Design and Simulation of Two Stroke Engines

Chapter 4 - Combustion in Two-Stroke Engines
4.1.3 Effect of scavenging efficiency on flammability
In Sec. 3.1.5 there is mention of the scavenging efficiency variation with scavenge ratio.
As the engine load, or brake mean effective pressure, bmep, is varied by altering the throttle
opening thereby producing changes in scavenge ratio, so too does the scavenging efficiency,
SE, change. It will be observed from Fig. 3.10, even for the best design of two-stroke engines,
that the scavenging efficiency varies from 0.3 to 0.95. Interpreting this reasonably accurately
as being "charge purity" for the firing engine situation, it is clear that at light loads and low
engine rotational speeds there will be a throttle position where the considerable mass of exhaust
gas present will not permit ignition of the gasoline vapor-air mixture. When the spark
occurs, the mass of vapor and air ignited in the vicinity of the spark is too small to provide an
adequate release of heat to raise the surrounding layer of unburned mixture to the auto-ignition
temperature. Consequently, the flame does not develop and combustion does not take
place. The effect is somewhat similar to the lean burning misfire limit discussed above.
During the next scavenging process the scavenging efficiency is raised as the "exhaust
residual" during that process is composed partly of the unburned mixture from the misfire
stroke and partly of exhaust gas from the stroke preceding that one. Should the new SE value
prove to be over the threshold condition for flammability, then combustion does takes place.
This skip firing behavior is called "four-stroking." Should it take a further scavenge process
for the SE level to rise sufficiently for ignition to occur, then that would be "six-stroking,"
and so on. Of course, the scavenging processes during this intermittent firing behavior eject
considerable quantities of unburned fuel and air into the exhaust duct, to the very considerable
detriment of the specific fuel consumption of the engine and its emission of unburned
hydrocarbons.
Active radical combustion
It was shown by Onishi [4.33] that it is possible under certain conditions to ignite, and
ignite with great success, an air-fuel mixture in high concentrations of residual exhaust gas,
i.e., at low SE values. This was confirmed by Ishibashi [4.34] and the engine conditions
required are to raise the trapping pressure and temperature so that the exhaust residual remains
sufficiently hot and active to provide an ignition source for the air-fuel mixture distributed
through it. Hence the term "active radical," or AR, combustion. The ensuing combustion
process is very stable, very efficient, and eliminates both the "four-stroking" skip firing regime
and the high exhaust emissions of hydrocarbons that accompany it, as described above.
It is certainly a design option open for the development of the simple carburetted engine and
one which has not been sufficiently exploited. There is some evidence that several motorcycle
engines of the 1930s had low speed and low load combustion processes which behaved
in this manner.
The effect must not be confused with detonation, as discussed in the next section.
4.1.4 Detonation or abnormal combustion
Detonation occurs in the combustion process when the advancing flame front, which is
pressurizing and heating the unburned mixture ahead of it, does so at such a rate that the
285