Design and Simulation of Two Stroke Engines

Chapter 3
Scavenging the Two-Stroke Engine
3.0 Introduction
In Chapter 1, particularly Sec. 1, there is a preliminary description of scavenging for the
various types of two-stroke engines. This chapter continues that discussion in greater depth
and provides practical advice and references to computer design programs to aid the design
process for particular types of engine.
3.1 Fundamental theory
The original paper on scavenging flow was written by Hopkinson [3.1] in 1914. It is a
classic paper, written in quite magnificent English, and no serious student of the two-stroke
engine can claim to be such if that paper has not been read and absorbed. It was Hopkinson
who conceived the notion of "perfect displacement" and "perfect mixing" scavenge processes.
Benson [1.4] also gives a good account of the theory and expands it to include the work of
himself and Brandham. The theory used is quite fundamental to our conception of scavenging,
so it is repeated here in abbreviated form. Later it will be shown that there is a problem in
correlation of these simple theories with measurements.
The simple theories of scavenging all postulate the ideal case of scavenging a cylinder
which has a constant volume, Vcy, as shown in Fig. 3.1, with a fresh air charge in an isothermal,
isobaric process. It is obvious that the real situation is very different from this idealized
postulation, for the reality contains gas flows occurring at neither constant cylinder volume,
constant cylinder temperature, nor constant cylinder pressure. Nevertheless, it is always important,
theoretically speaking, to determine the ideal behavior of any system or process as a
marker of its relationship with reality.
In Fig. 3.1 the basic elements of flow are represented. The incoming scavenge air can
enter either a space called the "displacement zone" where it will be quite undiluted with
exhaust gas, or a "mixing zone" where it mixes with the exhaust gas, or it can be directly
short-circuited to the exhaust pipe providing the worst of all scavenging situations.
In this isothermal and isobaric process, the incoming air density, pa, the cylinder gas
density, pc, and the exhaust gas density, pex, are identical. Therefore, from the theory previously
postulated in Sec. 1.5, the values of purity, scavenge ratio, scavenging efficiency, trapping
efficiency, and charging efficiency in this idealized simulation become functions of the
volume of the several components, rather than the mass values as seen in Eqs. 1.5.1-1.5.9.
The following equations illustrate this point.
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