Design and Simulation of Two Stroke Engines

Chapter 3 - Scavenging the Two-Stroke Engine
ior by me [3.7] and [3.13], Asanuma [3.8] and Booy [3.9] in actual firing engines will
demonstrate. Such experiments are useful and informative, but they do not assist with
the assignment of theoretical SRpcj and o values to the many engine cylinders described in
those publications. An assignment of such parameters is vital if the results of the experiments
are to be used to guide engine simulations employing mass-based thermodynamics and gas
dynamics.
3.2 Experimentation in scavenging flow
Since the turn of the 20th Century, the engineers involved with the improvement of the
two-stroke engine took to devolving experimental tests aimed at improving the scavenging
efficiency characteristics of the engines in their charge. This seemed to many to be the only
logical methodology because the theoretical route, of which Sec. 3.1 could well be described
as the knowledge base pre-1980, did not provide specific answers to specific questions. Many
of these test methods were of the visualization kind, employing colored liquids as tracers in
"wet" tests and smoke or other visible particles in "dry" tests. I experimented extensively
with both methods, but always felt the results to be more subjective than conclusive.
Some of the work was impressive in its rigor, such as that of Dedeoglu [3.10] or Rizk
[3.11] as an example of liquid simulation techniques, or by Ohigashi and Kashiwada [3.12]
and Phatak [3.19] as an example of gas visualization technology.
The first really useful technique for the improvement of the scavenging process in a
particular engine cylinder, be it a loop- or cross- or uniflow-scavenged design, was proposed
by Jante [3.5]. Although the measurement of scavenging efficiency in the firing engine situation
[3.7, 3.8, 3.9] (Plate 3.2) is also an effective development and research tool, it comes too
late in terms of the time scale for the development of a particular engine. The cylinder with its
porting has been designed and constructed. Money has been spent on casting patterns and on
the machining and construction of a finished product or prototype. It is somewhat late in the
day to find that the SE-SR characteristic is, possibly, less than desirable! Further, the testing
process itself is slow, laborious and prone to be influenced by extraneous factors such as the
effect of dissimilarly tuned exhaust pipes or minor shifts in engine air-fuel ratio. What Jante
[3.5] proposed was a model test on the actual engine cylinder, or a model cylinder and piston
capable of being motored, which did not have the added complexity of confusing the scavenging
process with either combustion behavior or the unsteady gas dynamics associated
with the exhaust tuning process.
3.2.1 The Jante experimental method of scavenge flow assessment
The experimental approach described by Jante [3.5] is sketched in Fig. 3.4. A photograph
of an experimental apparatus for this test employed at QUB some years ago is shown in Plate
3.1. It shows an engine, with the cylinder head removed, which is being motored at some
constant speed. The crankcase provides the normal pumping action and a scavenging flow
exits the transfer ports and flows toward, and out of, the open cylinder bore. At the head face
is a comb of pitot tubes, which is indexed across the cylinder bore to provide a measured
value of vertical velocity at various points covering the entire bore area. Whether the pitot
tube comb is indexed radially or across the bore to give a rectangular grid pattern for the
recording of the pitot tube pressures is immaterial. The use of pitot tubes for the recording of
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