Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
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PRODUCTION tlLINC**
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FRIOUINCY (Hx)
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Fig. 8.17 Attenuation characteristics of an intake silencer for the
OMC V8 outboard (from Ref [1.12]).
This type of silencer is known as a low-pass device and has no silencing capability below
its lowest resonating frequency. The sharp drop in noise attenuation behavior associated with
this effect is clearly seen in Fig. 8.17 for the OMC V8 outboard motor, which has a lowest
resonating frequency of about 200 Hz (8 cylinders at a minimum full throttle operating speed
of 1500 rpm is equivalent to 200 Hz as a fundamental frequency).
The natural frequency of this type of silencer [1.8] is given by fj, where:
Ar
LeffVb
(8.5.17)
where the effective length of the intake pipe, Leff, is related to the actual length, Lp, and the
diameter, dp, by:
'eff L„ + (8.5.18)
The design of the silencer is accomplished by setting this natural frequency of the silencer,
fj, to correspond to the natural frequency of the induction pulses from the engine, fe.
Returning to the attenuation behavior of the intake silencer of the OMC V8 engine outboard
motor in Fig. 8.17, it can be seen that the peak transmission losses occur in two distinct
bands, i.e., at 100 Hz and 500 Hz, a ratio of five; the lower absorption frequency at 100 Hz
fundamentally corresponds to an engine speed of 750 rpm.
From this discussion, it is clear that the engine forcing frequency of direct design interest,
fe, is that corresponding to the engine speed of rotation and the number of cylinders,
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