On Mon, 24 Jan 2005 13:26:44 -0800, Tim Wescott
wrote:

Dave Hinz wrote:

On Sun, 23 Jan 2005 21:00:53 -0800, Tim Wescott wrote:

Why is it that reed valves are so successful in high-performance
motorcycle two strokes yet are only used in teeny 'sport' class model
airplane engines? Is it just because you can't get suitable reed
material in such small sizes?


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.


Inquiring minds want to know.


There ya go.

Dave Hinz

Well, they _do_ run at that low RPM, but they don't have to deliver
torque down there.

Makes sense -- thanks.

There may be another factor. Model engines commonly use the crank as
the inlet valve, with a hollow crank for the air-fuel mix passage into
the crankcase. The counterweight and crank throw are cantilevered.
The big end of the rod slides over the crank throw when the sleeve is
removed.

This is very simple and works well for low mass (small) engines. I
suspect by the time you're up to motorcycle size engines of high
output, the cantilevered design would not be favored compared to reed
valves and crank bearings on both sides.

Pete Keillor