Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
The empirical guidance to be given will range from the simplistic, such as the maximum
port width that the designer can select without encountering piston ring breakage, to the
complex, such as the design of the entire geometry of an expansion chamber to suit a given
engine. Actually, what will be regarded as simplistic by some will be considered by others as
inconceivable. To quote the apparently simple example above, the experienced engineer in
industry will have little problem in deciding that the maximum width of an exhaust port could
be as large as 70% of the cylinder bore dimension, provided that suitably large corner and top
edge radii are used to compress the piston ring inside the timing edges. On the other hand, the
archetypical undergraduate university student will not have that past experience to fall back
on as empirical guidance and has been known to spend inordinate amounts of time peering at
a computer screen which is awaiting an input, unable to make a decision.
There exists the danger that the empirical guidance to be given will be regarded as the
final design, the "quick-fix formula" so beloved by a human race ever desirous of cutting
corners to get to a solution of a problem. Let a word of caution be sounded in that case, for no
simplistic approach will ever totally provide an exact answer to a design question. A good
example of that is the use of the heat release model of combustion within the engine modeling
programs presented in Chapter 5. While the quality of the answer is acceptable in design
terms today, the thinking engineer will realize the limitations imposed by such an approach,
and will strive to incorporate ever more sophisticated models of combustion and heat transfer
into those engine design programs, particularly as CFD theory becomes more practical and
desk-top computers calculate ever faster.
The empirical advice will incorporate matters relating to the engine simulations shown in
Chapter 5. The following is a list of the topics to be covered:
(i) The estimation of the data for porting characteristics to meet a design requirement
for a given engine cylinder at a given engine speed to produce a particular power
output,
(ii) The translation of the data in (i) into port timings, widths, heights and areas for a
particular engine geometry,
(iii) The estimation of the data for the geometry of an expansion chamber exhaust system
for a high specific output engine so that it is tuned at a desired engine speed
and matched to the flow requirements of the engine,
(iv) The preliminary design of reed valve and disc valve induction systems for twostroke
engines.
6.1 Design of engine porting to meet a given performance characteristic
The opening part of this section will discuss the relationship between port areas and the
ensuing gas flow, leading to the concept of specific time area as a means of empirically
assessing the potential for that porting to produce specific performance characteristics from
an engine. The second part will deal with the specific time area analysis of ports in engines of
a specified geometry and the predictive information to be gleaned from that study. The third
part will examine the results of that study by its analysis through an engine modeling program.
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