Design and Simulation of Two Stroke Engines

1.1 -I
I
0 1 2
SCAVENGE RATIO, SRv
Fig. 3.13 Trapping efficiency characteristics.
Chapter 3 - Scavenging the Two-Stroke Engine
DISPLACEMENT
MIXING
•±— UNIFLOW
-n— GPBDEF
••— SCRE
A— QUBCR
-i LOOPSAW
description is not possible. The design approach is discussed in Sec. 3.5.2 and a
sketch of the layout is to be found in Fig. 3.32(a).
(d) A ported uniflow-scavenged cylinder of 302 cm 3 swept volume, called UNIFLOW. It
has a bore stroke ratio of 0.6, and the porting configuration is not dissimilar to that
found in the book by Benson [1.4, Vol 2, Fig. 7.7, p213]. The engine details are
proprietary and further technical description is not possible.
Then there are those cylinders shown in Figs. 3.12 and 3.13, with the UNIFLOW cylinder
repeated to provide a standard of comparison:
(e) A 409 cm 3 cross-scavenged cylinder, called GPBDEF. It is a prototype cylinder designed
to improve the scavenging of the same outboard engine described above as
CROSS. The technique used in the design is given Sec. 3.5.3. The detailed porting
geometry and the general layout is illustrated in Figs. 3.32(b) or 3.34(a).
(f) A loop-scavenged cylinder of 375 cm 3 swept volume, called SCRE. This engine has
three transfer ports, after the fashion of Fig. 3.38.
(g) A 250 cm 3 QUB cross-scavenged cylinder, called QUBCR, and previously described
in [1.10] in considerable detail. The detailed porting geometry is drawn in that paper.
The general layout is almost exactly as illustrated in Figs. 1.4 or 3.34(b).
(h) A loop-scavenged cylinder of 65 cm 3 swept volume, called LOOPSAW. This engine
has two transfer ports, after the fashion of the upper left sketch in Fig. 1.2 or Fig.
3.35. The engine is designed for use in a chainsaw and is very typical of all such
cylinders designed for small piston-ported engines employed for industrial or outdoor
products.
The disparate nature of the scavenging characteristics of these test cylinders is clearly
evident. Note that they all fall within the envelope bounded by the lines of "perfect displacement
scavenging" and "perfect mixing scavenging." By the end of 1994 at QUB, a large
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