Design and Simulation of Two Stroke Engines

Chapter 6 - Empirical Assistance for the Designer
The incorporation of the expansion coefficient, Cc, is very similar to that used for the reed
valve case and is in the range of 1.35-1.45. Actually, it should be noted that in the reed valve
design theory it is employed as the reciprocal number, at 0.70. In the computer solution presented
in Prog.6.5, the actual value encoded is 1.4.
Note, as with previous solutions of similar equations, if the data for swept volume in Eq.
6.4.7 are inserted in the conventional units of cm 3 , then the next line will fortuitously evaluate
the maximum port area, Amax, as mm 2 . If strict SI units are used, then Vsv is in m 3 in Eq.
6.4.7, and Eq. 6.4.8 will produce the output for Amax in the equally strict SI unit of m 2 .
However, I issue my familiar warning, stick to SI units throughout all design calculations and
units will never become an arithmetic problem!
6.4.2 A computer solution for disc valve design, Prog.6.5
So that the designer may concentrate more on the design process and less on the arithmetic
tedium of solution, the above equations are programmed into a computer program,
Prog.6.5, DISC VALVE DESIGN, available from SAE. As with the majority of the programs
referred to in this book, maximum advantage is taken of the screen graphics capability of the
desktop computer. An example of the use of the calculation is given in Fig. 6.29, which shows
the computer screen image of the program being used to design a disc valve option for induction
to a 125 Grand Prix engine, one of the power units used throughout these chapters as a
working example of a high specific output engine. The screen displays the correctly scaled
drawing of the disc valve and the intake port, based on both the input data dimensions inserted
by the designer and the output data as calculated by the program. Indeed, the computer
screen picture is almost identical to the view given of the left-hand cylinder in Plate 1.8, or as
sketched in Fig. 5.3.
The input data are listed in the upper half of the printed portion of the diagram. As discussed
above, the performance target requirements based on cylinder swept volume and the
SWEPT VOLLME, cc= 125 ENGINE BMEP, bar= 1 1 SPEED, rpm= 11500
DISC VALVE OPENS, deg.btdc= 1 40
DISC VALVE CLOSES, deg.atdc= 80
DISC Ml NIMLM RADIUS,'Rm in', mm= 2 8
PORT CORNER RADII, 'Rp', mm= 8
PORT ANGLE, 'PHIp', deg.= 56
OUTPUT DATA
MAXIMUM RADIUS, 'Rmax', mm= 58.
DISC ANGLE, 'PHId', deg=164.
CARBURETTER DIA., 'Dtv', mm= 39.
PORT HEIGHT, mm = 30.
PORT WIDTH at mid-height, mm= 36.
ASPECT RATIO, width/height=1.19
Fig. 6.29 Computer screen output from Prog.6.5, DISC VALVE DESIGN.
459