Design and Simulation of Two Stroke Engines

Chapter 5 - Computer Modeling of Engines
exactly as shown in Fig. 5.1(b), with the top section varying from 40 to 53 to 40 mm effective
width over some 10 mm, and then remaining virtually parallel at 40 mm down to the bdc
position. A further aspect of the exhaust port timing is that it has an exhaust control valve, as
sketched in Fig. 5.2. If the exhaust control valve perfectly sealed the cylinder at closing timings
of 85,90,95,100,105 and 110° btdc, then the trapped compression ratio would be raised
from 9.0 to 9.6, 10.3,11.1,11.8,12.4 and 13.1, respectively. The valve does not seal the port
in this ideal manner, but does so quite effectively, and in its fully lowered position closes the
port at 95° btdc.
The engine is liquid cooled and has a tuned exhaust system as sketched in Fig. 5.7. The
lengths Li to L7 are 83,189,209, 65, 78, 205 and 250 mm, respectively. The diameters di to
d7 are 37.5, 48.5, 100, 116, 116, 21 and 21 mm, respectively.
During the simulation, it is necessary to assume values for the mean wall temperatures of
the various elements of the ducting and the engine. The values selected are, in °C: cylinder
surfaces, 200; crankcase surfaces (which in this engine receive some coolant), 80; inlet duct
wall, 30; transfer duct wall, 100; exhaust duct walls, 350.
The combustion model employed is exactly as shown in Fig. 4.7(e) for a racing engine,
with an ignition delay of 12°, a combustion duration, b°, of 41°, and Vibe constants, a and m,
of 5.25 and 1.25, respectively. The actual engine uses aviation gasoline, the properties of
which are given in Sec. 4.3.6, at an air-to-fuel ratio of 11.5. It is spark-ignited with an ignition
timing of 20° btdc up to 11,500 rpm, when the system in practice retards the spark linearly
until it is at 14° btdc at 12,300 rpm. The simulation incorporates the experimental ignition
timing curve. The burn coefficient, Cburn. is 0.85.
The scavenge model used in the simulation is as given in Sec. 3.3.1 and is characterized
by the Ko, Ki and K2 coefficients numerically detailed for the "YAM14" cylinder in Fig. 3.16.
The particular racing engine cylinder has not been scavenge tested, so its precise behavior is
unknown. Therefore, the scavenging characteristics of a multiple port, loop-scavenged, motorcycle
engine, with relatively good quality scavenging, has been assumed. Within the simulation
these data are applied through the theory given in Sees. 3.3.1 to 3.3.3.
The friction characteristics assumed during the simulation are as described above in Eq.
5.4.1.
Correlation of simulation with measurements, Figs. 5.28-5.33
The measured performance characteristics of power and torque (as bmep) are shown in
Fig. 5.28 and compared to those computed by the simulation over the speed range of the
engine. The correlation for power and bmep is good. In Fig. 5.29, the measured and computed
behavior for delivery ratio, trapping efficiency and charging efficiency are shown; here, too,
the correlation is good both in amplitude and profile. The high values of delivery ratio and
charging efficiency, compared to the equivalent diagram for the chainsaw in Fig. 5.10, explains
the disparity in the bmep and power attained in each case. It does not explain how they
are achieved.
The simulation of the exhaust gas temperature, not just as a bulk mean value recorded in
the middle of the pipe system, but everywhere throughout the pipe at every instant of time, is
a vital issue if the simulation is to accurately phase the dynamic events within the long tuned
exhaust pipe. It is not possible to record temperature-time histories with the same accuracy as
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