Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
holes to pipe in a side-resonant silencer is normally less than unity. Expressed numerically,
the following are conventional empiricism:
N h A h ,
absorption silencer A J
side-resonant silencer A
A3
The experimental evidence is that the absorption silencer is very effective at attenuating high
frequencies, i.e., above 2 kHz, and is relatively ineffective below about 400 Hz. Most authorities
agree that the theoretical design of an absorption silencer, either by the methodology of
Coates [8.3] or by an acoustic procedure, is somewhat difficult because it depends significantly
on the absorption capability of the packing material. The packing materials are usually
a glass-reinforced fiber material or mineral wool.
You should attempt the design using Prog.8.1, but arrange for as many holes of an appropriate
diameter in the central pipe as is pragmatic. If the hole size is too large, say in excess of
3.5 mm diameter, the packing material within the cavity will be blown or shaken into the
exhaust stream. If the hole size is too small, say less than 2.0 mm diameter, the particulates in
the exhaust gas of a two-stroke engine will ultimately seal them over, thereby rendering the
silencing system ineffective. The normal hole size to be found in such silencers is between
2.0 and 3.5 mm diameter.
One detailed aspect of design which needs special comment is the configuration of the
holes in the perforated section of the silencer. The conventional manufacturing method is to
roll, then seam weld, the central pipe from a flat sheet of perforated mild steel. While this
produces an acceptable design for the perforated pipe, a superior methodology is to produce
the pipe by the same production technique, but from a mild steel sheet in which the holes have
been somewhat coarsely "stabbed," rather than cleanly excised. Indeed, the stabbed holes can
be readily manufactured in pre-formed round pipe by an internal expanding tool. The result is
shown in sketch form in Fig. 8.15 for the finished pipe section. This has the effect of reducing
the turbulent eddies produced by the gas flowing in either direction past the sharp edges of the
clean-cut holes of conventional perforations. This type of turbulence has an irritating highfrequency
content; it will be recalled that a whistle produces noise by this very edge effect.
Positioning an absorption silencer segment
The published literature [1.8] on the topic agrees on several facets of the design as commented
on above, but the principal function is to remove the high-frequency end of the noise
spectra. The high-frequency part of any noise spectrum emanates from two significant sources:
the first is due to the sharp pressure rise at the front of the exhaust pulse, always faster in a
two-stroke than a four-stroke engine (see Sec. 8.6.1), and the second is from the turbulence
generated in the gas particle flow as it passes sharp edges, corners and protuberances into the
gas stream. The considerable noise content inherent in turbulence is quite visible in Plate 2.4
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