Design and Simulation of Two Stroke Engines

Chapter 8 - Reduction of Noise Emission from Two-Stroke Engines
There are several other factors that contribute to the overall effectiveness of this silencer.
They are:
(i) The efflux of hot exhaust gas occurs through an annular ring into air which is at a
much lower temperature. There is a large contact area between the two. This gives
considerable damping of the turbulence in the exiting exhaust gas plume, thereby
reducing the high-frequency noise inherent from that source. This approach to silencing
turbulence noise is conventional practice for aircraft gas turbines.
(ii) The central body houses side-resonant cavities which can be used to tune out particular
frequencies with a high noise content. The flow passing the entrances to
these cavities is moving at right angles to those apertures, and at particle velocities
closer to acoustic levels than in the conventional silencers shown in Figs. 8.7-8.9.
Thus the assumptions inherent in acoustic theory for Helmholz resonators is approached
more closely and the application of that theory can be applied with more
confidence in the quality of the outcome.
(iii) The exhaust pressure pulsations entering the first chamber, a diffusing silencer
section, have the normal and considerable amplitude associated with such waves.
The first box reduces the magnitude of that pressure oscillation prior to it entering
the annulus en route to the atmosphere, passing the several resonant cavities as it
goes. However, the outside skin of the silencer does not experience the forces due
to the full magnitude of the original oscillation in the first box, as is the case in the
other silencers shown in Figs. 8.7-8.9, and so the outside skin vibrates less than
those shown in the sketches. Later, in Sec. 8.6, there is discussion that double skinning
of the outside of a silencer can be a vital issue to prevent that vibration from
being a significant source of noise.
8.5.5 Silencing the intake system
Probably the simplest and most effective form of intake silencer is of the type sketched in
Fig. 8.11. The geometry illustrated is for a single-cylinder engine but the same arrangement
can also apply to a multi-cylinder design, and you will find such a discussion in the paper by
Flaig and Broughton [1.12]. From that paper is reproduced the attenuation curve for a V8
two-stroke outboard motor, shown in Fig. 8.17. You can see that the peak attenuation required
is mainly in the band from 400-600 Hz. Note that this corresponds to a forcing frequencv
from the fundamental induction pulses of this eight-cylinder engine at 3000-4500 rpm.
The acoustic design of the low-pass intake silencer
Fig. 8.11 supplies the basic mechanism of induction silencing, being a volume connected
to the induction system, however many air intakes to the several cylinders there may be. On
the atmospheric side of this box is a pipe of length, Lb, and area, Ab, or with diameter, db, if
it has a circular cross-section. The air cleaner is placed within the box, helping to act as an
absorption silencer of the various high-frequency components emanating from the edges of
throttles or carburetor slides. Assuming that it is reasonably transparent to the air flow, this
has no real effect on the silencing behavior of the box volume, "VV
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