Design and Simulation of Two Stroke Engines

Design and Simulation ofTwo~Stroke Engines
tant to emphasize that slowing down the exit rate of exhaust gas flow during the blowdown
phase inevitably leads to longer port timing durations and some reduction of trapping efficiency.
This can be alleviated to some extent by making the exhaust port wider without unduly
increasing the total timing duration, as the shallower area-time profile necessary for
noise reduction also permits a wider port without endangering the mechanical trapping of the
piston ring by the timing edge.
Even with a suitably profiled exhaust port timing edge, the pressure-time rise characteristics
of an exhaust pulse in a two-stroke power unit is much faster than that in a four-stroke
cycle engine where a poppet valve is employed to release exhaust gas from the cylinder. As
already remarked, the noise spectrum from a rapidly rising exhaust pressure wave is "rich" in
high-frequency content, irritating to the human ear.
In Fig. 8.25 it can be seen that the shaping of the exhaust port can be conducted in the
same fashion as for the bottom of the inlet port. However, Johnston [8.14] shows more extreme
profiles than the simple radiusing and proposed the "Q" port shape shown on the same
figure, with a view to enhancing the effects already discussed.
Simulation of the effect by engine modeling
In the input data to the chainsaw engine simulation, both the exhaust and intake ports
have a maximum width of 28 mm. The simulation result, described above as F, is compared
with heavily radiused intake and exhaust ports, labeled as R. The original top and bottom
radii for the intake and exhaust ports is 2 and 3 mm, respectively. For the simulation of R, this
radius is increased to 10 mm in both cases. However, this reduces the port area and so it is
necessary to open the exhaust port 1 ° earlier, at 107° atdc, and to open the inlet port 3° earlier,
at 78° btdc, so as to provide for the same delivery ratio to pass through the engine. The result
of the simulation of design R is shown in Table 8.5, where it is compared with design F.
The reduction of intake and exhaust noise as forecast by the theory is achieved, even
though the modification of changing the port radii from 2 or 3 mm to 10 mm is a relatively
modest change, by comparison with the "Q port" shown in Fig. 8.25. The equality of delivery
i
NORMAL
§1
l-l-
E5 RADIUSED
STEEP FRONTED
PULSE
t
SHALLOWER
PROFILE
Fig. 8.25 Exhaust port profiling to reduce noise.
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