Design and Simulation of Two Stroke Engines

4 -i
3 -
LU
CC
D 2
CO '
CO
LU
CC
-1
0 -
1.17
PV =K
EXPERIMENTAL DATA
TDC
_U
Chapter 1 - Introduction to the Two-Stroke Engine
EXPERIMENTAL DATA
THEORETICAL CYCLE
BDC
3 4 5 6 7
LOG(VOLUME)
Fig. 1.15 Logarithmic plot of pressure and volume.
the actual compression process clearly begins before the official trapping point at exhaust
port closure, and this in an engine with no tuned exhaust pipe. The theoretical assumption of
a constant volume process for the combustion and exhaust processes is clearly in error when
the experimental pressure trace is examined. The peak cycle pressures of 54 bar calculated
and 36 bar measured are demonstrably different. In Chapters 4 and 5 a more advanced theoretical
analysis will be seen to approach the measurements more exactly.
The work on the piston during the cycle is ultimately and ideally the work delivered to the
crankshaft by the connecting rod. The word "ideal" in thermodynamic terms means that the
friction or other losses, like leakage past the piston, are not taken into consideration in the
statement made above. Therefore, the ideal work produced per cycle (see Eq. 1.5.24) is that
work carried out on the piston by the force, F, created from the gas pressure, p. Work is always
the product of force and distance, x, moved by that force, so, where A is the piston area:
Work produced per cycle = J Fdx = J pAdx = J pdV
(1.5.24)
Therefore, the work produced for any given engine cycle, in the case of a two-stroke
engine for one crankshaft revolution from tdc to tdc, is the cyclic integral of the pressurevolume
diagram in the cylinder above the piston. By the same logic, the pumping work required
in the crankcase is the cyclic integral of the pressure-volume diagram in the crankcase.
In both cases, this work value is the enclosed area on the pressure-volume diagram, be it a
theoretical cycle or the actual cycle as illustrated in Fig. 1.14. The above statements are
illustrated in Fig. 1.16 for the actual data shown previously in Figs. 1.14 and 1.15.
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