Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
The variations for the geometry of engine ports as conceived by practicing designers can
be quite diverse, but they fall into two categories, the simple or the complex. The simple port
layout is shown in Fig. 5.1(a) and the complex is illustrated in Fig. 5.1(b).
The simple port layout with piston crown control
This is sketched in Fig. 5.1(a) and typically applies to the exhaust and scavenge ports.
The crankshaft has turned an angle, 6, from the top dead center, tdc, position and the piston
has moved a length, xe, from the tdc point, having uncovered a port at an angle, 81, from tdc,
and will fully uncover it after turning an angle, 62; the piston travel lengths are xei and xe2,
respectively. For exhaust and scavenge (transfer) ports, almost universally, the value of 82 is
180° or at bdc, in which case the value of X02 is the stroke length, Lst.
The area of the port at any juncture is shown in the figure as Ae for a rectangular port of
width, xp, and top and bottom corner radii, rt and it,, respectively.
The width across the port, xpe, at any crankshaft angle, 6, will be determined by the
piston position being in the middle of the port, or across either of the corner radii, and is found
from simple geometrical considerations, thus:
if x0 < rt + xei then xp6 = xp - 2rt + 2^rt 2 - (rt - x6 + xei) (5.2.1)
if xe > rt + xei and xe < xe2 - rb then xp0 = xp
(5.2.2)
if x0 > xei - rb then xp0 = xp - 2rb + 2^rb - (rb - xe2 + xe) 2 (5.2.3)
The area of the port is then given by, where an incremental crankshaft movement, d8, is
accompanied by a corresponding incremental piston movement, dx:
e=e
dx
A0= Jxp9dx= J xp0— d8 (524)
x=xe, 6=0,
This is a relatively complex piece of calculus, and is much more readily solved on a
computer by conducting the calculation in even increments of angle, thereby conducting the
integral as a simple summation process for any port. The information is loaded into a data
file, and by linearly interpolating from that data file, the value of port area can be determined
at any given point within the computation. It is seen thus:
. ^ 9 x p9 + X p8+A6 / >>
A e - la I v x e+A8~ x eJ (5.2.5)
8=6,
The summation can be conducted equally effectively by employing even increments of
angle, A6, of 1 ° crankshaft, or even increments of piston movement, Ax, of 0.1 mm.
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