On Fri, 16 Mar 2012 13:58:03 -0700, Bill Martin wrote:
On 03/15/2012 04:14 PM, Tim Wescott wrote:
On Thu, 15 Mar 2012 14:11:59 -0700, Bill Martin wrote:
On 03/15/2012 10:14 AM, Tim Wescott wrote:
snip
The standard construction for model airplane engines of this sort is to
have a cylinder lining of steel or (if you have friends in the right
sorts of shop) hard chromed brass. It's a thin tube, usually with a
lip on top, that slides into the crankcase. The crankcase has passages
cast or milled into it for the transfer and for the exhaust.
Oh, ok. I see what you are doing now. you are making a sleeve, as
opposed to an actual cylinder, there is already a vertical path for gas
flow in the outer wall.
So the gas travels up from the crank case to the port along the outside
of the sleeve, then turns 90 degrees to go into the cylinder.
Meanwhile, the exhaust is exiting through its port without (we hope)
leaking into the crankcase because the liner is sealed to the case by a
close mechanical fit.
I'm not sure of the detailed heat-flow -- I just know that it must be
adequate, or all my engines would have burnt up long ago.
My experience is just with high output 2-strokes, as in Kart racing
engines. A steel sleeve would not do it there...you must match the
piston expansion with heat as closely as possible. Nikasil on Al is
pretty much the only game now, except for air cooled iron sleeve types.
The Al cylinders work much better, run tighter clearances, last longer.
Aluminum pistons in hard-chromed brass or aluminum sleeves is "it" for
model airplane engines. I'm pretty sure the piston is high-silicon,
because the idea is that the sleeve expands faster than the piston --
when cold, there's just a slight pinch at the top of the stroke that goes
away as soon as the engine warms up.
(And you never, ever, want to run them really rich -- then the fit gets
too loose).
I'll be doing a steel piston in a steel sleeve, then if I find myself
getting serious about it I'll see about making friends with a shop that
can hard chrome the inside of an itty bitty brass cylinder.
--
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