Design and Simulation of Two Stroke Engines

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Chapter 5 - Computer Modeling of Engines
3 CYLINDER SUPERCHARGED ENGINE AT 3500 RPM
EXHAUST
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CHARGING
EFFICIENCY
CYLINDER
0 100 200 300 400
CRANKSHAFT ANGLE, deg. atdc on cylinder no.3
Fig. 5.36(c) Open cycle pressures and charging for cylinder No. 3.
charging efficiency curve commencing at 150° atdc. Further information on this topic is given
in Fig. 5.37, for the purity and temperature in the scavenge ports at cylinder 1, an end cylinder.
The blowback into the exit from the scavenge ports is considerable, with the purity dipping
to 0.4 and the temperature rising to 600°C. It is almost bottom dead center before the
situation has been recouped and the purity has risen again to unity. It is a much more extreme
characteristic than that seen for a crankcase compression engine in Figs. 5.17 and 5.18. It also
illustrates a common failing of supercharged designs, in that they are rarely endowed with
adequate blowdown characteristics, for the cylinder backflow into the scavenge ports backs
up the compressor, increasing the pumping losses significantly. Designers of such engines
should re-examine Figs. 3.40 and 3.41, for it is possible you may have initially considered the
exhaust port width shown there as excessive. In the light of the evidence given here, you may
wish to reconsider that opinion.
The exhaust tuning of a three-cylinder engine
The pressure characteristics within the exhaust pipe at the exhaust port are also shown in
Figs. 5.36(a)-(c) for each of the three cylinders. Just prior to the closing of the exhaust port,
there is a "plugging" pulse reminiscent of that seen for the racing engine in Fig. 5.34. While
this engine does not have a tuned expansion chamber, the effect of employing the compact
manifold is almost the same. The "plugging" pulse effect in this case is caused by another
cylinder blowing down into the manifold. In the case of Fig. 5.36(a) for cylinder 1, the "plugging"
pulse emanates from the opening of the exhaust port of cylinder 3. In Fig. 5.32(b) it is
from cylinder 1; in Fig. 5.32(c) it is from cylinder 2. This information is gleaned from the
firing order being 1-3-2. The effect is to hold the charging efficiency at its plateau level, in
this case about 0.61. This behavior in the multi-cylinder engine is known as "cross-charging."
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