Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
ing efficiency, SE, for which no measurement is available for comparison purposes; the computation
predicts that it has a peak value of 0.8 at 6600 rpm.
With the simulation closely predicting air flow and power, the potential for accurately
simulating the measured brake specific fuel consumption, bsfc, and the emissions of hydrocarbons,
bsHC, is realized in Fig. 5.11.
500 -,
^ 400 -
g 300 -
CO
GQ
°3 200
O
LL
CO
m 100 4
CALCULATED
MEASURED
BRAKE SPECIFIC FUEL CONSUMPTION
BRAKE SPECIFIC HYDROCARBON EMISSION
CALCULATED
D B g Si £—g—g °~~Q •
MEASURED
0 —. 1 « 1 1 1 • 1 > 1 > 1
5000 6000 7000 8000 9000 10000 11000
ENGINE SPEED, rpm
Fig. 5.11 Measured and computed bsfc and bsHC characteristics of a chainsaw.
The closed cycle simulation, relying on the combustion and heat transfer theory of Chapter
4, is seen to give a more than adequate representation of that behavior in Figs. 5.12 and
5.13. In Fig. 5.12 are the measured and computed cylinder pressure diagrams at 9600 rpm,
and while the error on peak pressure is relatively small, the computation of the angular position
of peak pressure is completely accurate. In Fig. 5.13 at the same speed is the comparison
of measurement and calculation of the cylinder pressures during compression and expansion.
The Annand model of heat transfer in Sec. 4.3.4, and the fuel vaporization model in Sec.
4.3.5, can be seen to provide very accurate simulation of the compression and expansion
processes. The measured data are averaged over 100 engine cycles.
Design data available from the simulation, Figs. 5.14-5.27
The computation provides extensive information for the designer, much of which can be
measured only with great difficulty, or even not at all due to the lack of, or the non-existence
of, the necessary instrumentation. Much of this information is required so that the designer
can comprehend the internal gas-dynamic and thermodynamic behavior of the engine and the
influence that changes to engine geometry have on the ensuing performance characteristics.
The following is a sample of the range and extent of the design information which an accurate
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