[lkcl]

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.

[UUU]

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.

[lkcl]

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.

uuu, thank you for this. how did you derive this? 63.7-50 is a full 13.7mm which seems to be a heck of a lot. ok, so add 50 to 63.7, you get 113.7. 113.7 divided by 13.7 is 8.3 - is that the compression ratio? is that how it's calculated?

if so, then i have a bit of a complication: the calculation of the compression ratio has to take into account the stroke only from when the ports are sealed (because the piston rings went past it).

ok, in being slightly concerned about machining cast iron, could i ask you (or anyone else) for advice? there is an alternative arrangement, to use 4 cylinders. this would be a cylinder bore of about 40mm, as each cylinder would be about 62.5 (call it 64) cc.

is 64cc (and 40mm) within the realm of using an aluminium alloy? i'm aware that cast iron is still best for the rings, and so greatly appreciate the pointers you gave, uuu, on where to begin finding them.

many thanks,

l.