Design and Simulation of Two Stroke Engines

1.6
>& 1.4 -
LU
Q.
1.2 -
1.0
Chapter 2 • Gas Flow through Two-Stroke Engines
MEGAPHONE EXHAUST
0.6
0.00 0.01 0.02 0.03 0.04 0.05 0.06
Fig. 2.53 Measured and calculated pressures with Mach 0.7 criterion.
The result of the computations using the GPB modeling method are shown on the same
figures with the theoretical criterion for flow separation from the walls of a diffuser, as debated
in Sec. 2.15.1, differing in each of the three figures. Recall from the discussion in that
section, and by examining the criterion declared in Eq. 2.15.5, that gas particle flow separation
from the walls of a diffuser will induce deterioration of the amplitude of the reflection of
a compression wave as it traverses a diffuser. The taper of 8° included angle employed here
would be considered in steady gas flow to be sufficiently steep as to give particle flow separation
from the walls. The theory used to produce the computations in Figs. 2.51 to 2.53 was
programmed to record the gas particle velocity at every mesh within the diffuser section. The
Eq. 2.15.3 statement was implemented at every mesh at every time step, except that the computational
switch was set at a Mach number of 0.5 when computing the theory shown in Fig.
2.51, at a Mach number of 0.6 for the theory plotted in Fig. 2.52, and at a Mach number of 0.7
for the theory presented in Fig. 2.53. It will be seen that the criterion presented in Eq. 2.15.5
provides the accuracy required. It is also clear that any computational method which cannot
accommodate such a fluid mechanic modification of its thermodynamics will inevitably provide
considerable inaccuracy. Total reliance on the momentum equation alone for this calculation
gives a reflected wave amplitude at the pressure transducer of 0.5 atm. It is also clear
that flow separation from the walls occurs only at very high Mach numbers. As the criterion
of Eq. 2.15.5 is employed for the creation of the theory in Figs. 2.43 to 2.45, where the taper
angle is at 12.8° included, it is a reasonable assumption that wall taper angle in unsteady gas
flow is not the most critical factor.
The differences between measurement and computation are indicated on each figure. It
can be observed that the GPB finite system modeling gives a very accurate representation of
the measured events for the reflection of compression pressure waves at a tapered pipe ending
at the atmosphere.
2.19.8 A pipe with a gas discontinuity attached to the QUB SP apparatus
The experiment simulates an exhaust process with a straight pipe attached to the cylinder.
Fig. 2.54 shows a straight aluminum pipe of 5.913 m and 25-mm internal diameter attached to
the port and ending at a closed end with no exit to the atmosphere. There are pressure transducers
attached to the pipe at stations 1, 2 and 3 at the length locations indicated. However, at
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