Martin Whybrow wrote:
"Tim Wescott" wrote in message
...
Dave Hinz wrote:
On Mon, 24 Jan 2005 13:26:44 -0800, Tim Wescott
wrote:
Dave Hinz wrote:
Nope, not that reason. Reeds give the greatest benefit in low-RPM
torque,
in a 2-stroke engine. Since a model airplane engine never runs at that
low RPM, there's no need for them.
Well, they _do_ run at that low RPM, but they don't have to deliver
torque down there.
Good points.
Makes sense -- thanks.
It's really fascinating how a 2-stroke works. I've seen some computer
modeling of the flow characteristics of a few different engines at
varying RPM, which showed all of this in red/blue flow, with the
intensity of color showing the density of that location. Of course,
I can't google it up right now.
Yes, the Day cycle (did you know it was called that? Invented by a guy
named Day, of course) is pretty fascinating. They're doing some amazing
things with direct fuel injection and two-part gaseous mixtures to make
it clean, too -- basically they keep the same cycle, but the fuel (or
fuel-laden air) gets put into the cylinder after the cylinder is
scavenged with fresh air.
Not practical for model two-strokes, of course.
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
There's another reason; bike engines have timed ignition systems which
prevent the engine running backwards; most glow plug engines rely on the
induction timing to prevent reversal. The small reed induction engines (e.g
Cox 049) use a starter spring to ensure that they only start in the correct
direction (you hook it around the prop, wind it backwards a few turns and
let go), you couldn't easily do this with a larger engine. I know from
experience that if you try and start one of these reed induction engines
with your finger, they bounce off compression and start backwards; that's
led to a few very short flight on occasions ;-)
Martin
I have flown the Cox engines for years, and yes it's an issue (always
check by feeling for airflow). You can learn to flip the thing in such
a way that you neither start it backward or chop off your fingers by
exerting force on the front of the blade about 2" out from center and
following through (and using a finger guard).
Those dang starters rob power and wear out -- I don't use 'em.
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com