Design and Simulation of Two Stroke Engines

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Chapter 5 - Computer Modeling of Engines
The area of the port at this juncture is shown in the figure as Ae for a rectangular port of
width, xp, and top and bottom corner radii, rt and rb, respectively. Notice that the position of
the top and bottom port radii are in the same physical locations for the port, in a cylinder
which is considered to be sitting vertically. Thus the analysis for the instantaneous port area,
Ae, presented in Eqs. 5.2.1 to 5.2.5, is still applicable except that the values for the port radii
must be juxtaposed so as to take this nomenclature into account. The relationship for the
maximum port area, Amax, in Eq. 5.2.6 is correct as it stands.
Hence, during an engine simulation using the theory of Sees. 2.16 and 2.17, the effective
throat area of a port controlled by the piston crown at any juncture (see Eq. 2.16.4) is defined
by At> and the numerical value of At is that given by Ae in Eqs. 5.2.4 or 5.2.5, taking into
account the above discussion. To carry out this exercise during simulation, be it for a simple
regularly shaped port, or a more complex shape, all of the data listed in Fig. 5.1(c) are required
as input data to it.
5.2.2 The porting of the cylinder controlled externally
The use of valves of various types to control the flow through the ports of an engine was
introduced in Sec. 1.3. For the intake system, reed valves, disc valves and poppet valves are
commonly employed. For the exhaust system, poppet valves have been commonly used, particularly
for diesel engines. As a further refinement to the piston control of ports leading to
the exhaust system, timing edge control valves have been utilized, particularly for the highperformance
engines found in racing motorcycles. The areas of the ports or apertures leading
into the cylinders of the engine must be accounted for in the execution of a simulation model
of an engine.
The use of poppet valves
Poppet valves are not normally used in simple two-stroke engines, but are to be found
quite conventionally in uniflow-scavenged diesel engines used for marine applications or in
trucks and buses, such as those shown in Plates 1.4 and 1.7. The area for flow through a
poppet valve, at any juncture of its lift, is described in detail in Appendix A5.1.
Hence, during an engine simulation using the theory of Sees. 2.16 and 2.17, the geometric
throat area of a port controlled by the piston crown at any juncture (see Eq. 2.16.4) is defined
by At> and the numerical value of Ae is that given by At in Appendix A5.1 in Eqs. A5.4 and
A5.5. To carry out this exercise during simulation, all of the data shown in Fig. A5.1 are
required as input data to it.
The use of a control valve for the port timing edge
The common practice to date has been to apply timing control valves to the exhaust ports
of the engine. A typical arrangement for an exhaust port timing control valve is sketched in
Fig. 5.2. In Fig. 5.2(a) the valve is fully retracted so that the timing control edge of the valve
coincides with the top edge of the exhaust port. The engine has a stroke dimension of Lst and
a trapped stroke of length xts. When the valve is rotated clockwise through a small angle and
held at that position, as in Fig. 5.2(b), normally at a lower point in the engine speed range, the
trapped stroke is effectively changed to xets and the total area of the port is reduced. Clearly,
the blowdown area and the blowdown timing interval are also reduced. While such a valve
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