Design and Simulation of Two Stroke Engines

Chapter 3 - Scavenging the Two-Stroke Engine
vides vital information without which simulation of scavenging within a computer model
could not be conducted.
3.3.3 Connecting a volumetric scavenging model with engine simulation
The engine simulation by computer, as presented in Chapters 2 and 5, traces the thermodynamics
and gas dynamics of flow in, out and through the engine cylinder. The previous
section, and in particular Eq. 3.3.12, provided the vital piece of information regarding the
value of the purity of the charge leaving the cylinder. It is not an unreasonable assumption that
the gas leaving the cylinder is sufficiently well mixed that the exiting gas has a common
temperature, Te. It is also a very reasonable assumption that the pressure throughout the cylinder
is uniform and that the exiting charge has a pressure which is the same as the mean value
for all of the cylinder contents. However, the temperature of the exiting charge of a stratified
process must be composed of exhaust gas and fresh charge elements which are a function of
their physical position within the cylinder.
There are two ways of dealing with this problem of the determination of the exiting
charge temperature, one simple and one more complex.
The most complex method is actually the easiest to debate, for it consists of conducting a
CFD computation using a proprietary computer code, such as Phoenics or StarCD, and obtaining
the information very accurately and very completely for the entire scavenge operation.
The effectiveness of such a code is described in Sec. 3.4. At the same time, it must be
stated that CFD computations are slow, even on the fastest of computers, and the incorporation
of a CFD computation of scavenging into the ID engine simulation as described in Chapters
2 and 5 would change its computation time from several minutes into many days.
Thus, until the great day comes when CFD computations can be accomplished in minutes
and not days within an engine simulation model a simpler criterion is required to estimate the
temperatures of the air and exhaust components of the charge lost from the cylinder during
scavenging. I have presented such a simple criterion [3.35] consisting of the realistic assumption
that the temperature differential AT.^ at any instant between the air and the exhaust is
linked to the mean temperature in the cylinder. In other words, as the scavenge flow rate rises,
this reduces both the mean temperature within the cylinder, Tcy, and the temperature of the
exhaust gas, Tex; but also there is an increase of the temperature of the trapped air, Tta. The
arithmetic value of the temperature differential, ATax which links these literal statements
together is defined by a factor, Ctemp, thus:
Tex = Tta + ATax = Tta + QempTCy (3.3.13)
The temperature differential factor, Ctemp> is clearly a complex function of time, i.e., the
engine speed, the displacement/mixing ratio of the fluid mechanics of the particular scavenge
process, the heat transfer characteristics of the geometry under investigation, etc. Thus no
simple empirical criterion is ever going to satisfy the needs of the simulation for accuracy. My
best estimation for the arithmetic value of Ctemp is given by the following function based on
its relationship to (a) the attained value of cylinder scavenging efficiency, SEcy, computed as
being trapped at the conclusion of a scavenging process, and (b) the mixing caused by time
and piston motion, the effects of which are expressed by the mean piston speed, cp. The
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