Design and Simulation of Two Stroke Engines

Chapter 3 • Scavenging the Two-Stroke Engine
In Fig. 3.15, for two engine cylinders GPBDEF and SCRE, the SEV-SRV characteristics
have been curve fitted with a line of the form of Eq. 3.3.3:
loge(l - SEV) = K0 + iqSRy + K2SR
The purity of the charge leaving the exhaust port can be deduced from the appropriate differentiation
of the relationship in Eq. 3.3.3, or its equivalent in the format of Eq. 3.3.4, and
inserting into Eq. 3.3.11 resulting in:
ne = nas - (KJ + 2K2SRv)e Ko+KlSRv+K2SR (3.3.12)
The application of Eq. 3.3.12 to the experimental scavenging data presented in Fig. 3.16
for the curve fitted coefficients, Ko, KI and K2, for many of the cylinders in that table is given
in Figs. 3.18 and 3.19. Here the values of purity, ne, at the entrance to the exhaust port are
plotted against scavenge ratio, SRV. The presentation is for eight of the cylinders and they are
split into Figs. 3.18 and 3.19 for reasons of clarity.
The characteristics that emerge are in line with predictions of a more detailed nature,
such as the simple computational gas-dynamic model suggested by Sher [3.24, 3.25] and the
more complex CFD computations by Blair et al. [3.37]. In one of these papers, Sher shows
that the shape of the charge purity characteristic at the exhaust port during the scavenge
process should have a characteristic profile. Sher suggests that the more linear the profile the
worse is the scavenging, the more "S"-like the profile the better is the scavenging. The worst
scavenging, as will be recalled, is from one of the loop-scavenged cylinders, YAM 12. The
best overall scavenging is from the loop-scavenged SCRE and UNIFLOW cylinders and, at
low scavenge ratios, the cross-scavenged cylinder GPBDEF.
SCAVENGE RATIO, SRv
CROSS
GPBDEF
SCRE
YAM 12
Fig. 3.18 Purity at the exhaust port during scavenging.
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