Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
enged units (square symbol) and loop-scavenged designs (round symbol) and it is obvious
that there is no clearcut winner in the race for perfect scavenging and that many, indeed the
majority, of the units tested could withstand improvement by either experimental or by theoretical
methods.
You can observe from reading this chapter that the knowledge base on scavenging has
been greatly enlarged in the past two decades. As with many other branches of science and
engineering this can be attributed to the ever wider use of computation which has moved from
the mainframe to the desktop and laboratory bench level. Nowhere is this more true than for
R&D related to scavenging in two-stroke engines.
The increasing use of computational fluid dynamics holds out the hope for a complete
design process of the in-cylinder flow regime in terms of an accurate assessment of the SE-SR
characteristics, either prior to the manufacture of the cylinder or as part of the onward development
of existing engine designs.
The single-cycle gas testing apparatus, an experimental tool which became possible by
the development of the accurate paramagnetic oxygen analyzer, is actually devoid of any
complex computer control technology and could well have been employed for R&D much
earlier in history. However, the preparation of model cylinders for experimental work on this
rig has been greatly assisted by modern CAD/CAM techniques. Since 1990 it has been possible
to machine scavenge duct profiles directly in a low-melting-point alloy using surface (or
solid) modeling CAD software. The machined transfer duct core is immersed into a coldsetting
plastic compound in a mold. When the plastic has set, the machined duct core can be
"melted" out by immersion in hot water and the material itself recycled. A photograph of the
machined cores and the finished test cylinder are shown in Plate 3.4. The important issue here
is that when the test sequence on the single-cycle rig has been completed, the information on
the scavenge duct profiles which produced the best SEV-SRV characteristics is not a drawing
which can lead to misinterpretation, but a file of CAM data to permit the accurate transmission
of the finalized scavenge duct geometry into the manufacturing process. At QUB it is
possible to produce test cylinders and their scavenge ducts in approximately one week of
CAD and CAM activity, and another week to conduct the experimental optimization process
on the single-cycle test apparatus.
More advanced techniques are now available which speed up this process. The advent of
"rapid prototyping," otherwise known as stereo lithography, means that the entire cylinder
including its scavenge ducting can be modeled in 3D CAD software and manufactured by a
laser etching process in a resin which hardens to the point where it can be used directly on the
QUB single-cycle test apparatus. Plate 3.5 shows a prototype cylinder produced by stereo
lithography at QUB for use on the single-cycle test apparatus. It is possible to conduct this
entire prototype cylinder design and manufacturing process in two or three days and conclude
the entire optimization process for the scavenging in ten days total. The data file for the "rapid
prototyping" stage, and indeed the solid cylinder model it produces, can be used in the ensuing
manufacturing process to create cores or molds for the production castings.
Which then is the optimum method to employ for the development of optimized scavenging
for the two-stroke engine cylinder: single-cycle gas experiment or CFD theory, or should
both techniques be applied during R&D in series or parallel? What does the word optimum
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