The Effects of Piston-Head Shape, Cylinder-Head Shape, and Exhaust Restriction, on the Performance of a Piston-Ported Two-Stroke Cylinder Page: 10 of 42
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"N.A.C..A. Technical Note No. 756
B head, and' round-edge piston 4. Exhaust-port inserts
similar to the 09- inlet-port inserts were made up in two
sizes. When these.two inserts were placed in the bottom
of the exhaust ports, there remained 13/16 inch for the
one and 9/16 inch for the other between the top of tho
inserts and the top of the exhaust port. (See fig. 3.)
With no insert, the distance from piston top to port top
at bottom center was about 1.2 inches. Light spring -in-
d.icator cards were taken to determine the amount of super-
charging possible by this method (figs. 15 and 16).
Effect of swirl.- A plate, partially blocking off
the inlet ports on one side of the engine, and thus pro-
ducing a strong swirl, was tried in the flow model -and:
engine. The scavenge ratio was kept the same as without
the plate.
. RESULTS AND DISCUSSION
Effect of port arrangement.- The effect of port ar-
rangement on engine performance (with flat piston 3 and
shallow spherical head B) was reported in reference 1 and
is -summarized herewith by figure 17. The corresponding
flow patterns as determined in the flow model for the A
and E'ports (the poorest and the best as determined by
engine performance) are shown in figure 12.
Neither port arrangement shows evidence of unscav-
enged areas or serious flow peculiarities, such as re-
verse scavenging or violent swirl. It may be noticed that
with the. E porting, the flow is' characterized by a concen-
tration of the upward-flow areato a minimum at about 2
inches above the piston. At this point the upward-moving
area occupies about 25 percent of the total cylinder cross
section,. The upward-flow area subsequently widens out,
until it occupies about 40 percent 6f the cross-sectional
area at the 6-inch level. The contraction of the upflow
area with the A porting occurs at a higher level and does
not expand again.
The "A" ports show much more "short-circuiting" (that
is, flow from inlet to exhaust) than the E ports at the
lower levels. It seems probable that the "short-circuit-
ing" accounts for the poorer engine performance of the
"A" porting. An interesting difference between the two
arrangements is in the rotary motion of the charge in the
cylinder, which occurs with the "E" ports only. The up-9
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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/m1/10/: accessed July 17, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.