Design and Simulation of Two Stroke Engines

CD
1.2 -i
1.0 -
di „„
LU 0.8
cc
m
z
O 0.6 -
H
O
UL
CO
CO
<
0.4 -
0.2 ---r
0.0
350
125 cc GP MOTORCYCLE ENGINE, 10350 rpm
DR=1.15,SE=0.96
IGNITION 20 9 btdc
HEAT RELEASE, 8 B btdc, DELAY 12 B b e =41 e
Chapter 4 - Combustion in Two-Stroke Engines
MEASURED
Vibea=5.25, m=1.25
360
—r
370 380 390
CRANKSHAFT ANGLE, degrees
Fig. 4.7(e) Mass fraction burned characteristics of a racing engine.
The data are recorded by Cartwright [4.35] in the course of the preparation of that reference.
The measured data can be seen to be modeled by Vibe coefficients, a and m, of 5.25 and 1.25,
respectively. The burn period, b°, at an engine speed of 10,350 rpm in the middle of the power
band, and a scavenging efficiency of 0.95, is quite brief at 41°. Note that the fuel being consumed
is aviation gasoline with a motor octane number (MON) of 100, and a H/C molecular
ratio of 2.1; it is colloquially referred to as "avgas." The 50% burn position is at 9° atdc; the
periods of 0-20%, 0-50%, and 0-80% burn are recorded as 10°, 17° and 25°, respectively.
There are many common factors in these diagrams of mass fraction burned. One which is
almost universal is the siting of the 50% point for the mass fraction burned, B. The optimized
burn profile rarely has this point before 5° atdc nor later than 10° atdc. It can also be observed,
from Fig. 4.5 and Figs. 4.7(a), (b), (d), and (e) that the ignition delay for engines, with scavenging
efficiencies at 0.75 and above, are quite commonly between 10° and 14°, whereas at
the lower scavenging efficiency, to be found in Fig. 4.7(c), this rises to 23° crankshaft angle.
However, I would not wish this to become the universal "law of ignition delay," for the evidence
is too flimsy for that conclusion. The modeler should note that increasing the value of the
coefficient, a, and decreasing the value of the coefficient, m, advances the rate of combustion.
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