Design and Simulation of Two Stroke Engines

Chapter 6 - Empirical Assistance for the Designer
in Table 6.4. The standard transfer port timing is on the "target" value, so the timing variations
are such as to investigate greater and lesser values of specific time area. It can be seen
that the "target" value of blowdown specific time area is obtained by a transfer port timing of
125° atdc. Increasing the transfer time area reduces that for blowdown, and vice versa.
Table 6.4 Time areas for various transfer timings for a chainsaw
Transfer timings
117° atdc
119° atdc
121° atdc (std)
123° atdc
125° atdc
target values
"sve
0.00705
0.00705
0.00705
0.00705
0.00705
0.0095
ASvb
0.00007
0.00011
0.00015
0.00020
0.00026
0.00027
ASvt
0.00813
0.00739
0.00670
0.00603
0.00542
0.00660
The results of the simulation are presented in Figs. 6.12 to 6.15, for bmep, air flow,
specific fuel consumption and specific hydrocarbon emissions, respectively. In each figure
the performance parameter is shown with respect to the transfer port and blowdown time
areas and the transfer port timing is also indicated.
More transfer port area should give more air flow, but only if the rest of the design is
optimized to permit it. At a 117° timing there may be more transfer time area but the blowdown
is inadequate. The backflow reduces the air flow, as can be seen in Fig. 6.13. The air flow
peaks at transfer port timings of 121° and 123° atdc. The effect on bmep and power at this
speed is to give a peak at a transfer port timing of 123° atdc. When the equally important
specific fuel consumption is investigated in Fig. 6.14, and the even more important specific
hydrocarbon emissions in Fig. 6.15, the optimization at a transfer port timing of 123° atdc is
now evident. Much more importantly, the optimization is at specific time area values of trans-
3.8
S3 37 *
3.5
transfer time area
TO 117 s
i 1 r—| 1 1 r
50 60 70 80 90
Asvt, s/mx 10000
3.8
3.7
3.6 -
3.5
blowdown time area
0 1 2 3
Asvb, s/mx 10000
Fig. 6.12 Effect of transfer timing on engine torque (bmep).
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