Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
T RELEASE RATE, J/deg
HEA
i
I TDC 1
. 6 ___!__;_ i
ORG /
- 6 — -IV-r-f —
0 / | I
| 10 9 | 10 a | 20 2
>l b fi I b fi I b 9
i 6 1 6 I 3
I I I I i
-20 0 20
CRANKSHAFT ANGLE, deg. atdc
I i
TOTAL HEAT RELEASED
14b 9 QRB
36
I 20 2 |
I b 9 I
L bs
3 I I
40
I f I
60
Fig. 4.6 Possible model of heat release rate for combustion simulation.
The analysis of the experimental data in Fig. 4.5 is found to be fitted with coefficients a
and m of value 8 and 1.3, respectively, for a total burn period, b°, of 60° duration. This is the
"calculated" data referred to in Fig. 4.5. The fit can be seen to be good and when the heat
release rate is recalculated from this theoretical equation, and plotted in Fig. 4.4 as "calculated"
data, the good correspondence between measurement and calculation is maintained.
Thus it is possible to replace the simple line model, as shown in Fig. 4.6, with the Vibe
approach.
Further data for spark-ignition engines are found in the paper by Reid [4.31] and a reprint
of some is in Fig. 4.7. The data show mass fraction burned cuves for a hemispherical combustion
chamber on the engine at throttle area ratio settings of 100%, 25% and 10% in Figs.
4.7(a)-(c), respectively. These data come from an engine of similar size and type to the QUB
LS400, but with a bore-stroke ratio of 1.39. The engine speed is 3000 rpm and the scavenging
efficiency for the cylinder charge in the three data sets in (a) to (c) are approximately 0.8,0.75
and 0.65, respectively; the scavenge ratio was measured at 0.753, 0.428 and 0.241, respectively.
It is interesting to note the increasing advance of the ignition timing with decreasing
cylinder charge purity and the lengthening ignition delay which accompanies it. Nevertheless,
the common factor that prevails for these mass fraction burned curves (and the comment
is equally applicable to Fig. 4.5) is that the position of 50% mass fraction burned is almost
universally phased between 5° and 10° atdc. In other words, optimization of ignition timing
means that, taking into account the ignition delay, the burn process is phased to provide an
optimized pressure curve on the piston crown and that is given by having 50% of the fuel
burned by about 7.5° atdc. The 50% value for the mass fraction burned, B, usually coincides
with the peak heat release rate, QRQ .
310