Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
and radius input data values of 65 and 120, respectively, give the deflector flow area, A^ef, as
1231 mm 2 . Consequently, the CA ratio is calculated as 1.16, which would be regarded as a
satisfactory value. The value of deflector radius, selected here as 120 mm, and known to
provide good scavenging for this particular bore dimension of 70 mm [1.10], can vary over a
wide range from 30% to 80% greater than the cylinder bore dimension without unduly affecting
the quality of scavenging.
It is difficult to over-emphasize the importance of the deflection ratio in the design of
cross-scavenged engines, and further information with respect to the QUB type of cross scavenging
is presented in Fig. 3.33. This shows the experimental results for trapping efficiency
for two QUB-type engines, one with a deflection ratio of 1.15 and the other with 1.44. The
one with the CA value of 1.15 is the QUBCR cylinder discussed in Sec. 3.2.4 and Figs. 3.12
and 3.13. The higher CA ratio lowers the trapping efficiency of the QUB type of cross-scavenged
cylinder down to a level which places it equal to an optimized example of a classic
cross-scavenged design. As further evidence, the QUB type with the higher CA ratio of 1.44
was built as a firing engine, not just as a model cylinder for experimentation on the singlecycle
scavenge rig, and the resulting performance characteristics fell at least 10% short of the
behavior reported for the QUB 400 loop-scavenged research engine in Chapters 5 and 7; in
other words, they were somewhat disappointing. To emphasize the point, a QUB-type deflector
piston engine of the square dimensions of 70 mm bore and stroke and with a deflection
ratio of 1.16, with a transfer port layout very similar to that shown in Fig. 3.34(b), was constructed
and experimentally produced very similar levels of brake specific fuel consumption,
bmep, trapping efficiency and exhaust emission characteristics to the QUB 400 loop-scavenged
engine [3.39]. The application of the design into other areas ranging from direct-injection
to mopeds and biogas engines can be found in other publications [3.39-3.44].
As described previously for the classic cross-scavenged cylinder, the deflection ratio criterion
for the QUB type is the same:
1.1