Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
of gas constant, Ru, specific heat at constant volume, Cyy, and the ratio of specific heats, yu,
may also be determined from Sec. 2.1.6. The average properties of the entire cylinder space at
the commencement of the time step, and which are assumed to prevail during it, may be found
as:
Rl = BiRb - (1 - B,)RU
Cvi = BAb - (1 - B!)CVU
(A4.2.5)
(A4.2.6)
Y^Btfb-fl-Bifru (A4.2.7)
Using heat release data, or a mass fraction burned approach as described in Sec. 4.4.2, the
overall behavior for the entire cylinder space may be found as before, using Eq. 4.4.1, or even
more simply below if the time interval is sufficiently short. A sufficiently short time interval
is defined as 1 ° crankshaft.
T SQR-oQL+meC^-p^-V!)
12 ~
m C
c vl
(A4.2.8)
and p2 = ° 1 2 (A4.2.9)
V 2
For the properties within the two zones, a simple solution is possible only if some assumption
is made regarding inter-zone heat transfer. Without such an assumption, it is not
possible to determine the individual volumes within each zone, and hence the determination
of individual zone temperatures cannot be conducted, as the thermodynamic equation of state
must be satisfied for each of them, thus:
Tn2=*£f- and Tb2=A (A4.2.10)
m R
u2 u
m R
b2 b
Some researchers [4.29-4.31] have employed the assumption that there is zero heat transfer
between the zones during combustion. This is clearly unrealistic, indeed it is inter-zonal
heat transfer which induces detonation. The first simple assumption which can be made to
instill some realism into the solution is that the process in the unburned zone is adiabatic. At
first sight this also appears too naive, but this is not the case as the alternative restatement of
the adiabatic assumption is that the unburned zone is gaining as much heat from the burned
zone as it is losing to the surfaces of the cylinder and the piston crown. The more this assumption
is examined, in terms of the relative masses, volumes, surface areas and temperatures of
the two zones, the more logical it becomes. With it, the solution is straightforward:
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