Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
The diffuser is designed as a "horn" and the exponent, kj,, is called the "horn coefficient."
It is self-evident from any of the discussion on gas flow, that all diameters are the internal
diameters of the exhaust pipe, and that all lengths are distances along the centerline of a pipe
which may be curved for some segments.
Some years ago, I presented a paper concerning the empirical design of expansion chambers
[2.18]. It is instructive to compare the above discussion with that paper and to note that
the empiricism has actually become simpler with the passage of both time and experience.
For those who wish to have these calculations performed automatically, the Appendix
Listing of Computer Programs includes Prog.6.2v2, EXPANSION CHAMBER. This carries
out the useful dual function of deducing all of the above dimensions and data and the computer
screen and line printer will show a dimensioned sketch of the exhaust system. An example
of that is shown for a prediction of an expansion chamber for the 125 racing motorcycle
engine used as one of the working examples in Chapter 5. The screen/line-print output
is shown in Fig. 6.22. If the data predicted by the empirical design program are compared
with that given in Sec. 5.5.2, a considerable level of close correspondence will be observed.
Recall that the engine modeling program predicted in Fig. 5.28 a high specific power performance
for an engine with a very similar exhaust system. This should give you some confidence
that this simple program, Prog.6.2v2, albeit overlaid with some considerable practical
experience, enables the rapid and efficient preparation of the geometry of an expansion chamber
for further complete analysis by an engine simulation. Prog.6.2v2 permits you to vary the
diffuser profile by changing the horn coefficient. In Fig. 6.22, the values of d2 and d3 are
displayed as 66 and 99 mm, respectively, for a horn coefficient of 1.5. If the horn coefficient
GRAND PRIX PIPE
038
056 089 0116 022 022
TUNED LENGTH FROM PISTON TO TAIL PIPE ENTRY IS 832
DESIGN LENGTHS AND DIAMETERS OF AN EXPANSION CHAMBER FOR A TWO-STROKE ENGINE
BORE= 5 6 STROKE= 50.6CON-ROD= 110 EXHAUSTOPENS, B atdc= 81 EXHAUST PORTS= 1
WIDTH EACH PORT= 43 EXHAUST PORT TOP RADIUS= 8 AND BOTTOM RADIUS= 8
ENGINESPEED= 11750 EXHAUSTTEMPERATURE, B C= 600 HORNEXPONENT= 1.5
AREA RATIOS FOR PORTTO PIPE=1.051 AND PORT TO Ml D-SECTION= 9.79
Fig. 6.22 Expansion chamber design for a racing engine using Prog.6.2v2.
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