for the cylinders, and for the piston rings, with aluminium pistons.
    i saw some discussions people were having in other posts, so i saw that cast iron unlike steel plays well with aluminium (which i wasn't previously aware of).

    so going on from here, how easy is cast iron to work with, and what tools would i need? (assume i am intelligent enough to follow precise and complex instructions needed to operate those tools )

    bear in mind that i will need to have 4 holes in the side of the cylinder, about... 10mm in diameter, each hole. ok, possibly up to 8 holes (all at about the same level, as shown in the diagram). is cast iron amenable to careful drilling? how do you work it (with recommended tools) so that the inner bore will fit the rings accurately? if there are answers to these questions already, i would greatly appreciate references to pre-existing threads and/or links to "howtos" preferably with photographs in order to both keep this thread compact and also save you time!

    or, how about this: are there any aluminium alloys that would be suitable (and are easy to work with)?

    btw: one thing i'd very much like to be able to do is to buy in off-the-shelf tubing that i can work with, rather than have to get parts custom-made, or rather than getting a rectangular block and having to machine the whole damn thing!

    You've discussed the sealing requirements of the rings - but you also have the gland to consider where your piston rod goes through.
    yes. bourke's design used leather seals (because that was what was easily available to him), and a tiny oil channel and half-flooded the crank compartment with oil. this was used to drip-feed oil into the under-side (fuel-air pre-mixing) chamber, which then of course went up the side-wall etc.

    unlike bourke's design, the one i'm following does not have *any* side-wall loading, nor piston rod loading, *at all*. bourke used a scotch yoke, and he had to take into consideration some small side-loading pressure on the rod assembly. an expired 1974 patent shows some enhancements that take into consideration what happens if a bourke design runs for a considerable period of time (years of use).

    but - yes. what would you say would be an easy way to do deliberate partial-sealing of the bit where the piston rod goes through? bear in mind that it has to resist about 2 to 3 atmospheres of pressure, as well as cope with decompression of about 0.5 to 0.333 atmospheres. let's assume that the central chamber with the crank is sealed (but contains half oil and half air).

    would the use of leather still be appropriate, here? (!) i like the idea because it means i don't have to be massively accurate on this part

    thanks uuu.

  6. #6
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    packrat, hi,

    after thinking overnight rather my initial puzzled immediate reaction to what you wrote: i'm going to say this - briefly - once - then i'm going to ignore any other questions or comments along the lines of "this can't possibly work". hope that's ok with everyone, that you'll have to do your own chemistry research.

    i understand where the confusion comes from over the question that you asked, but look up what the flame speed of air-fuel hydrocarbon mixtures is, at lower temperatures. at lower temperatures, when the combustion of hydrocarbon fuels is predominantly carbon-based, the flame speed is between 25 and 75 ft per second (which is why otto-cycle 2 and 4 stroke engines get a flame coming out of the exhaust, and a turbo unit has to be used to recover some of that as useful work).

    at higher temperatures - above 1800F - the hydrogen in the hydrocarbons starts to burn. the flame speed here is two orders of magnitude faster: i heard it's something mad like 5,000 ft per second - hence the nickname "detonation".

    in an otto cycle engine, detonation means that something is seriously wrong (or that something is about to go seriously wrong) because the amounts of fuel required to get an otto cycle engine to operate with any power would, if detonated (burned at over 1800F), release far too much energy for the mechanical design to withstand. side-wall loading pressures would go up by an order of magnitude for a start.

    by contrast, in this design, there *is* no side-wall loading - so that is a non-issue - and the amount of fuel is greatly reduced (because it's not needed, because it's burned more efficiently), so likewise the pressures and temperatures produced are reduced to within acceptable mechanical tolerances of the piston chamber and the piston rings.

    now with that as background in mind, it's possible to answer the specific concern that you raised. firing at 90 degrees pre-TDC when the 30:1 air-fuel mixture has been compressed to an approximate 15:1 mixture will result in an initial slow burn, because the temperatures will be in the range where carbon-oxygen burning is predominant. this you can confirm by working out the speed at which the piston will be moving: it should be (or more specifically *needs* to be!!) faster than 25-75 ft per second (but not significantly so).

