Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
(iv) Two-stroke engines use ball, roller, and needle roller element bearings and these
tend to be noisy by comparison with pressure-fed hydrodynamic bearings.
The advantages of the two-stroke engine
(i) The engine with the tuned exhaust pipe produces a high specific output, but this is
achieved by choking the final outlet diameter, thereby simplifying the design of an
effective exhaust silencer.
(ii) The crankcase pump induces air by pumping with a low compression ratio and, as
this reduces the maximum values of air intake particle velocity encountered in its
time history, this lowers the higher-frequency content of the sound produced.
(iii) The peak combustion pressures are lower in the equivalent two-stroke cycle engine,
so the noise spectrum induced by that lesser combustion pressure is reduced
via all of the transmission components of the cylinder, cylinder head, piston and
crankshaft.
8.4 Some fundamentals of silencer design
If you study the textbooks or technical papers on acoustics and on silencer design, such as
many of those referenced below, you will find that the subject is full of empirical design
equations for the many basic types of silencers used in the field of internal combustion engines.
However useful these may be, you will get the feeling that a fundamental understanding
of the subject is not being acquired, particularly as the acoustic theory being applied is
one oriented to the propagation of acoustic waves, i.e., waves of infinitesimal amplitude,
rather than finite amplitude waves, i.e., waves of the very considerable amplitude to be found
in the inlet and exhaust systems of the internal-combustion engine. The subject matter to be
found in the acoustic treatment of the theory is somewhat reminiscent of that for the topic of
heat transfer, producing an almost infinite plethora of empirical equations for which the authors
admit rather large error bands for their implementation in practice. This has always
seemed to me as a most unsatisfactory state of affairs. Consequently, a research program was
instigated at QUB some years ago to determine if it was possible to predict the noise spectrum
emanating from the exhaust systems of internal-combustion engines using the approach of
the calculation of the propagation of finite amplitude waves by the method of characteristics
as described in Ref. [7.36]. This resulted in the technical publications by Blair, Spechko and
Coates [8.1-8.3]. A much more complete exposition of the work of Coates is to be found in his
doctoral thesis [8.17].
8.4.1 The theoretical work of Coates [8.3]
In those publications, a theoretical solution is produced [8.3] that shows that the sound
pressure level at any point in space beyond the termination of an exhaust system into the
atmosphere is, not empirically but directly, capable of being calculated. The amplitude of the
nth frequency component of the sound pressure, pn, is shown to be primarily a complex
function of: (a) the instantaneous mass flow rate leaving the end of the pipe system, rh, and
(b) the location of the measuring microphone in both distance and directivity from the pipe
end, together with other parameters of some lesser significance.
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