Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
At a stoichiometric air-fuel ratio for gasoline, this means that a homogeneously charged
engine, followed by a homogeneous combustion process, would ingest 1 kg of octane with the
air. This situation is illustrated in Fig. 7.1(al) and (bl). The supplied air-fuel ratio and that in
the combustion space are identical at 15.
If the engine had stratified charging, but the ensuing mixing process is complete followed
by homogeneous combustion, then the situation is as illustrated in Fig. 7.1(a2) and (b2).
Although one of the entering air-fuel streams has a rich air-fuel ratio of 10 and the other is
lean at 30, the overall air-fuel ratio is 15, as is the air-fuel ratio in the combustion space during
burning. The supplied air-fuel ratio and that in the combustion space are identical at 15. In
effect, the overall behavior is much the same as for homogeneous charging and combustion.
If the engine has both stratified charging and combustion, then the situation portrayed in
Fig. 7.1(a3) and (b3) reveals fundamental differences. At an equal "delivery ratio" to the
previous examples, the combustion space will hold 15 kg of air. This enters in a stratified
form with one stream rich at an air-fuel ratio of 10 and the second containing no fuel at all.
Upon entering the combustion space, not all of the entering air in the second stream mixes
with the rich air-fuel stream, but a sufficient amount does to create a "burn zone" with a
stoichiometric mixture at an air-fuel ratio of 15. This leaves 3.75 kg of air unburned which
exits with the exhaust gas. The implications of this are:
(i) The overall or supplied air-fuel ratio is 20, which gives no indication of the air-fuel
ratio during the actual combustion process and is no longer an experimental measurement
which can be used to help optimize the combustion process. For example,
many current production (four-stroke) automobile engines have "engine management
systems" which rely on the measurement of exhaust oxygen concentration as
a means of electronically controlling the overall air-fuel ratio to precisely the stoichiometric
value,
(ii) The combustion process would release 75% of the heat available in the homogeneous
combustion example, and it could be expected that the bmep and power
output would be similarly reduced. In the technical phrase used to describe this
behavior, the "air-utilization" characteristics of stratified combustion are not as
efficient as homogeneous combustion. The diesel engine is a classic example of
this phenomenon, where the overall air-fuel ratio for maximum thermal efficiency
is usually some 30% higher than the stoichiometric value,
(iii) The exhaust gas will contain a significant proportion of oxygen. Depending on the
exhaust after-treatment methodology, this may or may not be welcome,
(iv) The brake specific fuel consumption will be increased, i.e., the thermal efficiency
will be reduced, all other parameters being equal. The imep attainable is lower with
the lesser fuel mass burned and, as the parasitic losses of friction and pumping are
unaffected, the bsfc and the mechanical efficiency deteriorate.
(v) An undesirable combustion effect can appear at the interface between the burned
and unburned zones. Tiny quantities of aldehydes and ketones are produced as the
flame dies at the lean interface or in the end zones, and although they would barely
register as pollutants on any instrumentation, the hypersensitive human nose records
them as unpleasant odors [4.4]. Diesel engine combustion suffers from this complaint.
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