Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
seizure. The criterion for port width, in order to eliminate ring pegging, is the same as that
debated for uniflow scavenging above, i.e., the port angle shown in Fig. 3.32 as the subtended
value, 0p, should be less than 25° in most circumstances.
The basic layout is shown in Fig. 3.32(a) with further amplification in Fig. 1.3. Fig.
3.32(a) contains a plan section with a view on the piston crown and also an elevation section
through the scavenge and exhaust ports at the bdc position. Further amplification is also given
by Plates 1.6 and 4.2. Fig. 3.32 shows no splitters in the scavenge duct separating the scavenge
ports and which would give directional guidance to the scavenge flow. While this is the
preferred geometry so that the gas flow follows the design directions, it is rarely seen in
practice and the open transfer duct from crankcase to the port belt, which is illustrated here, is
the common practice in the industry. The "ears" of the deflector are usually centered about a
diameter or are placed slightly toward the scavenge side. The edge of the ears are normally
chamfered at 45° and given some (0.5 to 1.0 mm) clearance, xc, from the bore so that they
may enter the combustion chamber above the tdc point without interference. Contrary to
popular belief, I [1.10] showed that while this apparently gives a direct short-circuit path to
the exhaust port, the actual SE-SR characteristic was hardly affected. Consequently, any loss
of engine performance caused by chamfering 50% off the ears of the deflector, and giving
them the necessary side clearance, must be due to a further deterioration of the combustion
efficiency by introducing even longer and more tortuous flame paths. The "ears" of the solid
deflector are chamfered to prevent the corners from glowing and inducing pre-ignition (see
Chapter 4).
In Chapters 6 and 7 there will be discussions on the determination of port timings and
port heights for all types of engines. In terms of Fig. 3.32(a), it will be assumed that the
scavenge port height at bdc is hs, and there are a number of scavenge ports, Ns. The most
common value for Ns is three, but multiple port designs with five or more scavenge ports have
been seen in practice. If the ports are rectangular in profile, the maximum scavenge flow area,
Asp, and including corner radii, rc, is given by:
Asp = Ns(hsxs - rc 2 (4 - %)) (3.5.2)
Where the maximum area of the ports at bdc are segments of drilled holes, the designer is
forced to calculate that area for use in Eq. 3.5.2. There are two important design criteria, one
being the match of the maximum scavenge flow area, Asp, to the deflector flow area, A