Design and Simulation of Two Stroke Engines

Chapter 8 • Reduction of Noise Emission from Two-Stroke Engines
system would need to be quietened, as extra noise reduction for the intake side can have little
bearing on the final outcome.
When the engine has intake silencer, S, fitted, then the opposite is the case. Tackling the
exhaust system for noise reduction in this case is pointless until the intake system noise level
is reduced, at least to equality. Such arguments are reinforced in the concluding remarks to
this chapter.
Silencing the untuned exhaust system of a chainsaw
The design of an untuned exhaust system for a two-stroke engine is discussed in Chapter
5, particularly in connection with the example of the chainsaw engine, where the small bulk
required of the entire powerplant precludes the availability of adequate space for effective
silencers, be they intake or exhaust. The problems in this regard are sketched in Fig. 8.18.
There are other types of two-stroke engines with untuned or relatively untuned exhaust pipes,
and these are to be found on agricultural and electricity-generating equipment where powerplant
space may not be at the same premium as it is on a handheld power tool such as a chainsaw or
a brushcutter. Another relatively straightforward example is the outboard motor, where the
virtually unsilenced exhaust gas is directed underwater into the propeller wash which provides
very effective exhaust silencing without any particular acoustic skills being employed
by a designer. Whether this underwater racket is offensive to fish is not known.
The most difficult engine for which to design an adequate silencing characteristic is the
handheld power tool, i.e., the chainsaw or similar device. It is self-evident that, as noise is a
functi . of the square of the dp, or pressure, fluctuation as seen in Eq. 8.1.2, then the larger
the silencer volume into which the exhaust pulses are "dumped," the lesser will be the dp
value transmitted into the atmosphere. The designer of the chainsaw is continually looking
for every available cubic centimeter of space to become part of the exhaust silencer. The
space may be so minimal that the two-box design shown in Fig. 8.18 becomes an impossible
luxury. The designer is left with the basic option of trimming out the fundamental firing
frequency using a diffuser type or side-resonant type of silencer as described in Sec. 8.5.
As a rule of thumb, the designer of a silencer for the handheld power tool will know that
there is going to be real silencing difficulties if there is not a total volume available for a
silencer which is at least twelve to fifteen times greater than the cylinder swept volume. If that
is not the case, then the only design methodology left open is to choke the exhaust system by
a restrictive silencer, thereby reducing the delivery ratio, and accept the consequential loss of
power. Further design complications arise from the legal necessity in many applications for
the incorporation of a spark arrestor in the final tail-pipe (see Fig. 8.18) before entry to the
atmosphere [8.22]. By definition, a spark arrestor is a form of area restriction, as it has to
inhibit, and more importantly extinguish, any small glowing particles of carbon leaving the
silencer. This need is obvious for engines such as chainsaws and brushcutters, where the
device is being used in an environment with a known fire hazard.
In any event, of equal importance is the necessity to damp the vibrations of the skin of the
silencer, for this compact device is exposed to the normal "impact" of the unattenuated exhaust
pulses. Consequently, double-skinning of the silencer surface is almost essential.
The design example chosen is the same chainsaw used several times before in various
chapters. The data for the chainsaw in Sec. 5.4.1 give the geometrical details of the exhaust
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