Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
at 11,500 rpm. Using Eq. 1.6.6, this translates into the production of 11 bar bmep. All
of the cylinder porting is to be piston controlled and the induction by a reed valve
directly into the crankcase.
Upon inserting the information on the bmep level for each engine into Eqs. 6.1.9-6.1.13,
or using Prog.6.1, the following information on specific time area is obtained for each engine.
All units are s/m.
Table 6.1 Specific time areas for a chainsaw and a racing engine
Specific time area,
s/m
65 cm 3 chainsaw
125 cm 3 racer
ASve
0.0095
0.0162
ASVb
0.00027
0.00113
ASvt1
0.0057
0.0086
ASvt2
0.0066
0.0185
ASvi
0.0071
0.0162
Note that the specific time area requirements for the porting of the racing engine are
much larger than for the chainsaw engine. Although the cylinder sizes are only 25% different,
the larger cylinder of the racing engine is expected to produce about three times the torque at
an engine speed that has only 70% of the time available for filling it. To assist with the
visualization of what that may imply in terms of porting characteristics, a computer-generated
sketch of a cylinder of a 125 racing engine of "square" dimensions, complete with a
piston-controlled induction system of the requisite size, is shown in Fig. 6.7. A comparison
with the similar sketch for the chainsaw engine in Fig. 6.2 reveals the considerable physical
differences in both port timings and area. It is clear that the large ports needed for a Grand
Prix engine are open much longer than they are closed.
The next step is to analyze the porting characteristics of the two engines discussed in
Chapter 5, and to determine if the criteria noted above have any relevance to those which are
known to exist.
6.1.2 The determination of specific time area of engine porting
Discussion of the calculation of port areas in a two-stroke engine has been a recurring
theme throughout this book, most recently in Sec. 5.2 with respect to engine simulation. The
calculation procedure for specific time area is encapsulated within any program that will
compute the area of any port in an engine as a function of crankshaft angle, G, or time. The
value required is for Ae, i.e., the areas in Fig. 6.1 for inlet, transfer, exhaust or blowdown. As
it is unlikely that the Asv will be determined by a direct mathematical solution due to the
complexity of the relationship between the instantaneous value of A as a function of 0, the
computer solution by summation at crankshaft intervals of one or two degrees, i.e., ZAedG,
will provide adequate numerical accuracy. When the ZAedG is determined, that value is inserted
into the appropriately combined Eqs. 6.1.7 and 6.1.8 as follows:
8=ep
5>ed9
Specific time area, Asv = -2=2 s/m (6.1.14)
6Vsvrpm
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