Design and Simulation of Two Stroke Engines

Chapter 5 - Computer Modeling of Engines
Nevertheless, the explanation of the mechanism of the achievement of high specific performance
characteristics in the simple, ported two-stroke engine is quite straightforward. The
fundamental principles are (i) provide a "plugging" pulse prior to trapping to seal in the
cylinder charge and prevent spillage, (ii) provide a "suction" pulse around the bdc position to
assist the scavenging flow in reaching a high charging efficiency prior to sealing, (iii) provide
a "suction" pulse around the bdc position to extract air from the crankcase and lower its
pressure so as to commence induction vigorously, and (iv) harness the exhaust pulse resonance
effects to enhance the basic mechanisms of (i), (ii) and (iii), if that is possible within
the design configuration. This process is significantly related to engine speed, as the further
illustrations from this simulation of cylinder and exhaust pipe pressure show in Figs. 6.18 to
6.20 for engine speeds of 9600, 11,200 and 12,300 rpm, respectively. They are near the bottom,
near the peak, and at the upper end of the useful power band of this highly tuned engine.
The basic tuning and charging action seen in Fig. 5.32 is observed to appear in Figs. 6.18-6.20
to a greater or lesser degree. Further discussion occurs in Sec. 6.2.5 with respect to the design
of the expansion chambers for two-stroke engines and their speed-related tuning characteristics.
A discussion on speed-related tuning is also in Sec. 6.2.5.
The behavior of the reed valve induction system, Fig. 5.35
Fig. 5.35 shows the results of computer simulation of the reed valve motion, the delivery
ratio, and the forcing pressures to create those effects, namely within the crankcase and the
superposition pressure at the entrance to the reed block which is at the crankcase end of the
inlet tract. The reed valve motion is plotted as the tip lift ratio, Crcjt-
It can be seen that the reed is open for most of the cycle, but it is impelled into action by
a strong pressure ratio across it which lasts from about 190° atdc to 240° atdc. The pressurecreated
impulse is significant, for it coincides with (i) the crankcase pressure having been
lowered below atmospheric pressure by the gas-dynamic tuning action emanating from the
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