Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
The conclusion from this calculation is that a silencing pipe 125 mm in length and 15 mm
internal diameter inserted into a box of 500 cm 3 volume would provide an adequate intake
silencing characteristic for this particular chainsaw engine. If the pipe is to be re-entrant into
the box, shown as partly so in Fig. 8.11, you should remember that it is the effective internal
box volume which is to be employed as input data.
It is interesting to note that these acoustic criteria are also satisfied by a pipe length of 160
mm and a diameter of 17 mm, and again by another of 200 mm length and 19 mm diameter,
respectively. As the pipe diameters are larger, these silencers should be less restrictive and
pass air more readily from the atmosphere into the silencer box. Their relative effectiveness is
determined by a much more rigorous simulation method below.
It is important to emphasize that this is an acoustic, not an absolute, design calculation
and the experienced designer will know that this prediction is normally a prelude to an intensive
period of experimental development of the actual intake silencing system [1.12].
8.5.6 Engine simulation to include the noise characteristics
Noise characteristics may also be obtained by simulation, using the same software code
employed earlier in Chapters 5-7 and incorporating the theory of Sec. 8.4.1. The example
chosen is the chainsaw engine featured immediately above and frequently throughout Chapters
5, 6 and 7. The layout of the intake and exhaust silencers, and their connection to the
engine cylinder, is shown in Fig. 8.18.
The intake system of the chainsaw
The silencer geometries are more completely seen in Fig. 8.18, which ties together the
previous sketches shown as Figs. 5.5, 5.6 and 8.11. The standard intake system is, like many
on industrial engines, somewhat rudimentary, whereas that shown in Fig. 8.11 or 8.18 is more
sophisticated, as is the data acoustically acquired above for the intake silencer. The filter
gives little obstruction to the flow as it has an effective flow diameter, df, of 30 mm. The filter
EFFECTIVE AIR FILTER
NEEDED TO STOP INGRESS
OF PARTICLES SUCH AS
SAWDUST
LOW-PASS INTAKE SILENCER
DOUBLE-SKINNED SILENCER BOX
TO AVOID RADIATION OF NOISE
FROM SILENCER SURFACE
SPARK ARRESTOR
REQUIRED FOR
CHAINSAW
/
DIFFUSING SILENCERS TUNED
TO ABSORB DIFFERING FREQUENCIES
Fig. 8.18 Compact silencer designs needed for engines such as chainsaws where the bulk
of the entire engine must be minimized.
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