Design and Simulation of Two Stroke Engines

Chapter 6 - Empirical Assistance for the Designer
This equation applies to all ports and to all gas flow through the engine. Within the engine
and gas-dynamic models of Chapters 2 and 5, this summation exercise is carried out at each
calculation step so that the values of delivery ratio, scavenge ratio, etc., are ultimately determined.
The term, Ae, which is the instantaneous area of the port at any instant, employed in
Eq. 6.1.3, is determined using the theoretical approach detailed in Sec. 5.2.
As most two-stroke engines have the symmetrical porting geometry shown in Fig. 6.1,
and as the variations of density and gas velocity with time are somewhat similar from engine
to engine, there might exist some mathematical function linking the remaining terms to the
gas flow transmitted. Perhaps more importantly, as the gas flow which is transmitted is related
to that which is trapped and burned with fuel to produce power, the relationship could
extend to a more direct linkage to engine performance.
For example, the application of Eq. 6.1.3 to the inlet port within an engine model predicts
the total air mass ingested into the engine on each cycle. The point has already been made
above that this must have a direct relationship to the work output from that cycle when that air
is burned with fuel in the correct proportions. As already pointed out, this can be seen in Figs.
5.9 and 5.10 from the close correspondence between the delivery ratio and bmep with engine
speed for the chainsaw engine.
The net work output per cycle, from Fig. 1.16, is given by:
Work/cycle = Vsv bmep (6.1.4)
If the relationship is linear, then the division of air mass ingested by work output should
be "a constant":
i Jpe A e c e dt
airingested = ^ = a constant (6-1 -5)
Vsvbmep Vsvbmep
On the assumption that the temporal variations of density, pe, and particle velocity, CQ,
are somewhat similar, even for different engines, incorporating these so-called constants into
the right-hand side of Eq. 6.1.5 reduces it to:
This can also be written as:
9=0
,=t P
|A0dt
m oc bmep (6.1.6)
V
v sv
jAe-de
3 - - bmep (6-1-7)
6=0
v Ysv
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