Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
exhaust pipe, and (ii) an intake system pressure pulsation which is above atmospheric pressure.
At any other period the overall pressure differential across the reed petals is relatively
small. It will be seen that it is this initial "jab" of force on the reed petal, from 190° atdc to
240° atdc, which provides the energy to swing the reed off its seat to a tip lift ratio of 0.25.
The delivery ratio at that point, with the reed lifted to 80% of its maximum lift, is miniscule at
0.15 and is even falling slightly. The momentum given to the reed petal continues the lifting
process, with some minor pressure assistance, until the peak lift point occurs at a Crdt value of
0.3 at about 310° atdc. For a 38-mm-long reed, that means that the tip has lifted some 11.4
mm. At that point, with some further minor pressure differential continuing to flow air past it
and acting against its closure, the "spring" forces in the petal material dominate and the reed
petal reseats itself. During the period from maximum reed lift, the pressure difference across
it may be small, but it lifts the delivery ratio, DR, from 0.3 at 60° btdc to 1.2 at 75° atdc, a
period of 135°. The intake system provides a modest ramming action, up to 1.25 atm by 90°
atdc, but eventually the increasing crankcase compression pressure supersedes it and backflow
occurs, taking some 10% off the delivery ratio. The reed finally reseats itself at 115° atdc. As
it had opened at 200° atdc, the total opening period of the reed is 275°.
The designer should note most carefully that the reed action is controlled by the joint
action of the exhaust system and the intake system dynamics. To attain a high delivery ratio,
the intake system must provide two ramming actions, the conventional one which prevents
significant backflow as the reed is closing and another which assists in impelling the reed
rapidly off its seat as early as possible.
This information can be compared to that given in Chapter 1 in Figs. 1.8(c) and (d), and
the discussion will supplement that found later.
Further simulation involving this racing motorcycle engine
In Chapter 6, where the focus is on empirical design assistance for the physical geometry
of the engine and its ducting, this reed-valved engine and its tuned exhaust system is used to
illustrate the discussion. In Sec. 6.1.2, the dimensions of the cylinder porting are shown to fit
into the same basic design rules regarding specific time areas as for the chainsaw. In Sec.
6.2.5, where simple design rules for the tuned expansion chamber are expressed, there are
further simulation results for the pressure-time histories in the exhaust system of the racing
engine at 8500, 9600, 11,200 and 12,300 rpm, shown in Figs. 6.18-6.21, respectively. That in
Fig. 6.21 is particularly interesting, for it graphs the pressure behavior in the exhaust pipe at
8500 rpm, when the exhaust timing valve is lowered to open at 100° atdc, and provides the
necessary insight into the bmep-speed relationship seen here in Fig. 5.31. The other diagrams
have the exhaust timing valve raised, in Figs. 6.18-6.20, so they illustrate the exhaust pressure
wave control of a bmep curve which starts at 9600 rpm and concludes at 12,300 rpm.
5.5.3 The simulation of a multi-cylinder engine
This discussion concerns a multi-cylinder, spark-ignition engine with direct in-cylinder
injection of gasoline. Such engines have been built in recent times as prototypes for automobiles
[5.23] and simulation has been attempted for such units using the homentropic method
of characteristics theory [5.24]. The simulation here uses the non-isentropic theory presented
in Chapter 2, together with the scavenging and combustion theory given in Chapters 3 and 4.
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