Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
time from zero to a maximum value and then returns to zero at transfer port closure. If one
attempts to simulate the scavenging flow by holding the piston stationary, and subjects the
cylinder to steady flow of either gas or liquid, then that flow will form attachments to either
the piston crown or the cylinder walls in a manner which could not take place in the real
velocity-time situation. This effect has been investigated by many researchers, Rizk [3.11],
Percival [3.16], Sammons [3.15], Kenny [3.18] and Smyth [3.17], and they have concluded
that, unless the test is being conducted in steady flow for some specific reason, this is not a
realistic simulation of the scavenging flow. As more recent publications show Jante test results
conducted in steady flow it would appear that not all are yet convinced of that conclusion
[3.50].
3.2.3 Absolute test methods for the determination of scavenging efficiency
To overcome the disadvantages posed by the Jante test method, it is preferable to use a
method of assessment of scavenging flow that will provide measurements of scavenging,
trapping and charging efficiency as a function of scavenge ratio. It would also be preferable to
conduct this test isolated from the confusing effects of combustion and exhaust pipe tuning
characteristics. This implies some form of model test, but this immediately raises all of the
issues regarding similarity discussed in Sec. 3.2.2. A test method which does not satisfy these
criteria, or at least all of the vitally important dimensionless criteria, is unacceptable.
It was Sammons [3.15] who first postulated the use of a single-cycle apparatus for this
purpose. Because of the experimental difficulty of accurately measuring oxygen concentrations
by gas analysis in the late 1940s (a vital element of his test method), his proposal tended
to be forgotten. It was revived in the 1970s by Sanborn [3.13], who, together with researchers
at QUB, investigated the use of a single-cycle apparatus using liquids to simulate the flow of
fresh charge and exhaust gas. Sanborn continued this work [3.21] and other researchers investigated
scavenging flow with this experimental methodology [3.22].
At QUB there was a growing realization that the occasional confusing result from the
liquid-filled apparatus was due to the low Reynolds numbers found during the experiment,
and that a considerable period of the simulated flow occurred during laminar or laminarturbulent
transition conditions. Consequently, the QUB researchers reverted to the idea postulated
by Sammons [3.15], for in the intervening years the invention of the paramagnetic
analyzer, developed for the accurate determination of oxygen proportions in gas analysis, had
realized the experimental potential inherent in Sammons' original technique. (As a historical
note, for it illustrates the frailty of the human memory, I had forgotten about, or had relegated
to the subconscious, the Sammons paper and so reinvented his experimental method.)
However, as will become more evident later, the QUB apparatus incorporates a highly
significant difference from the Sammons experimental method, i.e., the use of a constant
volume cylinder during the scavenge process.
The QUB apparatus sketched in Fig. 3.6, and appearing in Plate 3.3, is very thoroughly
described and discussed by Sweeney [3.20]. The salient features of its operation are a constant
volume crankcase and a constant volume cylinder during the single cycle of operation
from tdc to tdc at a known turning rate. The equivalent rotational rate is 700 rev/min. The
cylinder is filled with air to represent exhaust gas, and the crankcase is filled with carbon
dioxide to represent the fresh air charge. Here, one similarity law is being well satisfied, in
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