The Effects of Piston-Head Shape, Cylinder-Head Shape, and Exhaust Restriction, on the Performance of a Piston-Ported Two-Stroke Cylinder Page: 11 of 42
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N.A.C.A. Technical Note No. 756
ward flow, keeping near the wall, twists about 900 by the
time it reaches the top of the cylinder; then as the flow
turns downward, the twist reverses. The twist occurred
in either direction depending on how it started. It is
to be emphasized that the not rotation is zero, the result
being what amounts to a 900 twist to the top of the in-
verted "U" of flowing air . There is a marked horizontal
flow of air from the region of upward flow into the down-
ward region, all the way up the advancing edge of the up-
ward current. There is no evidence of downflow being
picked up by the upflow, however.
The "C" and "Dn ports, also good. performers in the
engine, showed flow characteristics very similar to those
of arrangement E, while the "B1 port arrangement, a poor
performer, gave flow patterns nearly like those of the "A"
arrangement.
Pistons.- Pistons I and 2..(se figs. 6 and 6a) were
developed independently by two investigators, using as a
guide the type of-flow which was found to be characteris-
tic of the best port arrangements. The process consisted
of gradually modifying a clay piston head, until the de-
sired flow was obtained in the flow model. The "B' (00)
ports and spherical B head were used during this work.
Particular effort was made to-eliminate short-circuiting
and eddies. Figure 13 shows the flow obtained in the
model with piston 2. The flow with piston 1 was the same,
as nearly as could be observed. Owing probably to the
symmetry of the piston, the twist in the flow loop is ab-
sent. The expansion of the upblast above the 2-inch level
was also absent. The use of a deflector surface very
close to the inlet ports to control the upblast, was con-
sidered, but was not tried because of the resulting re-
striction to the flow which would require a high scaveng-
ing pressure. There is a notable absence of "short-air-
cui-ting," which led to the hope of excellent engine per-
formance for these pistons.
The f-lows obtained with piston 3 (flat) and 4 (nearly
flat with edges rounded to 5/8-inch radus) are also shown
in figure 13, using the E port arrangement in each case.
The general nature of the flow with these two pistons is
quite similar, and both show much more "short-circuiting"
and "mixing" (that is, flow between the upgoing and down-
going air) than with pistons 2. They also show the "twist"
mentioned above.10
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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/m1/11/: accessed July 17, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.