Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
is Te which is found from the following approximate consideration of the enthalpy of the
exiting mixture of trapped air, Tta, and the exhaust gas, Tex.
T - em P ta ta v emJ^pex^ex /o o i o-v
* ^em^p ta "*" U — * *em A'p ex
Further properties of the gas mixture are required and are determined using the theory for
such mixtures, which is found in Sec. 2.1.6.
Incorporation of the theory into an engine simulation
This entire procedure for the simulation of scavenging is carried out at each step in the
computation by a ID computer model of the type discussed in Chapter 2.
In Chapter 5, Sec. 5.5.1, is a detailed examination of every aspect of the theoretical scavenging
model on the in-cylinder behavior of a chainsaw engine. In particular, Figs. 5.16 to
5.21 give numerical relevance to the theory in the above discussion.
Further discussion on the scavenge model, within a simulation of the same chainsaw
engine, is given in Chapter 7. Examples are provided of the effect of widely differing scavenging
characteristics on the ensuing performance characteristics of power, fuel economy
and exhaust emissions; see Figs. 7.19 and 7.20 in Sec. 7.3.1.
3.4 Computational fluid dynamics
In recent years a considerable volume of work has been published on the use of Computational
Fluid Dynamics, or CFD, for the prediction of in-cylinder and duct flows in (fourstroke
cycle) IC engines. Typical of such publications are those by Brandstatter [3.26], Gosman
[3.27] and Diwakar [3.28].
At The Queen's University of Belfast, much experience has been gained from the use of
general-purpose CFD codes called PHOENICS and StarCD. The structure and guiding philosophy
behind this theoretical package has been described by their originators, Spalding
[3.29] or Gosman [3.27]. These CFD codes are developed for the simulation of a wide variety
of fluid flow processes. They can analyze steady or unsteady flow, laminar or turbulent flow,
flow in one, two, or three dimensions, and single- or two-phase flow. The program divides the
control volume of the calculated region into a large number of cells, and then applies the
conservation laws of mass, momentum and energy, i.e., the Navier-Stokes equations, over
each of these regions. Additional conservation equations are solved to model various flow
features such as turbulent fluctuations. One approach to the solution of the turbulent flow is
often referred to as a k-epsilon model to estimate the effective viscosity of the fluid. The
mathematical intricacies of the calculation have no place in this book, so the interested reader
is referred to the publications of Spalding [3.29] or Gosman [3.27], and the references those
papers contain.
A typical computational grid structure employed for an analysis of scavenging flow in a
two-stroke engine is shown in Fig. 3.20. Sweeney [3.23] describes the operation of the program
in some detail, so only those matters relevant to the current discussion will be dealt with
here.
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