Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
2 and 5, then this section will permit a better initial selection of the data for use within such a
model.
6.4 Empirical design of disc valves for two-stroke engines
This subject is one which I rarely discuss without a certain feeling of nostalgia, because it
was the motorcycle racing engines from MZ in East Germany, with their disc valve inlet
systems and expansion chamber exhaust pipes, which appeared on the world's racing circuits
during my undergraduate student days. They set new standards of specific power performance
and, up to the present day, relegated the then all-conquering four-stroke power units
into second place. It was, literally, a triumph of intellect and professional engineering over
lack of finance, resources, materials and facilities. For many years thereafter, the disc valve
intake system was acknowledged to be the superior method of induction control for high
specific output engines. However, it was not long before it was learned how to produce equal,
if not superior, performance from piston control of the intake system. The current technical
position is that the reed valve has supplanted the piston-controlled intake port for racing
engines and is probably the most popular intake control method for all engines from the
cheapest 2 hp brushcutter to the most expensive 300 hp V8 outboard motor. Nevertheless, the
disc-valved engine, in the flat form made popular by MZ, and identical to the Rotax design
shown in Plate 1.8, is still very successful on the racing circuits. Racing engine design has a
fashion element, as well as engineering logic, at work; and fashion always comes full circle.
Therefore, it is important that a design procedure for disc valve induction be among the
designer's options and that engine modeling programs are available to predict the behavior of
the disc-valved engine. This enables direct comparisons to be made of engine performance
when fitted with all of the possible alternatives for the induction process.
As with other data required for engine modeling programs, the designer needs to be able
to establish empirical factors so that a minimum of guesswork is required when faced with the
too-numerous data bank of a major computer program or when rapid optimization is required
for the design. The following discussion sets out a logic procedure which by now should be
familiar, for it repeats the same basic empirical methodology used for the piston-controlled
intake and the reed valve induction systems.
6.4.1 Specific time area analysis of disc valve systems
Figs. 1.7(a), Plate 1.8, and Figs. 6.28 and 6.29 show the mechanical positioning, porting
control and timing of the disc valve intake device. The earlier explanations in this chapter
regarding the influence of the specific time area on the bmep attainable are just as relevant
here, as the disc valve merely gives an asymmetrical element to the inlet port area diagrams of
Fig. 6.1 for piston-controlled ports. Consequently, Fig. 6.28 shows that it is possible to (i)
open the intake port early, thereby taking full advantage of induction over the period when the
crankcase experiences a sub-atmospheric pressure, and (ii) shut it early before the piston
forces out any trapped air charge on the crankcase compression stroke. Actually, this is an
important feature, for the intake pressure wave is weaker by virtue of taking place over a
longer time period, and so the ramming wave is not so strong as to provide the vigorous
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