Thanks, Brian,
I
have written a speadsheet, presently in MS Excel, that takes measurements of rod length, bore, stroke, piston 'length' crown to skirt, upper port edges (measured from the top edge of the sleeve), and flange top to piston crown at TDC. So, that part of it is in hand. The spreadsheet also alerts if modifications to accomplish the desired timing cause the piston skirt to rise above the exhaust port bottom edge - as sub-piston induction usually does not go well with mufflers and muffler pressure set-ups.
As an exercise, I also worked out a variant that calculated the skewed timings and durations for a DeSaxe layout. It would actually have fit within my basic spreadsheet as another input value, but I didn't bother. No recent engines, to my knowledge, use the DeSaxe layout.
(Refresher for those unfamiliar with it - 'DeSaxe layout' has the cylinder axis offset from the crankshaft axis, usually toward the power stroke side. The idea, intuitively, is that this reduces the rod angle during the power stroke, so there's less side load pressing the piston towards the sleeve, causing more friction and wear.
-(There are other consequences of this layout than rod angle. The timing of port openings is shifted, and the time-duration of 'port open' conditions, power stroke, and bypass flow are all shifted. Several of these shifts seem strange, like where - exactly -
IS top dead center? Is it when the crankpin and the rod are in a perfectly straight line? ...depends on the amount of offset, which side that is on, and the measurements of stroke radius, rod length, etc. ... Does a longer 'time-duration' on the power stroke and exhaust port phase, than the 'time-duration' of the bypass flow phase and port opening/closing help or not? Again, the individual dimensions are needed and the results need study.
-(I don't know why we use the name, 'DeSaxe' for this - possibly some engine inventor by that name either made an engine of this type, or discussed it publicly.
-(A plausible reason that this layout is not in common use is that it simply did not make enough difference to justify the much more complex and demanding machining operations, compared to 'ordinary' engines in which the shaft and cylinder centerlines intersect, at right angles. ... particularly in the days before CNC... ..and now, with CNC available, the same basic reason may still apply - not enough benefit to justify even writing a CAM script to do the CNC.)
Oh, and Brian, about 30 years ago I wrote some stuff for PAMPA
Stunt News (may I say that out loud, here?
) in which I described blowdown as you did in your last post. I.e., the lag in shaft degrees between exhaust-opens and bypass-opens (and with a symmetrical engine, port closing times); the time any residual pressure trapped in the combustion chamber has, to vent, before the bypass opens to start the transfer phase. Some guy from Texas, USA, one George M. Ardritch, Aldred, Aldrich? Uh, yeah, I guess he spelled it "Aldrich," took me to task about that. Publicly... One tends to remember such...
But it matched engine theory I had learned in a school or two, up here in the US of America, anyway: Blowdown is defined, somewhat arbitrarily, as the TOTAL difference between the shaft° durations of exhaust and transfer periods. In a 4-cycle poppet valve engine, the time both valves are open is called 'overlap,' and the lag between exhaust opens and intake opens is largely irrelevant, unless it is insufficient time for exhaust residual pressure to drop to effective zero, allowing the descending piston to draw in the air-fuel charge for the next combustion.
In our two-cycle engines, most of the port open time is overlapped, inherently. At least we have a timed shaft inlet period (which also overlaps with the sleeve porting times) to help make things more efficient. In the old piston-ported sparkers of the 1930's and part of the 1940's, both inlet and sleeve ports were timed symmetrically, and with quite short port-open durations in both shaft degrees and time-duration. The only thing that kept the sparkers of that layout running the right direction was the orientation of the breaker points!
FYI, I studied the actual clock-time duration of similar, high performance, engines to determine a practical 'time duration' for port lag. Engines rated at 18,000 RPM, with a measured dimensional difference in port opening times, suggested the "clock-time" needed to allow residual combustion pressure to drop to insignificant. That time, corrected to shaft° at the RPM of interest, gave me the shaft°
lag I needed.
Not knocking, or trying to 'score points' - as I appreciate all constructive additions to what I had already known, and now can learn from. I hope any differences in description are more in the way of same meaning/slightly differing words than getting any of us hung up on the fact that some words (of identical meaning) are actually different words. (I've seen too much of that in too many aspects of life in my brief 70 years aboard planet Earth.)
Topic: ENYA 35 LINER MODS ? (Read 3120 times)
