Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
where the parameter, I, is the second moment of area given in Eq. 5.2.16. The maximum tip
lift ratio, CrCjt, for this empirical calculation is then:
r - Xti P
c rdt - — (6.3.7)
Consequently, for implementation in Eq. 6.3.1, the time area of the reeds, for a number of
reeds, nr, is given by:
j A d e ^ " ^ ^ m 2 deg (6.3.8)
where 6p can been assigned a value of 200° for the isosceles triangle of lift to xtjp.
The above equations, referring to a combination of time areas and the mechanics of reed
lift, i.e., Eqs. 6.3.1-6.3.8, should be strictly carried out in SI units, or arithmetic inaccuracies
will be the inevitable consequence.
The last geometrical dimension to be calculated is the stop-plate radius which should not
permit the tip of the reed to move past a Crcjt value of 0.3, but should allow the reed petal to be
tangential to it should that lift actually occur. That a lift limit for Crcjt of 0.3 is realistic is seen
in Fig. 5.35 for a racing engine. This relationship is represented by the following trigonometrical
analysis, where the normal limit criterion is for a tip lift ratio of 0.3:
(l-C?dt)Lr
r sP = 9r (6-3.9)
6.3.3 The design process programmed into a package, Prog.6.4
This calculation is not as convenient to carry out with an electronic calculator as that for
the expansion chamber, as a considerable number of cycles of estimation and recalculation
are required before a matched design emerges. Therefore, a computer program has been added
to those presented and available from SAE, Prog.6.4, REED VALVE DESIGN. This interactive
program has a screen output which shows a plan and elevation view to scale of the reed
block and reed petal under design consideration. A typical example is that illustrated in Fig.
6.27, which is also a design of a reed block and petals for a 125 racing engine which has
already been discussed in Chapter 5 as a design example. The top part of the sketch on the
computer screen shows an elevation section through one-half of the reed block, although it
could just as well represent a complete block if that were the design goal. The lower half of
the sketch shows a projected view looking normally onto the reed port and the petal as if it
were transparent, with the darkened area showing the clamping of the reed by the stop-plate.
The scaled dimension to note here is the left-hand edge of the dark area, for that is positioned
accurately, while some artistic license has been used to portray the extent of the clamped area
rightward of that position!