Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
fulfill that need. This is particularly helpful, principally because the conventional two-stroke
engine has a simple cylinder head, so the designer can conceive of an almost infinite variety
of combustion chamber shapes. This is obvious from an examination of some of the more
basic shapes shown in Fig. 4.13.
There have been several attempts to produce a simple analysis of squish velocity, often
with theory more empirically based than fundamentally thermodynamic. One of the useful
papers which has been widely quoted in this area is that by Fitzgeorge [4.22]. Experimental
measurements of such phenomena are becoming more authoritative with the advent of instrumentation
such as laser doppler anemometry, and the paper by Fansler [4.23] is an excellent
example of what is possible by this accurate and non-intrusive measurement technique. However,
the following theoretical procedure is one which is quite justifiable in thermodynamic
terms, yet is remarkably simple.
Figs. 4.2 and 4.12 represent a compression process inducing a squished flow between the
piston and the cylinder head. The process commences at trapping, i.e., exhaust port closure.
From Sec. 1.5.6, the value of trapped mass, mt, is known and is based on an assumed value for
the trapped charge pressure and temperature, pt and Tt. At this juncture, the mass will be
evenly distributed between the volume subtended by the squish band, Vst, and the volume
subtended by the bowl, Vbt- The actual values of Vsi and Vbi at any particular piston position,
as shown in Fig. 4.12, are a matter of geometry based on the parameters illustrated in
Fig. 4.2. From such input parameters as squish area ratio, csq, the values of squish area, As,
bowl area, At,, and bowl diameter, db, are calculated from Eq. 4.2.1.
For a particular piston position, as shown in Fig. 4.12, the values of thermodynamic state
and volumes are known before an incremental piston movement takes place.
PISTON MOVES ON COMPRESSION STROKE
Figure 4.12 Simple theoretical model of squish behavior.
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