Design and Simulation of Two Stroke Engines

Chapter 6 - Empirical Assistance for the Designer
mately, the acid test of dynamometer testing of a firing engine. At the same time, it is an
unwise move to totally ignore recommendations which are based on the accumulation of past
history and experience. In that regard, examination of that which already exists in production
always provides a guide to the possible, and a study of the compendia of design data on twostroke
engines from the Technical University of Graz [3.32] can assist in the design process.
6.2,5 The selection of the exhaust system dimensions
The subject of exhaust systems for two-stroke engines is one of considerable complexity,
not simply because the unsteady gas dynamics are quite difficult to trace within the human
mind, but also because the optimum answer in any given application is arrived at from a
combination of the desirable gas dynamics and the necessity of physical constraints caused
by the market requirements. A good example is the chainsaw engine discussed as a design
example within this book. The conclusion in Chapter 5 is that the compact exhaust system
necessary for a handheld tool is a serious limitation on the engine power output. The designer
of that unit is well aware of the problem, but is unable to silence the engine adequately within
the constraint of the given package volume without impairing performance.
As is evident from the above statements and from the discussion in Chapter 5, the complexity
of the pressure wave reflections in a tuned system are such that it is difficult to keep
track of all of the possible combinations of the pulsations within the mind. However, as Chapter
5 demonstrates, the computer will carry out that task quite well, but only after the numbers
for the lengths and diameters have been inserted into it for subsequent analysis. That puts the
problem of selecting those numbers right back into the human domain, which implies some
form of basic understanding of the process. This applies equally to the design of the compact
chainsaw system as it does to the expansion chamber for the racing engine.
The exhaust system for an untuned engine
The first objective is to allow the creation of the exhaust pulse without restriction, thus
the downpipe diameter should be at least as large in area as the total exhaust port effective
area. If the system is untuned then there is almost certainly a silencer to be incorporated
within it. As exhaust noise is a function of exhaust pulse amplitude and also of the rate of rise
of that exhaust pulse, there is little point in having a downpipe from the engine which is the
minimum criteria diameter as stated above. Thus the downpipe can be more comfortably
sized between 1.2 and 1.4 times the port area. The silencer box, if it has a volume at least 10
times the volume of the cylinder swept volume and does not have an outlet pipe which is
unduly restrictive, can be situated at an appropriate distance from the engine so that a suction
reflection, of whatever sub-atmospheric amplitude, can assist the engine to dispose of the
exhaust gas and promote a stronger flow of fresh charge through the motor. The length of the
downpipe from the piston face to the entrance to the silencer box is labeled in Fig. 5.6 as Li,
and employed in mm units for convenience in this empirical calculation. This pipe length can
be easily assessed from an empirical knowledge of the following: the total exhaust period in
degrees, 8ep; the probable mean exhaust gas temperature in °C, Texc; the engine speed of
rotation in rev/min, rpm.
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