Design and Simulation of Two Stroke Engines

Chapter 8 - Reduction of Noise Emission from Two-Stroke Engines
90 -i CHAINSAW ENGINE, 9600 rpm
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Fig. 8.22 Effect of modified exhaust system on the (dBA) noise spectra.
reduces the overall figure by 2 dBA, supporting the previous contention that reducing the
exhaust noise would rapidly lower the total, which is still somewhat higher than the intake
noise. Observe that the intake noise of Z is slightly higher, by some 0.2 dB, than that for
system F. There is a slight penalty from the exhaust choking on the intake noise spectra.
From Fig. 8.21, the exhaust outlet choking is seen to dampen the mass flow oscillations,
from whence comes the reduced noise levels observed in either Table 8.3, or in the (dBA)
spectra in Fig. 8.22.
It was observed previously that an acoustic analysis of this exhaust silencer by Prog.8.1
showed noise attenuation only above 800 Hz. In Fig. 8.22 that is indeed seen very clearly, but
there appears to be equally strong damping below 800 Hz; so is Prog.8.1 sufficiently adequate
for design purposes, as this information apparently contradicts it? The answer to that query is
given in Fig. 8.23, where both the total noise (as dBlin) and the A-weighted spectra are presented.
The matter is now clarified. It is the A-weighting characteristic which is providing the
reduced noise spectrum profile below 800 Hz. The total noise spectrum is relatively high and
flat until 800 Hz when the diffusion behavior begins to take effect. In other words, Prog.8.1
does not include the effect of a noise weighting scale.
The relevance of exhaust silencer volume for a chainsaw
The space to accommodate an adequate volume of exhaust or intake silencer for a simple
two-stroke engine is often a critical design element. Much comment on this issue has already
been made above. To illustrate this point, and of the effectiveness of simulation in the design
process, the modeling of the design, referred to above as F, is expanded to include slightly
larger exhaust silencers with approximately 10 and 25% greater volumes. These designs are
referred to as P and Q, and the changes to the exhaust box volumes are summarized in Table
8.4, with the nomenclature as in Fig. 5.6. The computations are run at the single engine speed
of 9600 rpm.
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