Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
silencer and its ducting, in terms of Fig. 5.6. The individual box volumes, VA and VB, are 300
and 260 cm 3 , respectively, and it is clear that this is very small as it is not even ten cylinder
volumes. The use of the acoustic program for a diffusing silencer, Prog.8.1, shows that there
is absolutely no attenuation of sound below 800 Hz.
For this simple silencer, it has two diffusing silencer segments. The question which the
designer asks is the extent to which the final outlet can be throttled before unacceptable
power loss occurs. The simulation answers this type of query with precision. The "standard"
outlet pipe diameter is 12 mm. As a design exercise, this outlet pipe is crimped at the very end
to give equivalent end orifices of 11, 10 and 9 mm diameter, labeled as X, Y and Z, respectively.
The GPB simulation models these effects, using the intake silencer, F, in each case.
The result of so doing is shown for the mass flow-time history at the exhaust exits in Fig.
8.21, the noise spectra in Fig. 8.22, and the overall effect on noise and performance characteristics
in Table 8.3.
Name
F
X
Y
Z
Table 8.3 Performance and exhaust silencing for the chainsaw
Intake
(dBA)
82.9
83.0
83.1
83.1
Exhaust
(dBlin)
92.6
91.3
89.8
88.2
Exhaust
(dBA)
89.0
88.4
87.4
85.9
Total
(dBA)
89.8
89.5
88.7
87.7
bmep
(bar)
3.79
3.71
3.61
3.50
DR
0.539
0.535
0.524
0.509
The expected results are achieved, namely choking the exhaust drops the delivery ratio
and loses bmep. The amount is 0.29 bar, apparently a meager drop, but nearly 10% of the
power output has actually been lost. The exhaust noise is lowered by some 3 dBA, and that
0.012 -,
o.ooo
CHAINSAW ENGINE, 9600 rpm
100 200 300
CRANKSHAFT ANGLE, 2 atdc
Fig. 8.21 Effect of modified exhaust system on the exit mass flow rate.
574
F
400