Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
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90 -i CHAINSAW ENGINE, 9600 rpm
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FREQUENCY, Hz
Fig. 8.26 Effect of box volume on the exhaust (dBA) noise spectra.
pipe diameter, d6 or d-j in Fig. 5.7, is always about half that of the initial exhaust pipe diameter,
d]. In other words, the area of the exhaust pipe which left the engine cylinder has been
reduced by a factor of about four. This is quite unlike the situation for a four-stroke engine
where any restriction of the exhaust pipe almost inevitably leads to a drop in delivery ratio
and a consequential linear fall in power output. As a direct result, the silencing of a fourstroke
engine is a more delicate design procedure. Equally, it implies that a high-performance
street motorcycle with a two-stroke engine can be designed relatively easily to be legally
quiet and the manufacturers of all production motorcycles ensure that this is the case. The
somewhat noisy after-market, or even "diy," devices which cause irritation to the general
public are the result of modifications by the owner, aimed more at machismo than machine
efficiency!
The earlier design discussion on tuned expansion chambers did not dwell on the material
to be used in its mechanical construction, but custom and practice show that this is typically
fabricated from mild steel sheet metal which is about 1.1-1.3 mm thick. This may be acceptable
for a racing engine where the mass of the exhaust system, as a ratio of the total machine
mass, must be as low as possible. For a silenced system, as the exhaust pulses of a two-stroke
engine are particularly steep-fronted, a thin metal outer skin of an expansion chamber and
silencer will act as the diaphragm of a metal loudspeaker, causing considerable noise transmission
to the atmosphere [8.26]. Therefore, the designer must inevitably introduce doubleskinning
of the most sensitive parts of the system, and often a layer of damping material is
inserted between the double-skinned surface. Occasionally it is found to be adequate to line
the internal surface of the silencer with a high-temperature plastic damping compound, and
this is clearly an economic alternative in many cases.
8.6.1 A design for a silenced expansion chamber exhaust system
To reinforce the points made above, consider the design of a silenced and tuned expansion
chamber system for an imaginary 125 cm 3 road-going motorcycle engine. The machine
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