Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
reason is that the friction losses of the valve gear and oil pump begin to assume considerable
proportions as the cylinder size is reduced, and this significantly deteriorates the mechanical
efficiency of the engine.
In Sec. 7.2 there are options listed which are open to the designer, and the remainder of
this section will be devoted to a closer examination of some of the options for the optimization
of an engine. In particular, the computer simulation will be used to illustrate the relevance
of some of those assertions. For others, experimental data will be introduced to emphasize
the point being made. This will reinforce much of the earlier discussion in Chapter 5.
7.3.1 The effect of scavenging on performance and emissions
In Chapter 3, you are introduced to the experimental and theoretical methods for the
improvement of the scavenging of an engine. For the enhancement of the performance characteristics
of any engine, whatever its performance target, be they for a racing engine or one
to meet legislated demands on exhaust emissions, better quality scavenging always translates
into more air and fuel trapped with less of it lost to the exhaust system. To further comprehend
this point, refer to the experimental data in the thesis by Kenny [3.18], or his associated
papers listed in Chapter 3. To demonstrate the potential effect on emissions and performance
by improving scavenging over a wider spectrum than that given experimentally by Kenny, a
theoretical simulation is carried out. The simulation is for the chainsaw at full throttle as
given in Sec. 5.4.1, at full throttle at 7200 rpm at an air-fuel ratio of 14.0 on premium unleaded
gasoline. The scavenging is changed successively from the standard LOOPS AW characteristic,
to UNIFLOW, SCRE, YAM 12, and GPBDEF quality scavenging as defined in
Table 3.16 and shown in Figs. 3.10-3.15. While it may be totally impractical to consider the
incorporation of uniflow scavenging in a chainsaw, it is highly instructive to determine the
effect on emissions and power of the employment of the (so-called) optimum in scavenging
characteristics. It is equally useful to know the impact on these same performance-related
parameters of the use of the worst scavenging, namely the YAM 12 characteristics. Recall that
the others are in between these extremes, with SCRE being a very good loop-scavenging
system, while GPBDEF is an unconventional form of cross scavenging.
The results of the simulation are shown in Figs. 7.19 and 7.20. Fig. 7.19 gives the results
for bmep, bsfc and bsHC. In Fig. 7.20 are the output data for delivery ratio, and the scavenging,
trapping and charging efficiencies. It is not surprising that UNIFLOW scavenging gives
the best performance characteristics. By comparison with YAM12, it is 16% better on bmep
and power, 19% better on bsfc, 31 % better on bsHC, 7% better on bsCO, 7.7% better on SE,
and 11.4% better on trapping efficiency. The effect on hydrocarbon emissions is quite dramatic.
As comparison with uniflow scavenging is somewhat unrealistic, it is more informative
for the designer to know what order of improvement is possible by the attainment of the
ultimate in loop-scavenging characteristics, namely the SCRE scavenging quality. In making
this comparison recall that the quality of the LOOPSAW scavenging is already in the "very
good" category. Comparing the SCRE simulation results with those calculated using
LOOPSAW scavenging, it is 5% better on bmep and power, 5.4% better on bsfc, 14% better
on specific hydrocarbon emissions, 2% better on bsCO, 2.5% better on scavenging efficiency,
and 3.4% better on trapping efficiency. The effect on bsHC is still considerable.
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