Design and Simulation of Two Stroke Engines

Chapter 6 - Empirical Assistance for the Designer
6.1.1 Specific time areas of ports in two-stroke engines
Fig. 6.1 is a graphical representation of the port areas as a function of crank angle in a
two-stroke engine. The actual data presented is for a chainsaw engine design somewhat similar
to that discussed in Chapter 5 and with the geometrical data given in Fig. 6.2. On the
grounds that engineers visualize geometry, the cylinder is presented in Fig. 6.2, as seen by a
computer-generated sketch based on the numerical data. Therefore, it is the movement of the
piston past those ports, be they exhaust, transfer or inlet, as the crankshaft turns which produces
the area opening and closing relationships presented in Fig. 6.1.
Some of the numbers generated are of general interest. Note that the maximum area of the
inlet port and the exhaust port are about equal. The transfer port total area is clearly less than
the inlet and exhaust areas. The area reached by the exhaust port at the point of transfer port
opening, i.e., at the official end of the blowdown period, is about one-quarter of the maximum
area attained by the exhaust port. The flat top to the inlet area profile is due to that particular
design being fully uncovered for a period of 52° around tdc. The symmetry of the port area
diagrams around the tdc or bdc positions is created by the piston control of all gas flow access
to the cylinder; this would not be the case for ports controlled by a disc valve, reed valve or
poppet valve mechanism (see Sec. 1.3).
It has already been demonstrated that the performance characteristics of an engine are
related to the mass flow rates of gas traveling through the ports of the engine. For example, in
Figs. 5.9 and 5.10, a chainsaw engine is shown to produce bmep which is directly related to
the work produced per cycle, virtually as a direct function of the mass of air which it breathes
per cycle, i.e., its delivery ratio. The following simplified theory for gas flow through a port
or valve is fundamental to a discussion of that cylinder filling process. Let AQ be the instantaneous
area of an open port at any crank angle G after its opening point, and let CQ be the
CM
E
500 -,
400 -
E
.- 300 -
<
UJ
DC
< 200 -
h-
QC
O
D.
100 -
DATA FROM A CHAINSAW ENGINE
BORE 56mm STROKE 41 mm ROD 73mm
EO 90 a & TO 120 a atdc IO 80 fi btdc
0 100 200 300
CRANKSHAFT ANGLE, e atdc.
INLET
Fig. 6.1 Port areas by piston control in a two-stroke engine.
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400