Design and Simulation of Two Stroke Engines

Name
F
R
Chapter 8 • Reduction of Noise Emission from Two-Stroke Engines
Table 8.5 Effect of radiused porting on performance and noise
Intake
(dBA)
82.9
82.5
Exhaust
(dBA)
89.0
88.3
bmep
(bar)
3.75
3.61
bsfc
(g/kWh)
480
488
bsHC
(g/kWh)
127
134
DR
0.539
0.530
ratio is virtually achieved and so the power performance is reasonably similar, yet is significantly
different in a very important area. The trapping efficiency has been deteriorated so
there is a rise in specific fuel consumption and a reasonably significant increase in hydrocarbon
emissions. Thus the trade-off could be regarded as unacceptable by a manufacturer who
must meet emission standards as a first priority, and may well feel that more effective engine
silencing can be achieved by other means. Under pressure from emissions legislation, even
the unthinkable may become acceptable, i.e., somehow make room on the device for larger
volume intake and exhaust silencers. Before the millenium, as an exhaust catalyst may have
to be installed to satisfy exhaust emissions legislation, that manufacturing bullet just may
have to be bitten. The effective silencing of handheld power tools should have been carried
out by their manufacturers many years ago and it is ironic that it is the latest U.S. and other
exhaust emissions standards which are finally going to accomplish it.
8.6 Silencing the tuned exhaust system
You may find the following section useful as an exercise in data collection which could
be the starting point for an engine simulation program to optimize any design, or the commencement
of an experimental program for the same purpose. A design for a tuned pipe
system is proposed and the data for it are collected using the acoustic theories given earlier.
The muffling of a tuned exhaust pipe is relatively straightforward and poses little difficulty
in attaining the dual aims of good silencing while retaining the pressure wave tuning
action necessary to attain a high bmep and high specific power output. In Sec. 7.3.4 it is
shown how even the introduction of a catalyst can be accommodated in a tuned pipe without
deterioration of the many design goals. Fig. 7.28 shows how that is accomplished, including
the presence of a silencer after the tuned section. This explanation is expanded here by Fig.
8.26. The design of the tuned section of the pipe is obtained using a simulation as in Chapter
5, or the empiricism in Chapter 6, and the acoustic design of the silencing section can be
carried out by the recommendations in this chapter. After the tuned section of the pipe, depending
on the availability of space within the application for the power unit, there will be
several chambers dedicated to the removal of specific acoustic frequencies or frequency bands.
Typical of such chambers are those defined as diffusing silencers, side-resonant silencers,
and absorption silencers. The detailed discussion of the noise suppression behavior of each of
these types is found in Sec. 8.5.
Perhaps the most important point to be made at this introductory stage is that, having
reduced the area of the exhaust pipe deliberately to attain a "plugging" reflection pulse for
power performance reasons, the first silencing design action has already been taken to the
benefit of the specific power output of the engine. To reinforce the point, the optimum tail-
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