Environmental Engineering Reference
In-Depth Information
The combustion process in the SI engine is initiated by a timed electric spark that starts a flame
front propagating through the air-fuel mixture. As it sweeps through the combustible mixture,
heating the reactants to the point where they burn extremely rapidly, it converts fuel to products of
combustion, with each fuel molecule being processed in about 1
s. When the flame front reaches
the walls of the combustion chamber, it is extinguished, but not until all except a tiny proportion
of the combustible mixture next to the wall will have been reacted to form combustion products.
7
However, in order to propagate a reliable, rapidly moving flame, the combustible mixture must
not be too rich or too lean; that is, it must not have too much or too little fuel, compared to the
amount of air, than is needed to completely consume all the fuel and oxygen present, called the
stoichiometric mixture. It is for this reason that the fuel and air are mixed in carefully controlled
proportions prior to or while entering the engine cylinder. To reduce engine power, the amount
of fuel burned per cycle is lowered by reducing the pressure, and hence density, of the incoming
charge; its proportions of fuel and air, as well as its temperature, remaining unchanged. Thus the
favorable high-speed flame propagation rate and rapid combustion are retained in SI engines, even
down to idling conditions.
Fuel combustion in the CI engine proceeds quite differently, without flame propagation, albeit
with comparable speed. Fuel injected into the very hot air within the cylinder is quickly evaporated,
and the fuel vapor then becomes mixed with the surrounding air and burns spontaneously without
the necessity of spark ignition. The surrounding air swirls past the injector nozzle, providing a flow
of oxygen needed to oxidize the evaporating fuel droplets as they emerge from the nozzle. When
less power is needed, less fuel is injected into the fixed amount of air in the cylinder, consuming
less oxygen and reducing the pressure rise in the cylinder. Thus the fuel-air ratio in the CI engine
is variable and lean. At maximum power, some excess air is required to burn all the fuel because
mixing conditions are not ideal.
At the molecular level, the combustion process is much more complex than might be inferred
from the overall stoichiometry of the reaction. For example, the complete oxidation of one octane
molecule (C
8
H
18
) requires 8
ยต
2 oxygen molecules (O
2
), forming 8 CO
2
and 9 H
2
O molecules.
The rearrangement of the carbon, hydrogen, and oxygen atoms among the reacting molecules
occurs one step at a time, requiring numerous individual changes as single H and C atoms are
stripped away from the fuel molecule and oxidized. These changes are aided by very reactive
intermediate species called radicals, such as H, O, OH, C
2
, CH, CH
2
, and so on, that act to
accelerate the molecular rearrangement but that disappear once the reaction is completed. Never-
theless, the combustion process is not perfect, so that small amounts of unreacted or imperfectly
oxidized products may remain; molecules that reached dead end paths and were unable to attain
the complete thermochemical equilibrium of the bulk of the reactants. These molecules are dis-
persed among the principal combustion products and, unless removed, enter the atmosphere as air
pollutants.
Nitric oxide (NO) is an important air pollutant that is a byproduct of the combustion process. It
is formed from nitrogen and oxygen because it is thermochemically favored at the high temperature
of the newly formed combustion products. It is produced rapidly by the two following reaction
+
9
/
7
Under certain conditions it is possible for the combustible mixture to ignite spontaneously and burn
explosively, in an uncontrolled and destructive manner. This is called combustion knock, which is avoided by
controlling the chemical properties of the fuel.
