Design and Simulation of Two Stroke Engines

Chapter 4 - Combustion in Two-Stroke Engines
velocities observed in various combustion chambers must bear some correspondence with
squish velocity. Kee [1.20] reports flame speeds measured in a loop-scavenged engine under
the same test conditions as already quoted in Sec. 4.2.3. The chamber was central and the
measured flame speed was 24.5 m/s. With the data inserted for the combustion chamber
involved, Prog.4.1 predicted maximum squish velocity and squished kinetic energy values of
11.6 m/s and 2.61 mJ, respectively.
In further tests using a QUB-type deflector piston engine of the same bore and stroke as
the loop-scavenged engine above, Kee [1.20] reports that flame speeds were measured at the
same speed and load conditions at 47 m/s within the chamber and 53 m/s in the squish band.
The average flame velocity was 50 m/s. With the data inserted for the deflector combustion
chamber involved, Prog.4.1 predicted maximum squish velocity and squished kinetic energy
values of 38.0 m/s and 50.2 mJ, respectively. In other words, the disparity in squish velocity
as calculated by the theoretical solution is seen to correspond to the measured differences in
flame speed from the two experimental engines. That the squish action in a QUB-type crossscavenged
engine is quite vigorous can be observed in Plate 4.3.
If the calculated squish velocity, cSq, is equated to the turbulence velocity, ctrb, of Eq.
4.3.1 and subtracted from the measured flame velocity, Cfl, in each of Kee's experimental
examples, the laminar flame velocity, qf, is predicted as 12.9 and 12.0 m/s for the loopscavenged
and QUB deflector piston cases, respectively. That the correspondence for qf is so
close, as it should be for similar test conditions, reinforces the view that the squish velocity
has a very pronounced effect on the rate of burning and heat release in two-stroke engines. It
should be added, however, that any calculation of laminar flame velocity [4.2, 4.5] for the
QUB 400 type engine would reveal values somewhat less than 3 m/s. Nevertheless, flame
speed values measured at 12 m/s would not be unusual in engines with quiescent combustion
chambers, where the only turbulence present is that from the past history of the scavenge flow
and from the motion of the piston crown in the compression stroke.
The design message from this information is that high squish velocities lead to rapid
burning characteristics and that rapid burning approaches the thermodynamic ideal of constant
volume combustion. There is a price to be paid for this, evidenced by more rapid rates of
pressure rise which can lead to an engine with more vibration and noise emanating from the
combustion process. Further, if the burning is too rapid, too early, this can lead to (a) high
rates of NOx formation (see Appendix A4.2) and (b) slow and inefficient burning in the latter
stages of combustion [1.20,4.29]. Nevertheless, the designer has available a theoretical tool,
in the form of Prog.4.1, to tailor this effect to the best possible advantage for any particular
design of two-stroke engine.
One of the beneficial side effects of squish action is the possible reduction of detonation
effects. The squish effect gives high turbulence characteristics in the end zones and, by inducing
locally high squish velocities in the squish band, increases the convection coefficients for
heat transfer. Should the cylinder walls be colder than the squished charge, the end zone gas
temperature can be reduced to the point where detonation is avoided, even under high bmep
and compression ratio conditions. For high-performance engines, such as those used for racing,
the design of squish action must be carried out by a judicious combination of theory and
experimentation. A useful design starting point for gasoline-fueled, loop-scavenged engines
with central combustion chambers is to keep the maximum squish velocity between 15 and 20
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