Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
library of information on this subject has been amassed with up to four QUB single-cycle gas
rigs conducting tests on over 1300 differing cylinder geometries. During the course of this
accumulation of data, there have been observed test points which have fallen below the "perfect
mixing" line. Remember when examining these diagrams that the higher the scavenge
ratio then so too is the potential bmep or torque that the engine may attain. An engine may be
deliberately designed to produce a modest bmep, such as a chainsaw or a small outboard, and
hence the quality of its scavenging characteristics above, say, a SRV value of 1.0 is of no
consequence. If an engine is to be designed to produce high specific power and torque, such
as a racing outboard or a motocross engine, then the quality of its scavenging characteristics
below, say, a SRV value of 1.0 is equally of little consequence.
Figs. 3.10 and 3.11 present what could be loosely described as the best and worst of
scavenging behavior. As forecast from the historical literature, the UNIFLOW cylinder has
the best scavenging characteristic from the lowest to the highest scavenge ratios. It is not the
best, however, by the margin suggested by Changyou [3.4], and can be approached by optimized
loop and cross scavenging as shown in Figs. 3.12 and 3.13. The two Yamaha DT 250
cylinders, discussed in Sec. 3.2.2, are now put in context, for the firing performance parameters
given in Fig. 3.7 are obtained at scavenge ratios by volume in excess of unity. This will
be discussed in greater detail later in Chapter 5.
It will be observed from the trapping efficiency diagram, Fig. 3.11, that the superior
scavenging of the CROSS engine is more readily observed by comparison with the equivalent
SE-SR graph in Fig. 3.10. This is particularly so for scavenge ratios less than unity. While the
trapping of the UNIFLOW engine at low SRV values is almost perfect, that for the CROSS
engine is also very good, making the engine a good performer at idle and light load; that
indeed is the experience of both the user and the researcher. The CROSS engine does not
behave so well at high scavenge ratios, making it potentially less suitable as a high-performance
unit.
That it is possible to design loop scavenging poorly is evident from the characteristic
shown for YAM 12, and the penalty in torque and fuel consumption as already illustrated in
Fig. 3.7.
Figs. 3.12 and 3.13 show that it is possible to design loop- and cross-scavenged engines to
approach the "best in class" scavenging of the UNIFLOW engine. Indeed, it is arguable that
the GPBDEF and QUBCR designs are superior to the UNIFLOW at scavenge ratios by volume
below 0.5. Put very crudely, this might translate to superior performance parameters in a
firing engine at bmep levels of 2.5 or 3.0 bar, assuming that the combustion efficiency and
related characteristics are equivalent.
Observe in Figs. 3.12 and 3.13 that both the LOOPSAW and SCRE designs, both loop
scavenged, are superior to either YAM 12 or YAM 14, and approach the UNIFLOW engine at
high SR values above unity. For the SCRE, a large-capacity cylinder designed to run at high
bmep and piston speed, that is an important issue and indicates the success of the optimization
to perfect that particular design. On the other hand, the LOOPSAW unit is designed to
run at low bmep levels, i.e., 4 bar and below, and so the fact that it has good scavenging at
high scavenge ratios is not significant in terms of that design requirement. To improve its
firing performance characteristics, it can be observed that it should be optimized, if at all
possible, to approach the GPBDEF scavenging characteristic at SRV values below unity.
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