Design and Simulation of Two Stroke Engines

PORT PLAN LAYOUT
Chapter 1 - Introduction to the Two-Stroke Engine
Fig, 1.3 Deflector piston of cross-scavenged engine.
thereby reduce the manufacturing costs of the cross-scavenged engine by comparison with an
equivalent loop- or uniflow-scavenged power unit.
One design of cross-scavenged engines, which does not have the disadvantages of poor
wide-open throttle scavenging and a non-compact combustion chamber, is the type designed
at QUB [1.9] and sketched in Fig. 1.4. A piston for this design is shown in Plate 1.6. However,
the cylinder does not have the same manufacturing simplicity as that of the conventional
deflector piston engine. I have shown in Ref. [1.10] and in Sec. 3.2.4 that the scavenging is as
effective as a loop-scavenged power unit and that the highly squished and turbulent combustion
chamber leads to good power and good fuel economy characteristics, allied to cool cylinder
head running conditions at high loads, speeds and compression ratios [1.9] (see Plate 4.3).
Several models of this QUB type are in series production at the time of writing.
1.2.3 Uniflow scavenging
Uniflow scavenging has long been held to be the most efficient method of scavenging the
two-stroke engine. The basic scheme is illustrated in Fig. 1.5 and, fundamentally, the methodology
is to start filling the cylinder with fresh charge at one end and remove the exhaust gas
from the other. Often the charge is swirled at both the charge entry level and the exhaust exit
level by either suitably directing the porting angular directions or by masking a poppet valve.
The swirling air motion is particularly effective in promoting good combustion in a diesel
configuration. Indeed, the most efficient prime movers ever made are the low-speed marine
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