Design and Simulation of Two Stroke Engines

Chapter 5 - Computer Modeling of Engines
Plate 5.1 The QUB 500 single-cylinder 68 bhp engine with the expansion chamber
exhaust slung underneath the motorcycle (photo by Rowland White).
demonstrated to contain a significant potential for tuning to provide high-performance characteristics
not possible in a similar arrangement for a four-stroke engine.
The sketches in Fig. 5.8 show the lengths and diameters necessary as input data for a
simulation to be conducted. The systems contain an exhaust box which, in the case of the
outboard engine, is also the transmission housing leading to the propeller and can be seen in
the photograph of the OMC V8 engine in Plate 5.2. That same type of engine, sketched in Fig.
5.8(a), has an exhaust manifold referred to as a "log" type in the jargon of such designs, and
contains branches which are effectively T junctions. The manifolds may contain splitters
which assist the flow in turning toward the exhaust box and, if effective in that regard, the
appropriate branch angles can be inserted into the input data file as is required for the solution
of the non-isentropic theory set out in Sec. 2.14. Should the bend at cylinder number 1, i.e.,
that at the left-hand end of the bank of cylinders sketched, be considered to be tight enough to
warrant an appropriate insertion of loss for the gas flow going around it, then the theory of
Sec. 2.3.1 can be employed.
The design shown in Fig. 5.8(b) is more appropriate to that used for an automotive engine,
be it for an automobile or a motorcycle, or for a diesel-engined vehicle, where space is
not at such a premium as it is for an outboard engine. The gas flow leaving the manifold
toward the exhaust box, which may contain a catalyst or be a silencer, does so by way of a
four-way branch. The theory for this flow regime requires an extension to that presented in
Sec. 2.14. The sketch shows the junction at a mutual 90° for each pipe, but in practice the
pipes have been observed to join at steeper angles, such as 60°, to the final exit pipe. This may
not always be a design optimum, for the closer coupling of the cylinders normally takes
precedence over the branch angle as a tuning criterion, as what may appear to provide an
easier flow for the gas to exit the manifold may reduce the inter-cylinder cross-charging
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