Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
In the example quoted by Kee [1.20], the flame travel time to the chamber extremity is
33°, as from Eq. 4.1.1 the value of 6fl is given by:
6 x x 3000
9 = 1000 = 33 06o
24.5
That does not mean that the combustion process has been completed in 33°, but it does
mean that initiation of combustion has taken place over the entire combustion space for a
homogeneous charge. In Sec. 4.2.3, it will be shown that the travel time, in this case cited as
33°, coincides approximately with the maximum rate of heat release from the fuel.
4.1.2 Air-fuel mixture limits for flammability
The flammability of the initial flame kernel has a rather narrow window for success
[4.25, 4.26]. The upper and lower values of the proportion by volume of gasoline vapor to air
for a flame to survive are 0.08 and 0.06, respectively. As one is supplying a "cold" engine
with liquid fuel, by whatever device ranging from a carburetor to a fuel injector, the vaporization
rate of that gasoline due to the compression process is going to be highly dependent on
the temperatures of the cylinder wall, the piston crown and the atmospheric air. Not surprisingly,
in cold climatic conditions, it takes several compression processes to raise the local
temperature sufficiently to provide the statistical probability of success. Equally, the provision
of a high-energy spark to assist that procedure has become more conventional [4.6]. At
one time, ignition systems had spark characteristics of about 8 kV with a rise time of about 25
us. Today, with "electronic" or capacitor discharge ignition systems those characteristics are
more typically at 20 kV and 4 us, respectively. The higher voltage output and the faster spark
rise time ensure that sparking will take place, even when the electrodes of the spark plug are
covered in liquid gasoline. Spark duration, and even multiple sparking, also assist flame kernel
growth and this is even more applicable for the ignition of a spray of liquid fuel in a
stratified charging approach using direct fuel injection.
Under normal firing conditions, if the fuel vapor-air mixture becomes too lean, e.g., at the
0.06 volume ratio quoted above, then a flame is prevented from growing due to an inadequate
initial release of heat. When the spark occurs in a lean mixture, the mass of fuel 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. In this situation, intermittent
misfire is the normal experience as unburned mixture forms the bulk of the cylinder contents
during the succeeding scavenge process and will supplement the fuel supplied by it.
Under normal firing conditions, if the fuel vapor-air mixture becomes too rich, e.g., at the
0.08 volume ratio quoted above, then the flame is prevented from growing due to insufficient
mass of air present at the onset of ignition. As with any flame propagation process, if an
inadequate amount of heat is released at the critical inception point, the flame is snuffed out.
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