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UNIT- 2 :
MECHANISM OF POLLUTANT FORMATION IN ENGINES
INTRODUCTION
NITROGEN OXIDES..
Nitric oxide is the major component of NOx (NO+NO2) emissions
from the I C
engines. NO2 accounts 1 2 % only of the total NOX emissions in
the S I engines,
while substantial amount of NO2 are emitted by the C I engines.
Its
concentration is high in diesel engines than that of spark
ignited engines. These
oxides of N2 are formed during combustion at high temp. in an
internal
combustion engine.
It has an very harmful affect over our nervous system. And of
course it is toxic.
NO is formed during combn. in the following three ways :
(i) Formation of thermal NO by oxidation of atmospheric nitrogen
at
high temperatures.
(ii) Oxidation of fuel-bound nitrogen at relatively low
temperatures to
form fuel NO.
(iii) NO formed at the flame front by a mechanism other than the
earlier
two mechanisms, prompt NO.
Thermal NO is the dominant source of nitrogen oxides in I C
engines. NO is
formed in the high temperature burned gases behind the flame
front. The rate of
formation of NO increases exponentially with the burned gas
temperature.
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Fuel NO is formed by combustion of the fuels with chemically
bound nitrogen.
The species or the intermediate nitrogen and the reactive like
HCN, NH3, CN, NH
etc are oxidized to NO by the O2 containing species.
Prompt NO is the significant amount of NO, formed rapidly in the
front of the
flame. It is formed in the flame by reaction of intermediate
chemical species of CN
group with O and OH radicals.
CH + N2 HCN + N
CH2 + N2 HCN + NH
C + N2 CN + N
KINETICS OF NO FORMATION.
The following three principal reactions govern the formation of
thermal NO.
O + N2 NO + N
N + O2 NO + O
N + OH NO + H
The rate of formation of NO using the three reactions can be
expressed by the
following equation,
d/dt [NO] = + k1[O][N2] K I[NO][N] + K2[N][O2] K -2[NO][O] +
K3[N][OH]
- K -3[NO][H]
Where, k1 , k2 and k3 are rate constants for the forward
reaction respectively,
the (-)ve sign denotes the rate constants for the reverse
reactions.
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FORMATION OF NO2 ..
Nitrogen dioxide in the exhaust gases of the S I engines is
negligibly small
compared to nitric oxide. In S I engines the amount of NO2 is
generally less than
2%, whereas in diesel engines it is 10 30 % of nitrogen oxides.
NO2
concentration is significantly higher in diesel engines than
spark ignition
engines.
NO2 is rapidly formed in the combustion zone by reaction of NO
with HO2 radial.
Subsequently, in the post flame region NO2 is converted back to
NO and O2 on
reaction with atomic O2. However, if the high temperature burned
gases rapidly
mix with colder air, or air fuel mixture responsible for
conversion of NO2 back to
NO. Such a situation would result in relatively high NO2
concentrations.
NO FORMATION IN S.I. ENGINES :-
For estimation of NO formation in SI engines, thermodynamic
state and
equilibrium composition of the burned gases must be known.
Temperature of
the burned gases can be calculated from the measured cylinder
pressure-time
history or from the calculated pressure-time traces using
empirical burn rates or
more fundamental combn models.
In one combustion model combustion model, after combn the post
flame
burned gas can be assumed to mix instantaneously with the gases
burned
earlier so that all the burned gas at a given instant (a very
short time), is
uniform in composition and temperature. This is commonly to as
the fully mixed
model.
Another combn model at the extreme is an unmixed multi-zone
model where no
mixing occurs between the mixture elements that burn at
different instants in
the cycle. In the unmixed model, each burned gas element
maintains its
separate identity and undergoes isentropic compression and
expansion as the
cylinder pressure changes. The elements that burn early get
compressed to peak
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pressure after combustion and reach higher temperatures than
those burning
later.
Using unmixed model, kinetically formed NO increases initially
with time. In the
early burned elements as the peak temperature is reached due to
compression
to peak cylinder pressure; the rate controlled NO reaches to
near equilibrium
value. Later as the temp. starts falling due to expansion NO
starts decomposing.
The rate of decomposition is controlled by the backward reaction
of NO kinetics
until the NO chemistry freezes due to expansion. Mass fraction
of NO in the
exhaust can be calculated by summing up of the frozen mass NO
fraction over
all the burned gas elements as below:
Where , x is a function of crank angle
NO FORMATION IN C.I. ENGINES:-
In the compression ignition engines, rapid combustion in
pre-mixed phase is
followed by , diffusion combustion process. The rate at which
fuel and air are
mixed controls the diffusion combustion process. The reaction
kinetics leading
to form NOx . As the fuel is injected in the hot compressed air,
the fuel spray
entrains air and non-uniform fuel distribution exists in the
combn chamber. Air-
fuel ratio varies widely from very rich at the core of spray to
very lean at the
spray boundaries. A fuel spray injected radically outward in
swirling air is shown
in figure. Spray core contains mostly liquid fuel and very rich
mixture exists in
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the nearby of spray core. Thus, NO is formed at varying rates
depending upon
the local air-fuel ratio and temperature.
Thus, NO is formed at varying rates depending upon the local a/f
ratio and
temperature. As the combn progresses, the already burned gases
keep on
mixing with colder air and fuel vapor changing its composition
and temperature.
Formation of NO occurs mainly in the burned gases produced
during premixed
combustion phase in the lean flammable region. These gases are
compressed to
a higher pressure and temp. and hence NO formation rates are
high.
