On Thu, 15 Mar 2012 14:07:34 -0500, jim wrote:

Tim Wescott wrote:

On Thu, 15 Mar 2012 12:10:40 -0600, Pete C. wrote:

Tim Wescott wrote:

On Thu, 15 Mar 2012 10:20:42 -0600, Pete C. wrote:

Tim Wescott wrote:

How would one make nice sharp-cornered, clean-sided (I'm not sure
of the surface finish, but mirror-bright would be nice) square
holes, about 0.1" on the sides, off center from the axis of the
bar (so not square to the bar surface), in a round steel bar
about 0.75" in diameter?

? ?? Alternately, how might one make those same holes in a cylinder
?? ?? machined out of that same bar, with a wall thickness of about
0.06", ?? ?? without distorting the cylinder by more than a couple of
thousandths ?? ?? (I am assuming that one would have to do some
post-operations to ?? ?? clean up the cylinder after making the holes,
unless one hand-filed ?? ?? them).
?? ??
?? ?? This is a thought experiment for making cylinder liners for
2-stroke ?? ?? engines of about 0.2 in^3 displacement; the holes would
be the ?? ?? transfer ports, and the cylinder liners need to have their
diameter ?? ?? controlled to about 0.001" on the outside and less than
that on the ?? ?? inside for proper sealing (or if not controlled, then
at least ?? ?? matched to the crankcase that they slide into, and the
piston that ?? ?? slides in them).
?? ??
?? ?? --
?? ?? My liberal friends think I'm a conservative kook. My conservative
?? ?? friends think I'm a liberal kook. Why am I not happy that they
have ?? ?? found common ground?
?? ??
?? ?? Tim Wescott, Communications, Control, Circuits ? Software ?? ??
http://www.wescottdesign.com
?? ?
?? ? Wouldn't this normally be just a milled slot? Square on the top
and ?? ? bottom (relative to the piston stroke) with the edges running
off the ?? ? cylinder leaving square "knife edges" at the ends? At 0.1"
perhaps a ?? ? pass with a slotting saw rather than a super small and
delicate end ?? ? mill.
??
?? Schnuerle ported engines want transfer ports that direct the gas
toward ?? the cylinder wall away from the exhaust port -- that tends do
a good ?? job of blowing out mostly spent combustion products, while
retaining ?? the most possible unburned fuel/air mix. More power,
better fuel ?? efficiency, cleaner burn -- what's not to like? (Except
for the ?? difficulty in machining, of course). ??
?? --
?? My liberal friends think I'm a conservative kook. My conservative ??
friends think I'm a liberal kook. Why am I not happy that they have ??
found common ground?
??
?? Tim Wescott, Communications, Control, Circuits ? Software ??
http://www.wescottdesign.com
?
? I'm having a little difficulty picturing exactly what you're trying
to ? do. Perhaps drill and broach? You can make your own broach a bit
more ? easily than the ECM and EDM lines of thought. Come to think of
it, ? perhaps rotary broaching might do what you need.

http://en.wikipedia.org/wiki/Schnuerle_porting. See the picture in the
upper right. Now imagine that instead of having two transfer ports
feeding gas to the cylinder, there's four; the two shown, plus two more
that are rotated more away from the exhaust. Then, if that's not
enough, a fifth "boost" port directly opposite the exhaust, and
pointing toward the top of the cylinder.

The whole idea is that the transfer gas hits the pocket at the
cylinder/ piston junction, then rises up the back of the cylinder in a
fairly unified mass; because this mass is increasing in volume, it
naturally pushes the spent gas out the exhaust port. Cross-flow
scavenging (the kind where there's a fence on the piston) tends to mix
the gases a lot more.

There's a lot of hand-waving explanation for why it works, but the
bottom line is that it works better than cross-flow scavenging!

I don't see why you need holes with square sharp corners to achieve
those port configurations.

Well, you don't, in general, which is why I'll probably just use round
holes to start. But if you want the maximum flow in the minimum space,
you do.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com