Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
selected at 60 mm is mechanically acceptable and that a potential width of 68 mm is available
with a little attention to design detail. Also observe that the rear transfer ducts must curve
around the inlet port, for the back bar is only 25 mm wide and the inlet port needs to be some
35 mm across at the widest point to accept a carburetor which will be about that flow diameter.
Because the widths of the ports are proportioned in the ratio of 5:3 in favor of the main
transfer port, the actual width of the main port is relatively narrow and cannot trap a piston
ring; the area can be maximized by inserting small corner radii, hence the selection of the 3
mm corner radii in the sketch drawn as Fig. 6.7. During the operation of Prog.3.4, always bear
in mind the advice given in Sec. 3.5.4.1 regarding the orientation of the main transfer ports. It
is clear that the use of reed or disc valve induction will free up more bore circumference for
exhaust and transfer ports; however, much further complexity may be added to the overall
design package, especially if it is a multi-cylinder unit.
Unpegged rings
If the design issue is durability, then the use of unpegged piston rings becomes a vital
topic, as discussed in Sec. 3.5.2. To employ unpegged rings the port should subtend no more
than 25° at the cylinder center. This is particularly important for diesel engines, for good
piston sealing during compression is a vital feature of its operation. Here the blown loopscavenged
engine, as shown in Fig. 3.40, can be readily designed to incorporate the relevant
port width criteria. For the spark-ignition engine, the cross-scavenged engine, as shown in
Fig. 3.32, is easily designed with unpegged rings. It is more difficult for the simple loopscavenged
engine to incorporate unpegged rings as the ports are normally wider and fewer in
number.
6.2.3 The port timing criteria for the engine
Having determined the port widths available for the various ports, the designer will then
index Prog.6.3 to calculate the specific time areas of those ports. Naturally, you have the
option of writing a personal numerical solution to the theory presented above for the derivation
of specific time area. The values for port timings are estimated and simply inserted as
data into Prog.6.3, then subsequently modified until the values for the deduced specific time
area match those predicted as being required by Prog.6.1, or Figs. 6.3-6.6 to provide the
requisite performance characteristics. The program also automatically produces the port height
and area information for all ports. This is also helpful, for the inlet port maximum area is a
very useful guide to carburetor size, i.e., its flow diameter. If one matches ports in this manner,
the carburetor flow area, for an engine with the inlet port controlled by the piston skirt,
can be set to between 85% and 95% of the inlet port flow area, as an approximate guide. For
engines with induction through reed valves or disc valves, a separate discussion on this topic
is found in Sees. 6.3.2 and 6.4.
6.2.4 Empiricism in general
Always remember that empirical design criteria are based on past history and experience.
By definition, the answers emanating from empiricism are not always correct and the result of
any design decision should be checked by an overall engine modeling program and, ulti-
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