Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
ment of reed lift is evident, as is the resulting calculation and measurement of delivery ratio
over the entire speed range of the engine, to b6-seen in Fig. 6.26. The timing of opening and
closing of the reed petal at the low speed of 5430 rpm shows the reed opening at 160° btdc and
closing at 86° atdc, which is an asymmetrical characteristic about tdc.
At the "high" engine speed of 9150 rpm, the reed petal opens at 140° btdc and closes at
122° atdc. This confirms the initial view, expressed in Sec. 1.3.4 and Figs. 18(c) and (d), that
the reed petal times the inlet flow behavior like a disc valve at low engine speeds and as a
piston-controlled intake port at high engine speeds. In other words, it is asymmetrically timed
at low speeds and symmetrically timed at high speeds.
Within this same paper by Fleck etal. [1.13] there is given a considerable body of experimental
and theoretical evidence on reed valve characteristics for steel, glass-fiber- and carbon-fiber-reinforced
composites when used as reed petal materials. The paper [1.13] gives
the dimensions of the engine with its tuned expansion chamber exhaust system and associated
measured power data, which you will find instructive as another working example for the
comparison of Prog.6.2v2 with experiment.
6.3.2 The use of specific time area information in reed valve design
From the experimental and theoretical work at QUB on the behavior of reed valves, it has
been found possible to model in a satisfactory fashion the reed valve in conjunction with an
engine modeling program. This implies that all of the data listed as parameters for the reed
valve block and petal in Fig. 5.4 have to be assembled as input data to run an engine simulation.
The input geometrical data set for a reed valve is even more extensive than that for a
piston-controlled intake port, adding to the complexity of the data selection task by the designer
before running an engine model. This places further emphasis on the use of some
empirical design approach to obtain a first estimate of the design parameters for the reed
block and petals, before the insertion of that data set into an engine modeling calculation. I
propose to pass on to you my data selection experience in the form of an empirical design for
reed valves as a pre-modeling exercise.
In Sec. 6.1 there is a discussion on specific time area and its relevance for exhaust, transfer
and intake systems. The flow through a reed valve has to conform to the same logical
approach. In particular, the value of specific time area for the reed petal and reed port during
its period of opening must provide that same numerical value if the flow of air through that
aperture is to be sufficient to provide equality of delivery ratio with a piston-controlled intake
port. The aperture through the reed valve assembly is seen in Fig. 5.4 and, from the discussion
in Sec. 5.2, is to be composed of two segments: the effective reed port area in the flow direction
as if the reed petal is not present; and the effective flow area past the reed petal when it
has lifted to its maximum, caused either by the gas flow or as permitted by the stop-plate.
Clearly, there is little point in not having these two areas matched. For example, if the design
incorporates a large reed port area but a stiff reed which will barely lift under the pressure
differential from the intake side to the crankcase, very little fresh charge will enter the engine.
Equally, the design could have a large flexible reed which lifted easily but exposed only a
small reed port, in which case that too would produce an inadequate delivery ratio characteristic.
In a matched design, the effective reed port area should be larger than the effective flow
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