Design and Simulation of Two Stroke Engines

Chapter 8 - Reduction of Noise Emission from Two-Stroke Engines
splits the total box volume into two parts, labeled as Vi and V2, and the simulation models the
entire system as two complete boxes separated by the filter element in precisely the same
fashion that it does for the exhaust system. The geometry is detailed in Table. 8.1.
Name
S
E
F
G
df
(mm)
30
30
30
30
Table 8.1 Intake silencer geometry for the chainsaw
d2
(mm)
22
22
22
22
dp
(mm)
22
17
15
19
LP
(mm)
50
160
125
200
V1
(cm 3 )
200
200
200
200
v2
(cm 3 )
100
300
300
300
fi
(Hz)
240
88.9
88.3
80.1
It will be seen that those characterized in Table 8.1 above as E, F, and G are the various
alternatives as designed by the acoustic criteria, whereas that called S is the original "standard"
system.
The unsteady gas-dynamic simulation prepares an output file of the mass flow-time history
at the intake from the atmosphere. For the four systems analyzed, this is shown in Fig.
8.19 as the results of simulation at 9600 rpm, at a slightly different, i.e., leaner, air-to-fuel
ratio than that employed for the more complete data map in Fig. 5.9. The theory of Sec. 8.4.1
conducts a Fourier analysis of the data in Fig. 8.19 and produces the noise spectra shown in
Fig. 8.20, at a distance, rm, of 1.0 m from the intake and exhaust exit points, at an orientation
angle, 0, of zero. Not surprisingly, the more open silencer, S, has the greater mass flow fluctuations
and has a higher noise spectra profile than any of the more optimized silencers, E, F
or G. They are all reasonably similar, but the silencer, F, has the smoothest mass flow-time
history and comes out with the lowest peak noise level at 640 Hz, the fourth harmonic. Thus
the design, F, one of those determined above by the acoustic criteria, emerges as being superior
from a noise standpoint to the standard silencer, S. The overall noise levels, the summations
of these spectra, are given in Table 8.2.
Name
S
E
F
G
Table 8.2 Performance and intake silencing for the chainsaw
Intake
(dBlin)
103.6
92.2
90.1
93.8
Intake
(dBA)
98.5
87.2
82.9
89.5
Exhaust
(dBA)
89.0
88.8
88.8
88.8
Total
(dBA)
99.0
91.1
89.8
92.2
bmep
(bar)
3.65
3.82
3.79
3.79
DR
0.525
0.546
0.541
0.550
The intake geometry of F is overall quieter than E and G by 4 dB or more. That is very
much quieter, for, recall from Eq. 8.1.6, two equal sound sources give an increase of approximately
3 dB. The acoustic criteria gave no guidance on this matter. The standard silencer, S,
is actually noisier than the exhaust system.
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