Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
.o
uu
cc
D
w
HI
cc
0-
40-1
30-
20-
10-
TOTAL WORK ON PISTON
IS jpdV FOR TOTAL CYCLE
WHICH IS ENCLOSED AREA
ON THE pV DIAGRAM.
~n 1 1 > 1 • 1—
100 200 300 400
CYLINDER VOLUME, cm 3
Q.
0)
E
500
Fig. 1.16 Determination ofimep from the cylinder p-Vdiagram.
1.5.9 The concept of mean effective pressure
As stated above, the enclosed p-V diagram area is the work produced on the piston, in
either the real or the ideal cycle. Fig. 1.16 shows a second rectangular shaded area, equal in
area to the enclosed cylinder p-V diagram. This rectangle is of height imep and of length Vsv,
where imep is known as the indicated mean effective pressure and Vsv is the swept volume.
The word "indicated" stems from the historical fact that pressure transducers for engines used
to be called "indicators" and the p-V diagram, of a steam engine traditionally, was recorded
on an "indicator card." The concept of mean effective pressure is extremely useful in relating
one engine development to another for, while the units of imep are obviously that of pressure,
the value is almost dimensionless. That remark is sufficiently illogical as to require careful
explanation. The point is, any two engines of equal development or performance status will
have identical values of mean effective pressure, even though they may be of totally dissimilar
swept volume. In other words, Figs. 1.14, 1.15 and 1.16 could have equally well been
plotted as pressure-compression (or volume) ratio plots and the values of imep would be
identical for two engines of differing swept volume, if the diagrammatic profiles in the pressure
direction were also identical.
1.5.10 Power and torque and fuel consumption
Power is defined as the rate of doing work. If the engine rotation rate is rps, revolutions
per second, and the two-stroke engine has a working cycle per crankshaft revolution, then the
power delivered to the piston crown by the gas force is called the indicated power output, Wj,
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