Design and Simulation of Two Stroke Engines

Chapter 7 • Reduction of Fuel Consumption and Exhaust Emissions
7.1 Some fundamentals of combustion and emissions
The fundamental material regarding combustion is covered in Chapter 4, but there remains
some discussion which is specific to this chapter and the topics therein.
The first is to reiterate the origins of exhaust emission of unburned hydrocarbons and
nitrogen oxides from the combustion process, first explained in Chapter 4. Recall the simple
chemical relationship posed in Eq. 4.3.3 for the stoichiometric combustion of air and gasoline,
and the discussion wherein it is stressed that the combustion of fuel and air occurs with
vaporized fuel and air, but not liquid fuel and air. The stoichiometric combustion equation is
repeated here and expanded to include the unburned HC and NOx emissions.
CHn + ^m(02 + kN2) =
x}CO + x2C02 + x3H20 + x402
+x5H2 + x6N2 + x7CHb + x8NOx
,y j jx
It is shown in Chapter 4 that dissociation [4.1] will permit the formation of CO emission
simply as a function of the presence of carbon and oxygen at high temperatures. This is also
true of hydrocarbons, shown above as CHb, or of oxides of nitrogen, shown as NOx in the
above equation. Nevertheless, the major contributor to CO and HC emission is from combustion
of mixtures which are richer than stoichiometric, i.e., when there is not enough oxygen
present to fully oxidize all of the fuel.
Hydrocarbons are formed by other mechanisms as well. The flame may quench in the
remote corners and crevices of the combustion chamber, leaving the fuel there partially or
totally unburned. Lubricating oil may be scraped into the combustion zone and this heavier
and more complex hydrocarbon molecule burns slowly and incompletely, usually producing
exhaust particulates, i.e., a visible smoke in the exhaust plume.
A further experimental fact is the association of nitrogen with oxygen to form nitrogen
oxides, NOx, and this undesirable behavior becomes more pronounced as the peak combustion
temperature is increased at higher load levels or is focusea around the stoichiometric airto-fuel
ratio, as shown clearly in Appendices A4.1 and A4.2.
It is quite clear from the foregoing that, should the air-fuel ratio be set correctly for the
combustion process to the stoichiometric value, even an efficient combustion system will still
have unburned hydrocarbons, carbon monoxide, and actually maximize the nitrogen oxides,
in the exhaust gas from the engine. Should the air-fuel ratio be set incorrectly, either rich or
lean of the stoichiometric value, then the exhaust pollutant levels will increase; except NOx
which will decrease! If the air-fuel mixture is very lean so that the flammability limit is
reached and misfire takes place, then the unburned hydrocarbon and the carbon monoxide
levels will be considerably raised. It is also clear that the worst case, in general, is at a rich airfuel
setting, because both the carbon monoxide and the unburned hydrocarbons are inherently
present on theoretical grounds.
It is also known, and the literature is full of technical publications on the subject, that the
recirculation of exhaust gas into the combustion process will lower the peak cycle temperature
and act as a damper on the production of nitrogen oxides. This is a standard technique for
production four-stroke automobile engines to allow them to meet legislative requirements for
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