    at these temperatures, the amount of energy released is quite small (compared to hydrogen-oxygen "detonation" burning). also, the energy released is also well below the amount of energy required to cause back-firing, especially given the arrangement of the cams which are, simplistically expressed using the "lever" principle, working in favour of the flywheel at this point in the cycle, rather than the burning gases.

    by the time TDC has been reached however, the temperatures and pressures have both gone through the roof, so to speak. the resultant bang is over within milliseconds, and the amount of energy is far in excess of the amount of energy which was generated in the prior 90 degrees of the piston's cycle.

    thus, we address the concern that you raised. the burn chemistry of the cycle between 90-pre-TDC and TDC is *different* from the burn chemistry of the cycle between TDC and a few milliseconds thereafter; whilst the flywheel was having its momentum slowed fractionally during the previous 90 degrees (by the early and slow burn), it's *nothing* compared to the *increase* in angular momentum caused by the bang that occurred around TDC.

    personally, i have to say that i do have some concerns about firing a spark 90 degrees pre TDC, and i will be making sure that i use an optical disc on the flywheel, with computer-controlled ignition, and will initially be starting it up with tiny amounts of fuel and at standard otto-cycle-engine ignition sequences, moving it further back as i gain confidence in the design.

    however, i know for a fact that the engine is *not* designed to take that kind of abuse: it's not an otto-cycle design. the ports are not designed to have flames coming out of them: they're simply too small for a start.

    in other words, the engine is designed around a different type of combustion cycle (but of the exact same hydrocarbon and air mixture we use every day in otto cycle engines) and it's this engine design that i wish to explore, to find out if it's really true, rather than take someone else's word for it, one way or the other. plenty of people have done otto cycle engines to death. i'm not interested in following the status quo.

    so - that is the last word that i will say on the subject of the chemistry and background on this design. if anyone wishes to research it for themselves please feel free to do so, but please start a new discussion thread: i'd like to keep this one on topic.

    many thanks.

    l.

  7. #7
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    If you retain the 50mm stroke you need a bore of 56.4mm to get 125cc. Or if you want a "square" engine with the same bore and stroke, then you need 54.2mm. Or if you keep the 50mm bore, go for a 63.7mm stroke.

    I like machining cast iron. It's messy, as it produces a fine grey dust rather than conventional swarf, but it's quite forgiving to machine. It can be a bit abrasive on the tool, (particularly in a rough casting with sand on the surface), so carbide is preferred to high-speed steel. Ordinary drills will work just fine for the ports - or you might want to mill them on a 4th axis machine to get a slot with rounded ends.

    You can get continuously cast bar, lovely stuff. See The College Engineering Supply - e.g. a 100mm length of 60mm round is £12.36, or 80mm round is £21.00 - they do diameters up to 420mm! This means drilling out and boring - it could be what you have to do - a small lathe like a Myford would do it. A bigger one (Colchester, Harrison) would breeze it. Unless you can start with motorcycle cylinder barrels from a breaker. If you chose four stroke ones, then you could machine in the ports. And a used cylinder might have a nicely worn-in bore surface. I can't see this being a cheaper option, necessarily.

    Piston rings can be tricky things to make - you could research the "Clupet" name - these are a superior form of ring in a double-circle, a bit like a key-ring. They know what they're about, so might save wasted time experimenting.

    I happen to think you're mad, embarking on such an experimental project with so little experience. But you get nowhere without trying, so good luck to you.

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    I hope you will let me explain what I was meaning when I made my statements. It's not the hydrogen burning but the double ended cylinders.

    Two cylinders of the same size is not a way that works. Look at any two stage compressor. The first stage is larger diameter, it takes atmosphere and compresses it to the first point, say 10:1. Now if you transfer that to a second cylinder that is the same size the air will expand in that cylinder to fill the space, then again compress to 10:1. The first compression only heated the air by compression. If that first cylinder compresses its air into a cylinder that is smaller and just big enough to accept it, it will then be compressed further.

