Design and Simulation of Two Stroke Engines

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Chapter 4 - Combustion in Two-Stroke Engines
The fundamental thesis behind the calculation is that all state conditions equalize during
any incremental piston movement. Thus, the values of cylinder pressure and temperature are
pcl and Tc], respectively, at the commencement of the time step, dt, which will give an incremental
piston movement, dx, an incremental crank angle movement, d9, and a change of
cylinder volume, dV.
However, if equalization has taken place from the previous time step, then:
Pel =Psl =Pbl cl Tsl=Tbl Pel = Psl = Pbl
and V cl Vsi = Vb] mci = mt = msi + mbl
For the next time step the compression process will occur in a polytropic manner with an
exponent, n, as discussed in Sec. 4.2.2. If the process is considered ideal, or isentropic, that
exponent is y, the ratio of specific heats for the cylinder gas. Because of the multiplicity of
trapping conditions and polytropic compression behavior, it is more logical for any calculation
to consider engine cylinders to be analyzed on a basis of equality. Therefore, the compression
process is assumed to be isentropic, with air as the working fluid, and the trapping
conditions are assumed to be at the reference pressure and temperature of 1 atm and 20°C. In
this case the value of y is 1.4. The initial cylinder density is given by:
Pt
RT
101325
287 x 293
1.205 kg/ ITf
The value of the individual compression behavior in the squish and bowl volumes, as
well as the overall macroscopic values, follows:
Ps2
Psl
v sl
yVsu
Pb2 =
Pbl
Pc2
Pcl
v bl
v v b2y
If a squish action takes place, then the value of squish pressure, pS2, is greater than either pC2
or pb2, but:
V cl
'c2
Ps2 > Pc2 > Pb2
The squish pressure ratio, Psq, causing gas flow to take place, is found from:
T> _ Ps2
sq
Pb2
327
Pel
Pcl
Y cl
(4.5.1)