Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
The terms for bore, stroke, con-rod length and engine speed, i.e., BO, ST, CRL and RPM
are self-explanatory and, as with all of the units in the programs, they are in mm dimensions
unless otherwise stated.
The trapped compression ratio is TCR; the number of cylinders of the engine is NC Y; the
bmep expected at the RPM is BMEP in bar units.
The exhaust ports and transfer ports open at EPO and TPO °atdc, respectively; the subtended
port angles, 9p, of the exhaust ports is EXANG; the radial thickness of the cylinder
liner in the scavenge port belt is LINER.
The chordal values of the bars of the exhaust and scavenge (transfer) ports, Xb, are EBAR
and TB AR, while that for the bar between the last exhaust port and the first scavenge port is
coded as ETBAR, respectively. The value of FOCUS is the proportion of the cylinder radius,
from the cylinder center to the edge of the bore on the scavenge side, where the guiding edges
of the main scavenge ports index the centerline. The guiding edges in question are the trailing
edges of the first port, and the leading edge of the second backswept port. This is shown even
more clearly in Fig. 3.40, where they are reproduced in the same manner as the screen display
of the computer program. The side port is at right angles to the line of symmetry and the back
ports are aligned with it, facing the exhaust port. Naturally, it is essential in this design configuration
to elevate the back ports, usually by 50-60°. To allow the designer this flexibility,
both the side and the back ports can be elevated by setting angles SUP and BUP between 0
and (a maximum of) 60°.
The plan radius of a recessed cylinder bowl in the fashion of a DI diesel engine is RBOWL;
the corner radius of that bowl is RCV; the squish clearance, the piston lengths to crown and
skirt, and the gudgeon pin offset (+ in direction of rotation) are shown coded as SQCL, GPC,
GPS and GPOFF, respectively.
The port corner radii, rc, are presented as RTE, RBE, RTT and RBT, the top and bottom
corner radii of the exhaust and scavenge (transfer) ports, respectively.
The output data are also fairly self-explanatory. However, each line shows: the engine
total swept volume (cm 3 ) and power (kW); the mean widths of all of the exhaust and scavenge
ports; the port width ratio CA; the areas Axp and Asp of the exhaust ports and scavenge
ports, respectively; the exhaust pipe area and its diameter; the blowdown, exhaust and transfer
port time areas of those ports (vide Sec. 6.1); the blowdown, exhaust and transfer port time
areas of those ports (vide Sec. 6.1); the combustion chamber clearance volume (cm 3 ) and the
depth of that chamber; the heights, hx and hs, of the exhaust ports and scavenge ports, respectively;
the subtended port angles, 6p, of each of the scavenge ports for they are specifically
designed to be identical; the subtended angles, 0b, of each of the bars of the exhaust ports,
between the exhaust and the scavenge ports, and between each of the scavenge ports, respectively;
the slopes (deg) of each of the focusing edges of the main scavenge ports with respect
to the "horizontal" centerline of scavenge symmetry.
A change of data is invited by the program for any or all of the input data, as is their
recomputation. The code for that data change is the name of that particular input data parameter,
to be typed in by the user. A hard copy, i.e., a printout, is available of the entire screen
display, and this is drawn to scale from the input and output data. The "by eye" design process
is aided by the computer program immediately drawing the sketch of the piston and the porting
to scale on the computer screen as a function of the input data.
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