    Secondly why even use a spark when you are in the realm of diesel/compression ignition?

    And I meant no disrespect to you in my first post. From the way it sounded to me this was going to be your first machining job, and didn't want it to be a bad experience for you.

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    Theory is fine up to a point, wheras experience is an invaluable tool.

    I'd be inclined to advise you to go and build something simple first, maybe a little steam engine.

    See first hand just what is involved when people build engines from scratch and overcome any problems that may come up associated with their design or machining techniques.

    I think you underestimate the level of experience needed to achieve a final working result. With software you push a button, anyone can do it the same way. With machining, a totally different story.

    Not trying to downgrade software engineering in any way as I found out for myself how involved it can be. Just trying to point out there is a big difference.

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    Here is a little inside information that you have seem to have missed or have not said. The cylinder of an engine gas or diesel has a slight tapper to them. Second unless you can figure out how to get the fuel in at such a cool temperature or have a custom gas made. At 30:1 compression you will have nothing but denotation with today's pump gas. Now if you could inject it at the right moment of the cycle then you co get past that too.

    But Hey more power to you if you can make and get it to run.

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    Quote Originally Posted by packrat View Post
    I hope you will let me explain what I was meaning when I made my statements. It's not the hydrogen burning but the double ended cylinders.
    ah ok sorry that's down to the unclear (unannotated) diagram. each cylinder is not double-ended: the underside chamber is used as a mixing chamber, just like in any ported 2-stroke engine. i realise i haven't quite got the scale right: the underside chamber is larger (much lower compression ratio) than the firing chamber. i marked the intake and outtake with arrows, but didn't actually put any words - sorry!

    anyway whilst i can't find the picture (animated gif iirc) i've seen online of ported scavenging i at least found this - Two-stroke engine - Wikipedia (loop-scavenging)

    from the description in his book, i can see that it's the closest to what bourke used in his design. he didn't do crossflow scavenging, that's for sure. there's a clear description of how the air-fuel mixture goes into a vortex due to the shaping on the top of the cylinder that's presented right at the point where the intake port into the firing chamber is.

    Secondly why even use a spark when you are in the realm of diesel/compression ignition?
    apparently bourke's design could move into dieselling once it was warmed up. he was however the only person in the world who got his designs to do that, and he unfortunately didn't outline clearly enough how it happened. it was part of the commercial secrets that he kept back, even when later in life his wife's health failed and he began to realise that he wasn't going to realise his dream, and released as much documentation on the engine as he could, instead.

    bottom line: i'll have to experiment, and/or would greatly appreciate some advice here.

    And I meant no disrespect to you in my first post. From the way it sounded to me this was going to be your first machining job, and didn't want it to be a bad experience for you.
    he he - no problem. thank you packrat. your concern is appreciated.

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    Quote Originally Posted by dodger889 View Post
    Here is a little inside information that you have seem to have missed or have not said. The cylinder of an engine gas or diesel has a slight tapper to them.
    yes. there's some cross-posting here. i have the original designs and patents, plenty of photographs. and the.... wait... what? taper? on the cylinder?

    Second unless you can figure out how to get the fuel in at such a cool temperature or have a custom gas made. At 30:1 compression you will have nothing but denotation with today's pump gas.
    ... detonation's exactly what's needed! but yes. a) bourke used a combination of i think it was... white gasoline and stove oil - i'll have to re-read the documentation to say precisely what it was b) the amount of fuel used is reduced when compared to a standard otto engine... but yes, you have a valid point that i'm just going to have to trust the design on. and use the right fuel.

    Now if you could inject it at the right moment of the cycle then you co get past that too.

    But Hey more power to you if you can make and get it to run.


    the high compression ratio is rather essential to achieving the fuel economy, and injectors is a bit more complicated than the original and more than i'm prepared to take on initially. so i'm going to just... see what happens. if nothing else it will be some spectacular messy youtube videos, with me standing a looong way away.

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