Most large rotary valve two strokes
use a disk valve. Model high power
engines use this valve as well. In model
engines it has traditionally needed to fit
into the crankcase, restricting its
diameter. This isn’t necessary and a
larger disk can be fitted. This valve is
often called the Zimmerman disk valve
after the designer who popularized (but
did not invent) it. It has the advantage of
simple construction and moderate cost,
but adds friction and has a large diameter
for a given flow.
PROPWASH
10
October 2013
High Power Two Stroke Design - Part 3
(Continued from page 9)
Piston ports have been used that open 100 degrees or more before top dead center.
However, starting and part throttle operation become difficult. Intake tuning can
overcome the blow back over a narrow rpm range, but there is little power outside this
range.
Rotary valves overcome this limitation. They typically open around 150 degrees
before top dead center as the piston rises and close around 60 to 70 degrees after top
dead center as the piston descends. This gives a larger open duration of 210 to220
degrees. Another point is that the transfer ports open around 115 to 120 degrees before
top dead center. That means the intake is open for around 30 degrees at the same time
the transfer is open with a typical rotary valve. This allows the pipe to “suck” mixture
into the crankcase as well as into the cylinder from the crankcase.
The simple solution to the problem of inexpensive piston ports versus expensive
rotary valves has been to put a reed valve in the piston port. A lot of experimentation
has resulted in reed valve designs for larger engines that are almost as good as a rotary
valve. This has not been true so far for model size engines.
Inexpensive industrial engines still use piston ports to keep costs at a minimum.
However, only the very early model engines used piston port intakes. Several different
rotary valves have been tried in model engines. The most common is the crankshaft
rotary valve. This valve doesn’t require extra parts, cutting down on costs and friction.
It is limited in size by the largest bearings that will fit in the crankcase.
A very similar valve is what I call the “Russian” drum valve for its popularity in
the K engines. It was also used in the MAC engines. It allows a larger size valve than
the crankshaft rotary valve but with additional parts and friction. Like that valve, it
increases crankcase volume a long way from the transfer passages.
A different drum valve is more common. It has all the advantages of the “Russian”
valve but with lower crankcase volume. However, the oil in the mixture isn’t directed
over the rod big end bearing.
Model Reed Valve
Piston Port with Reed Valve
A crankshaft valve and the Russian drum valve
A drum valve
Model disk valve
Zimmerman disk on a Zenoah