The Effects of Piston-Head Shape, Cylinder-Head Shape, and Exhaust Restriction, on the Performance of a Piston-Ported Two-Stroke Cylinder Page: 5 of 42
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N.A..C.A. Technical Note No.. 756
Flow model.- The flow model shown in figure 10 was
carefully made to the same internal dimensions as the
actual engine. The model includes inlet passage, ports,
cylinder, cylinder head and piston. Walls of the inlet
passage, and the cylinder barrel, with the exception of tre
port ring, are made of a transparent plastic material
with polished surfaces. It is thus possible to observe
the entire interior of the cylinder. The piston and cyl-
inder head can be moved axially in the cylinder, and are
held airtight and in position by means of expanding rubber
rings. The "port timing," that is, the position of-inlet
and exhaust ports relative to the piston, to the inlet
manifold, and relative to each other, can be changed by
moving the cylinder and port ring vertically with respect
to the manifolds, and transferring flat inserts from the
top or the bottom of the exhaust ports at the same time.
In order to observe the air flow, a circular wire
frame was made, across which were stretched six fine piano
wires, as shown' in figures 10 and 11. Small glass beads
were threaded on the piano wire and spaced with hypodermic
tubing. A rubber thread 0.01 inch in diameter and 1/8
inch long was cemente. to each bead, and a fine tuft of
cotton attached to the other end of the' thread. The frame
contained a total of 48 tufts which could rotate freely
with the bead as a bearing and follow the air flow in a
plane perpendicular to the wire. The flexibility of the
fine rubber thread also permitted the tuft to follow com-
ponents of the air motion parallel to the wire. By means
of this frame, the direction of the air flow at any number
of horizontal sections of the cylinder could be observed.
The tufts were small and light enough, so that their ef-
fect on the flow was apparently negligible. Air was sup-
plied to the inlet manifold by a small Sirrocco fan. The
static pressure in the inlet passage was held constant for
all runs at 0.6 inch alcohol. This pressure was sufficient
to give a well-defined flow pattern, but not enough to
damage the flow frame. Some use was also made of frames
giving vertical cross sections of the flow, but the re-
sults from these were not as easy to interpret.
TEST PROCEDURE
Flow model tests.- Preliminary studies were made with
the flow model, using various air velocities, piston posi-
tions and port timings, in order to determine-whether
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Rogowski, A. R.; Bouchard, C. L. & Taylor, C. Fayette. The Effects of Piston-Head Shape, Cylinder-Head Shape, and Exhaust Restriction, on the Performance of a Piston-Ported Two-Stroke Cylinder, report, March 1940; (https://digital.library.unt.edu/ark:/67531/metadc54614/m1/5/?rotate=180: accessed July 17, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.