Design and Simulation of Two Stroke Engines

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Chapter 3 - Scavenging the Two-Stroke Engine
YAMAHA CYLINDER 12
1
SCAVENGE RATIO, SRv
SEv THEORY
TEv THEORY
SEv EXP
TEv EXP
Fig. 3.30 Comparison of experiment and CFD computation.
In terms of insight, Figs. 3.21-3.24 and 3.25-3.28 show the in-cylinder charge purity
contours for cylinders 14 and 12, respectively. In each figure are four separate cylinder sections
culled from the cells in the calculation. The cylinder section (a) is one along the plane of
symmetry with the exhaust port to the right. The cylinder section (b) is at right angles to
section (a). The cylinder section (c) is on the surface of the piston with the exhaust port to the
right. The cylinder section (d) is above section (c) and halfway between the piston crown and
the cylinder head. In Fig. 3.21(a) and Fig. 3.25(a) it can be seen from the flow at 39° bbdc that
the short-circuiting flow is more fully developed in cylinder 12 than in cylinder 14. The SEV
level at the exhaust port for cylinder 14 is 0.01 whereas it is between 0.1 and 0.2 for cylinder
12. This situation gets worse by 29° bbdc, when the SEV value for cylinder 14 is no worse than
0.1, while for cylinder 12 it is between 0.1 and 0.4. This flow characteristic persists at 9°
bbdc, where the SEV value for cylinder 14 is between 0.1 and 0.2 and the equivalent value for
cylinder 12 is as high as 0.6. By 29° abdc, the situation has stabilized with both cylinders
having SEV values at the exhaust port of about 0.6. For cylinder 12, the damage has already
been done to its scavenging efficiency before the bdc piston position, with the higher rate of
fresh charge flow to exhaust by short-circuiting. This ties in precisely with the views of Sher
[3.24] and also with my simple scavenge model in Sec. 3.3.2, regarding the ne-SRv profile at
the exhaust port, as shown in Fig. 3.18.
Typical of a good scavenging cylinder such as cylinder 14 by comparison with bad scavenging
as in cylinder 12 is the sharp boundary between the "up" and the "down" parts of the
looping flow. This is easily seen in Figs. 3.23 and 3.27. It is a recurring feature of all loopscavenged
cylinders with good scavenging behavior, and this has been observed many times
by the research team at QUB.
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