Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
Finally, there is a section on the latest theoretical developments in the analysis of combustion
processes, for it will not be many years before our understanding of engine combustion
will be on a par with, say, unsteady gas dynamics or scavenging flow. The par level
referred to is where the theory will design combustion chambers for two-stroke engines and
the resulting engine will exhibit the designed combustion performance characteristics. It is
my opinion that this level is approaching, but has not yet been achieved.
4.1 The spark-ignition process
4.1.1 Initiation of ignition
It is a well-known fact that a match thrown onto some spilled gasoline in the open atmosphere
will ignite the gasoline and release a considerable quantity of heat with a significant
rise in temperature. The gasoline is observed to burn as a vapor mixed with air above the
remaining liquid, but rapidly vaporizing, gasoline. The procedure, for those who have witnessed
it (although on safety grounds I am not recommending that the experiment be conducted),
commences when the lighted match arrives at the vapor cloud above the liquid, and
the ignition takes place with a "whoosh," apparently a major or minor "explosion" depending
on the mass fraction of the gasoline that has been allowed to evaporate before the arrival of
the match. This tends to leave us with the impression that the gasoline-air mixture within the
cylinder of an IC engine will "explode" upon the application of the spark at the sparking plug.
That the flammability characteristics of a petrol-air mixture are a decidedly critical phenomenon
should be obvious to all who have had difficulty in starting either their lawnmower or
their automobile!
What then are the requirements of an ignition process? Why does an engine fire up?
When does it not fire up? The technical papers on combustion and the engineering textbooks
tend to bypass such fundamental concepts, so I felt that a paragraph or two would not be
amiss, especially as more difficult concepts will only be understood against a background of
some fundamental understanding.
Fig. 4.1(a) depicts a two-stroke engine where a spark has ignited an air-fuel mixture and
has produced a flame front burning its way through the mixture. For this to happen, like the
match example before it, there had to be a gasoline vapor and air mixture, of the correct mass
proportions, within the spark gap when that spark occurred. The energy in the spark provided
a local rise in temperature of several thousand degrees Kelvin, which caused any gasoline
vapor present to be raised above its auto-ignition temperature. The auto-ignition temperature
of any hydrocarbon fuel is that temperature where the fuel now has sufficient internal energy
to break its carbon-hydrogen bond structure and be oxidized to carbon dioxide and steam. In
the case of gasoline the auto-ignition temperature is about 220°C. The compression process
prior to the ignition point helps to vaporize the gasoline, whose maximum boiling point is
about 200°C. The mass of gasoline within the spark gap, having commenced to break down in
an exothermic reaction, raises the local temperature and pressure. The reaction, if it were
stoichiometric, would be as given previously in Eq. 1.5.16. The actual reaction process is
much more complex than this, with the gasoline molecule breaking down in stages to methane
and aldehydes. Immediately after the ignition point, the initial flame front close to the
spark plug has been established and heats the unburned layers of gasoline vapor-air mixture
surrounding it, principally by radiation but also by convection heat transfer and by the mix-
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