Design and Simulation of Two Stroke Engines

Chapter 4 - Combustion in Two-Stroke Engines
Appendix A4.2 A simple two-zone combustion model
The combustion process
Sec. 4.4.2 details a single-zone combustion model. The physical parameters for the theoretical
solution of a combustion process in two zones are summed up in the sketch in Fig.
A4.1. The assumption pervading this approach is that the pressure in the unburned and burned
zones are equal at the beginning and end of a time step in a computation, which is represented
by a time interval, dt, or a crankshaft interval, d9. The piston movement produces volume
variations from N\ to V2 during this period, and so the mean cylinder conditions of pressure
and temperature change from Pi to P2, and Ti to T2, respectively. The total cylinder mass, me,
is constant but the masses in the unburned and burned zones, mb and mu, change with respect
to the increment of mass fraction burned, dB, during this time interval, thus:
dB = Bo -B, ='B Gj+de -B(
(A4.2.1)
and mb2 = m bl + dB x mc (A4.2.2)
m u2 = m ul -dB x rtic (A4.2.3)
also rric = mbi + m ul and rric = mU2 + mb2 (A4.2.4)
The purities in both the burned and unburned zones are known. That in the burned zone is
zero and at the initial temperature, Tbi, the theory of Sec. 2.2.6 can be deployed to determine
the gas properties at that temperature, with respect to the air-to-fuel ratio and the particular
hydrocarbon fuel being used, to find the numerical values of gas constant, Rb, specific heat at
constant volume, Cvb, and the ratio of specific heats, "ft,- Th e purity in the unburned zone is
that at the trapping condition, nt. In the unburned zone, at the temperature, Tui, the properties
PlT„i
VU1 mui
P2Tu2
Vu2 mu2
Fig. A4.1 Simple theoretical model for two-zone combustion.
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