Design and Simulation of Two Stroke Engines

Chapter 3 - Scavenging the Two-Stroke Engine
Figure 3.37 Together with Fig. 1.2 shows the port layouts for design
by Prog.3.4, LOOP SCAVENGE DESIGN.
Put another way, small and inexpensive engines for industrial applications are often designed
with "finger" ports which have no inner walls at all and which are open to the moving
piston surface. The normal experience is that they exhibit a very indifferent quality of scavenging
when measured on a single-cycle gas scavenge rig.
3.5.5.5 Effect of bore-to-stroke ratio on loop scavenging
The tradition has it that loop scavenging is most effective in any cylinder size when the
engine is of "square" dimensions. Experiments at QUB have shown that the most effective
band for loop scavenging is from a bore-stroke ratio between unity and 1.2.
3.5.5.6 Effect of cylinder size on loop scavenging
The tradition has it that the higher the engine swept volume, the better the scavenging
quality for loop scavenging. Experimental experience at QUB over a thousand plus experiments
shows that virtually all sizes of engine can be made to scavenge equally well if the
R&D effort is put into so doing. However, it is probably true to say that it is much more
difficult to make a small cylinder, say of 50 cm 3 , scavenge well by design or by development
than one of, say, 500 cm 3 .
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