Design and Simulation of Two Stroke Engines

Chapter 2 - Gas Flow through Two-Stroke Engines
It can be observed that the GPB finite system modeling gives a very accurate representation
of the measured events for the reflection of compression pressure waves at a steeply
tapered pipe segment within ducting.
2.19.6 A convergent tapered pipe attached to the QUB SP apparatus
The experiment simulates an exhaust process with a straight pipe incorporating a convergent
taper attached to the cylinder. Fig. 2.46 shows a straight aluminum pipe of 108 mm and
25-mm internal diameter attached to the port, followed by a 2.667 m length of 68-mm parallel
section pipe, then a steeply tapered pipe at 12.8° included angle convergent to 25-mm diameter
over a 195 mm length; the final length to the open end to the atmosphere is a 2.511 m
length of pipe of 25-mm internal diameter. This form of steep taper is very commonly found
within the ducting of IC engines, including the rear cone sections of expansion chambers of
highly tuned two-stroke engines. There are pressure transducers attached to the pipe at stations
1, 2 and 3 at the length locations indicated. The basic theory of pressure wave reflections
in tapered pipes is given in Sec. 2.15.
CYLC
STATIC )N 1 STATION 2
108
< *•
2
P
as
P
d=68mmtaper=12.8 e
420
"
2454
2775
^^-^^^^
^J
2970
2 *"
<
STATION 3
3279
jp| d=25 mm
5481
Fig. 2.46 QUB SP rig with a convergent cone attached.
The cylinder was filled with air and the initial cylinder pressure and temperature were 1.5
bar and 293 K. In Figs. 2.47 to 2.49 are the measured pressure-time records in the cylinder
and at stations 1,2 and 3. The result of the computations using the GPB modeling method are
shown on the same figures, and the correlation is very good. Where differences can be observed
between measurement and computation are indicated on the figure.
The converging tapered pipe acts as a contraction to the area of the pipe system and sends
a compression wave reflection back from the compression wave of the exhaust pulse. It is
observed arriving back at station 1 at 0.02 second in Fig. 2.47, albeit in a superposition condition
from the nearby contraction.
Actually, as it is a short, steeply tapered pipe there is a close resemblance between the
behavior of this pipe and the sudden contraction discussed in Sec. 2.19.4 and illustrated in
Figs. 2.38 to 2.41. The cylinder release conditions were identical in both cases so the exhaust
pulses which arrived at the discontinuity in areas was the same. Consequently there are great
similarities between Figs. 2.39 and 2.47 for the pressure transducer at station 1, and between
Figs. 2.40 and 2.48 for the pressure transducer at station 2. This close comparison also extends
to the pressure transducer at station 3, in the tapered cone version in Fig. 2.49 and in
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