Design and Simulation of Two Stroke Engines

Chapter 3 - Scavenging the Two-Stroke Engine
relevant criteria for dynamic similarity. It is a dynamic experiment, for the piston moves and
provides the realistic attachment behavior of scavenge flow as it would occur in the actual
process in the firing engine. Sweeney [3.20, 3.23] demonstrates the repeatability of the test
method and of its excellent correlation with experiments conducted under firing conditions.
Some of those results are worth repeating here, for that point cannot be emphasized too strongly.
The experimental performance characteristics, conducted at full throttle for a series of
modified Yamaha DT 250 engine cylinders, are shown in Fig. 3.7. Each cylinder has identical
engine geometry so that the only modifications made were to the directioning of the transfer
ports and the shape of the transfer duct. Neither port timings nor port areas were affected so
that each cylinder had almost identical SR characteristics at any given rotational speed. Thus,
the only factor influencing engine performance was the scavenge process. That this is significant
is clearly evident from that figure, as the bmep and bsfc behavior is affected by as much
as 15%. When these same cylinders were tested on the single-cycle gas scavenging rig, the
SEV-SRV characteristics were found to be as shown in Fig. 3.8. The figure needs closer examination,
so a magnified region centered on a SRV value of 0.9 is shown in Fig. 3.9. Here, it can
be observed that the ranking order of those same cylinders is exactly as in Fig. 3.7, and so too
is their relative positions. In other words, Cylinders 14 and 15 are the best and almost as
effective as each other, so too are cylinders 9 and 7 but at a lower level of scavenging efficiency
and power. The worst cylinder of the group is cylinder 12 on all counts. The "double
indemnity" nature of a loss of 8% SEV at a SRV level of 0.9, or a TEV drop of 9%, is translated
into the bmep and bsfc shifts already detailed above at 15%.
A sustained research and development effort has taken place at QUB in the experimental
and theoretical aspects of scavenging flow. For the serious student of the subject, the papers
published from QUB form a series, each using information and thought processes from the
preceding publication. That reading list, in consecutive order, is [3.6], [1.23], [3.13], [3.20],
[1.10], [3.23], [1.11], and [3.17].
3.2.4 Comparison of loop, cross and uniflow scavenging
The QUB single-cycle gas scavenge experiment permits the accurate and relevant comparison
of SEV-SRV and TEV-SRV characteristics of different types of scavenging. From some
of those previous papers, and from other experimental work at QUB hitherto unpublished,
test results for uniflow-, loop- and cross-scavenged engine cylinders are presented to illustrate
the several points being made. At this stage the most important issue is the use of the
experimental apparatus to compare the various methods of scavenging, in order to derive
some fundamental understanding of the effectiveness of the scavenging process conducted by
these several methodologies. In Sec. 3.3 the information gained will be used to determine the
theoretical relevance of this experimental data in the formulation of a model of scavenging to
be incorporated within a complete theoretical model of the firing engine.
Figs. 3.10-3.13 give the scavenging and trapping characteristics for eight engine cylinders,
as measured on the single-cycle gas scavenging rig. It will be observed that the test
results fall between the perfect displacement line and perfect mixing line from the theories of
Hopkinson [3.1]. By contrast, as will be discussed in Sec. 3.3, some of the data presented by
others [3.3, 3.4] lie below the perfect mixing line.
227