Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
Hence, the overall mean square sound pressure, Ps, can be obtained by simple addition of
that of the harmonics, using the procedure for the addition of sound energy in Eq. 8.1.5.
How successful that can be may be judged from some of the results presented by Coates
[8.3], although that work was carried out using the "homentropic method of characteristics"
for the unsteady gas flow along the duct, and employed isentropic pipe end boundary conditions,
both of which approaches are shown by Kirkpatrick [2.41, 5.20, 5.21] to be of a lesser
accuracy than the method presented in Chapter 2. The implication of that statement is that
Coates' ability to accurately calculate mass flow rates at the pipe exit, upon which accurate
noise computation is predicated, is impaired. Consequently, this research work is ongoing at
QUB using the theory given in Chapter 2, to investigate the order of improvement, if any, in
accuracy of prediction of noise transmitted into space from the ducting of engines.
8.4.2 The experimental work of Coates [8.3J
The experimental rig used by Coates is described clearly in Ref. [8.3], but a summary
here will aid the discussion of the experimental results and their correlation with the theoretical
calculations. The exhaust system is simulated by a rotary valve that allows realistic exhaust
pressure pulses of cold air to be blown down into a pipe system at any desired cyclic
speed for those exhaust pressure pulsations. The various pipe systems attached to the exhaust
simulator are shown in Fig. 8.2, and are defined as SYSTEMS 1-4. Briefly, they are as follows:
SYSTEM 1 is a plain, straight pipe of 28.6 mm diameter, 1.83 m long and completely
unsilenced.
SYSTEM 2 has a 1.83 m plain pipe of 28.6 mm diameter culminating in what is termed a
diffusing silencer which is 305 mm long and 76 mm diameter. The tail-pipe, of equal size to
the entering pipe, is 152 mm long.
SYSTEM 3 is almost identical to SYSTEM 2 but has the entry and exit pipes re-entering
into the diffusing silencer so that they are 102 mm apart within the chamber.
SYSTEM 4 has what is defined as a side-resonant silencer placed in the middle of the
1.83 m pipe, and the 28.6-mm-diameter through-pipe has 40 holes drilled into it of 3.18 mm
diameter.
More formalized sketches of diffusing, side-resonant and absorption silencers are found
in Figs. 8.7-8.9. Further discussion of their silencing effect, based on an acoustic analysis, is
in Sec. 8.5. It is sufficient to remark at this juncture that:
(i) The intent of a diffusing silencer is to absorb all noise at frequencies other than
those at which the box will resonate. Those frequencies which are not absorbed are
called the pass-bands.
(ii) The intent of a side-resonant silencer is to completely absorb noise of a specific
frequency, such as the fundamental exhaust pulse frequency of an engine.
(iii) The intent of an absorption silencer is to behave as a diffusing silencer, but to have
the packing absorb the resonating noise at the pass-band frequencies.
The pressure-time histories within these various systems, and the one-third octave noise
spectrograms emanating from these systems, were recorded. Of interest are the noise spectra
and these are shown for SYSTEMS 1-4 in Figs. 8.3-8.6, respectively. The noise spectra are
presented in the units of overall sound pressure level, dBlin, as a function of frequency. There
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