Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
ciency is seen here, for the charging efficiency drops from a peak of 0.47 to 0.40, which will
deteriorate the ensuing power and torque, bsfc and bsHC, by at least that amount, i.e., 15%.
The design question is, as written above: What can be done about it?
In Fig. 5.17 it is discovered where this spilled charge goes. After 190° atdc, the purity at
the cylinder end of the scavenge duct decreases from unity to a value of about 0.90, and, apart
from some localized mixing within the transfer duct, remains at that value when the port shuts
until the next time it is opened, at which point the aforementioned blowdown backflow takes
place and reduces it even further. At the exhaust port, and in the exhaust pipe, spillage causes
the purity there to rise from 190° atdc until exhaust closure. You should note the subtlety of
the point being made: scavenging has ceased but spillage takes over. The other apparent
contradiction is that, during this same period, the cylinder scavenging efficiency, i.e., purity,
continues to rise. The explanation is that the spilled charge, i.e., that at the exhaust port, is of
lesser purity than the cylinder mean and while mass is lost from the cylinder, the average
purity of the cylinder charge is improved.
The scavenge model from the theory given Sec. 3.3.2, and shown graphically in Fig. 3.19
for the "loopsaw" test cylinder, is seen in operation in Fig. 5.17, in that the purity at the
exhaust port tracks the cylinder purity and lagging behind it as dictated by Eq. 3.3.12.
The temperatures of the gas during scavenging, in various locations beside or within the
cylinder, are shown in Figs. 5.18 and 5.21. In Fig. 5.18, at the cylinder end of the scavenge
duct, the blowdown backflow causes an initial rise to 240°C, and then flow from the crankcase
drops it down to about 100°C by bdc, whence cylinder spillage, with gas at a lesser
purity and at some 300°C, increases it to nearly 200°C by transfer port closure. After that,
with mixing caused by internal pressure wave motion in the transfer duct, and by expansion
during the induction phase, the temperature decays to about 130°C by the port opening point.
The cylinder temperature shows more dramatic behavior, falling from over 1000°C at the
onset of scavenging, to a minimum at about 250°C with the influx of colder air from the
crankcase, and then it rises toward the beginning of compression.
In Fig. 5.18, the temperature in the exhaust pipe at the exhaust port has an interesting
profile. It tracks cylinder temperature but is always higher, as the gas which is leaving the
cylinder has a lesser purity and therefore a higher temperature than the mean of the cylinder
contents. This follows the discussion in Sec. 3.3.3 regarding the temperature differential model
for air and exhaust gas at this very juncture. The behavior of this model is shown in Fig. 5.21,
and the apparently inexplicable flat in the exhaust gas temperature profile in Fig. 5.18 is now
clarified. At the beginning of scavenging, the entering air is rapidly heated to nearly 600°C,
but falls as more enters and cools the entire cylinder contents. On the other hand, the exhaust
gas, during this early period of "perfect displacement" scavenging up to 160° atdc, is leaving
the cylinder without benefit of this cooling through mixing, which explains the "flat" on the
profile. The temperature differential model, as defined in Eqs. 3.3.13 to 3.3.18, is seen to
control the average in-cylinder air and exhaust gas profiles in a logical manner. The word
logical is employed as there is no means of ever confirming this statement with experimental
data. The apparently instantaneous equalization of temperatures at the trapping point is merely
the computer program informing the cylinder contents that they will be theoretically treated
as a homogeneous gas from that point onward in the compression process.
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