Design and Simulation of Two Stroke Engines

Chapter 6 • Empirical Assistance for the Designer
Observe that the time area values at an exhaust port timing of 108° are the same as those
given in Table 6.2 as measured for the chainsaw. It can be seen that the transfer time area is
virtually on its "target" value, that for blowdown it is at a 104° atdc timing, and the total time
area "target" for the exhaust port is somewhat higher than any to be examined.
The results of the simulation are presented in Figs. 6.8 to 6.11, for bmep, air flow, specific
fuel consumption and specific hydrocarbon emissions, respectively. In each figure the performance
parameter is shown with respect to the exhaust and blowdown time areas and the
exhaust port timing is also indicated. As far as power and torque are concerned, in Fig. 6.8 it
would appear that the target time area values are very relevant, for the trend is to peak at the
104° port timing value. The air flow, plotted as delivery ratio in Fig. 6.9, climbs toward the
104° exhaust port timing, but the scale is narrow and it is virtually constant. The specific fuel
consumption, shown in Fig. 6.10, would appear to reach a plateau at the 104 and 106° exhaust
port timings, but superior to that achieved at the standard 108° timing value. However, when
the hydrocarbon emissions are examined in Fig. 6.11, the basic trend is for their deterioration
with a longer exhaust period. The best HC emission value is seen to be at a 106° exhaust
timing. In modern times, with hydrocarbon emissions now controlled by legislation in many
countries, the designer can no longer merely optimize the design for power alone.
From this analysis, the standard port timing and area can be seen to be a compromise
between the acquisition of power and the reduction of HC emissions. It is also clear that a
designer using a time area analysis would have arrived into this area of engine geometry more
rapidly than by merely guessing port timings and widths, and subsequently indulging in an
experimental R&D program. The use of the engine simulation ensures that the design process
is even more rapid and more accurate.
A change of transfer port timing with others constant
Let the exhaust port timing remain at the "standard" value of 108° atdc and change the
transfer port timing from 117° atdc to 125° atdc in 2° steps. This will have the effect of
altering the pumping behavior of the crankcase and of changing the blowdown time area as
well as that of the transfer ports. The results of these geometrical alterations are summarized
1—
JO
bmep,
oo .
3.7 -
exhaust time area
/ EO104 a
/ EO 110 9
3.8
3.7 -
blowdown time area
EO 104*
EO110 9
3.6 - — i — i — i — i — i — 3.6
60 70 80 90 1 2 3
Asve, s/m x 10000 Asvb, s/m x 10000
Fig. 6.8 Effect of exhaust timing on engine torque (bmep).
All