Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
3.5.5.7 The use ofProg.3.4, LOOP SCAVENGE DESIGN
A useful design program for all of the porting layouts illustrated in Figs. 1.2 and 3.37 is
available from SAE. The program is self-explanatory and the typical outputs shown in Figs.
3.38 and 3.39 are exactly what you see on the computer screen. The opening screen display
asks you to decide on the number of transfer ports in the design, i.e., two, three, four or five,
and from then on the design procedure will take the form of changing a particular data value
until you are satisfied that the aims and objectives have been achieved. Such objectives would
typically be the satisfaction of the criteria in Sees. 3.5.5.1 to 3.5.5.3 and to achieve the desired
value of port width ratio, Cpb.
Upon completion of the design process, the printer will provide hard copy of the screen
and, as can be seen from Fig. 3.39, give some useful manufacturing data for the cylinder liner.
This becomes even more pertinent for four and five port layouts, where the structural strength
of some of the port bars can become a major factor.
It is interesting to note that in the actual design shown for a 70-mm-bore cylinder in this
three transfer port layout, the Cpb value of 1.06 is about the same as for the QUB deflector
piston engine; it was 1.03. It is clear that it would be possible to design a four or five port
layout to increase the Cpb value to about 1.3. Remember that the uniflow engine had an
equivalent value of 2.5, although if that uniflow engine is of the long stroke type, or is an
opposed piston engine, it will require a Cpb value at that level to achieve equality of flow area
with the other more conventional bore-stroke ratio power units.
CURRENT INPUT DATA FOR BUOK FIG.3.35
BORE DIAMETER= 70
LINER THICKNE5S= 5
MAXIMUM EXHAUST WIDTH= 40
MAIN PORT, UP= AT 5
TARGET MTU 33.5
TARGET MT2= 14.0
ANGLE AMU 65
ANGLE AM2= 50
REAR PORT, UPRa AT 55
REAR PORT PLAN WIDTH=30
OUTPUT DATA
MAIN PORT PLAN WIDTHz 26.6
TOTAL EFFECTIVE PORT WIDTHz 74.2
TOTAL EFFECTIVE WIDTH/BORE RATICbl.Ofi
Fig. 3.38 First page of output from Prog.3.4, giving the scavenge geometry of the ports.
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