Design and Simulation of Two Stroke Engines

Chapter 6 • Empirical Assistance for the Designer
On the left are all of the input and output data values of the calculation, any one of which
can be readily changed, whereupon the computer screen refreshes itself, virtually instantaneously,
with the numerical answers and the reed block image. Note that the values of Young's
Modulus and reed petal material density are missing from those columns. The data values for
Young's Modulus and material density are held within the program as permanent data and all
that is required is to inform the program, when it asks, if the petal material is steel, glass-fiber,
or carbon-fiber. The information is recognized as a character string within the program and
the appropriate properties of the reed material are indexed from the program memory.
The reed valve design program, Prog.6.4, is used to empirically design reed blocks and
petals in advance of using the data in engine modeling programs. This program is useful to
engineers, as it is presumed that they are like me, designers both by "eye" and from the
numerical facts. The effectiveness of such modeling programs [1.13] has already been demonstrated
in Chapter 5 and in Figs. 6.8-6.15, so it is instructive to examine the empirical
design for the 125 Grand Prix engine shown in Fig. 6.27 and compare it with the data declared
for the actual engine in Chapter 5, Sec. 5.5.2.
The glass-fiber reed has a natural frequency of 160 Hz. As the engine has a natural forcing
frequency of 192 Hz, i.e., 11,500 -f 60, this reed is in some long-term danger of fracture
from resonance-induced fatigue, for it passes through its resonant frequency each time the
machine is revved to 11,500 rpm in each gear when employed as a racing motorcycle engine.
Presumably, as the lifetime of a racing engine is rarely excessive, this design is acceptable.
Indeed, it is even desirable in that it will vibrate readily at the forcing frequency of the engine.
However, it should not be regarded universally as good design practice for an engine, such as
a production outboard, where durability over some 2000 hours is needed to satisfy the market
requirements.
Of some interest is the photograph in Plate 4.1, which is just such a 250 cc twin-cylinder
racing motorcycle engine, i.e., each cylinder is 125 cc just like the design being discussed. On
this engine is the reed valve induction system which can be observed in the upper right-hand
corner of the photograph. The plastic molding attaches the carburetor to the reed block and
locks it onto the crankcase. The interior of the molding is profiled to make a smooth area
transition for the gas to flow from the round section of the carburetor to the basically rectangular
section at the reed block entrance.
6.3.4 Concluding remarks on reed valve design
It is not uncommon to find flat plate reed valves being used in small outboard motors and
in other low specific power output engines. A flat plate reed block is just that, a single plate
holding the reed petals at right angles to the gas flow direction. The designer should use the
procedure within Prog.6.4 of declaring a "dummy" reed block angle, (j)rb, of 40° and proceed
with the design as usual. Ultimately the plan view of the working drawing will appear with
the reed plate, per petal and port that is, exactly as the lower half of the sketch in Fig. 6.27.
With this inside knowledge of the reed design and behavior in advance of engine modeling
or experimentation, the engineer is in a sound position to tackle the next stage of theoretical
or experimental design and development. Should you assemble a computer program to
simulate an engine and its reed valve motion, together with the theory presented in Chapters
455