Formation of NO2 ___
NO2 in the exhaust gases of the spark-ignition is negligibly
small generally less
than 60 70 ppm compared to several hundred to thousands of ppm
of nitric
oxide. In S.I. engines NO2 is generally less than 2% , while in
diesel engines NO2
can constitutes 10 to 30% of the total emissions of nitrogen
oxides.
Concentrations of NO2 and NO in the S I and diesel engines
exhaust are
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compared in the figs. NO2 concentrations in the diesel engine
are significantly
higher than for the S I engine.
NO2 is rapidly formed in the combustion zone by reaction of NO
with HO2.
However, if the high temp. burned gases rapidly mix with colder
air or air-fuel
mixture caused by high turbulence it may quench reactions
responsible for
conversion of NO2 back to NO. such a situation would result in
relatively high
NO2 concentrations.
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CARBON MONOXIDE ___
CO emissions result during combustion of fuel rich mixtures due
to deficiency of
oxygen. A two-step process may approximate complete combustion
of
hydrocarbon fuel to form finally the CO2.
First step is conversion of hydrocarbons to CO. During this step
several
oxidation reactions occur involving formation of intermediate
species like
smaller HC molecules, aldehydes, ketones etc.
The second step is conversion of CO to CO2 provided sufficient
oxygen is
available. One of the principal reactions for conversion of CO
to CO2 is,
CO + OH CO2 + H
The reaction is quite fast and at high temperatures is
continuously under
equilibrium. The CO increases sharply as the a/f ratio decreases
below
stoichiometric condition. For lean mixtures, CO concentration is
very small.
UNBURNED HYDROCARBON emission (HC)___
Unburned HC emissions arise as part of the fuel inducted in to
the engine
escapes combustion. The unburned HC are also called volatile
organic
compounds.
Most of the fuel HC are present in the exhaust. Thus, fuels
containing higher
fractions of aromatics & olefins produce exhaust
hydrocarbons also rich in
aromatics & olefins, which are more photochemically
reactive. Almost 400
hundred different organic compounds are present in the engine
exhaust.
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Methane is also present in significant amounts in the exhaust
gases particularly
of the SI engines.
FLAME QUENCHING___
For flame to prope, the energy during chemical reactions should
keep the
reaction zone at high enough temperature to sustain rapid
combustion
reactions. As a flame approaches walls in the combustion
chamber, the walls
being at lower temperature than the gases in the flame more and
more heat is
lost from the flame and hot gases to the walls. This results in
drop in the
temperature of reaction zone, which slows down rections thereby
lowering heat
release rate. Finally, this process leads to lowering of gas
temperature below
ignition point and flame quenching takes place.
For quenching of flame propagating normal to a single wall,
energy balance at
the instant of quenching gives,
k Tc /q = SL h
= SL pb Tf
where,
k = thermal conductivity of the unburned mixture,
TC = characteristics temp. difference for heat transfer,
q = quench distance,
= unburned mixture density,
SL = laminar flame speed ,
h = heat release per unit mass of the mixture burned,
pb = average specific heat of burned gases, and
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Tf = temp. rise on combustion in the flame.
Introducing thermal diffusivity, in the above eqn. , we get,
q = / SL . TC / Tf
HC EMISSIONS FROM S.I ENGINES__
In the s.i engines, several processes contribute to unburned
hydrocarbon
emissions. Main sources of hydrocarbon emissions in the
4-stroke, homogenous
charge spark ignition engines are :
(i) Flame quenching on the cylinder walls,
(ii) Crevices flame quenching,
(iii) Absorption and desorption in oil film on cylindre
walls,
(iv) Carbon deposits in the chamber,
(v) Misfired combustion or bulk gas quenching,
(vi) Liquid fuel in the cylinder,
(vii) Exhaust valve leakage,
(viii) Crankcase blow by.
HC EMISSIONS FROM CI ENGINES__
Diesel fuel has a higher boiling range and contains hydrocarbons
of higher
boiling point and molecular weight compared to gasoline. The
five main sources
of HC in diesel engines are :
(i) Over mixing of fuel and air beyond lean flammability
limits,
(ii) Under mixing to fuel-air ratios too rich for complete
combustion,
(iii) Spray over penetration,
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(iv) Bulk quenching of combustion reactions due to mixing with
cooler air
or expansion,
(v) Poorly atomized fuel from the nozzle.
OVERMIXING OF FUEL:
Simple diagram of a fuel spray injected radially outward into
the swirling air
before combustion.
The leading edge in the downwards swirl stream contains larger
numbers of small
droplets, than larger droplets in the core. In this region the
a/f ratio is much
higher than it should be in lean limit to cause HC
emissions.
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UNDERMIXING OF FUEL:
Another cause of high HC emission is undermixing of the fuel
with air. This can
happen for the fuel injected later in the cylinder or because of
overfueling of the
engine. The fuel left in the injector nozzle holes at the end of
the injection gets
fully or partially vaporized during expansion stroke. Therefore,
the later injected
fuels get less time to be mixed with air and could not get burnt
fully.
SOOT OXIDATION:
Oxidation of soot can occur at the stage of early in the combn.
process. Soot can
be oxidized on reaction with O, O2, OH.
CRANKCASE EMISSIONS
Crankcase emissions are made up of water, acids, unburned fuel,
oil fumes and particulates.
These emissions are classified as hydrocarbons (HC) and are
formed by the small amount of
unburned, compressed air/fuel mixture entering the crankcase
from the combustion area
(between the cylinder walls and piston rings) during the
compression and power strokes. The
head of the compression and combustion help to form the
remaining crankcase emissions.
Since the first engines, crankcase emissions were allowed into
the atmosphere through a road
draft tube, mounted on the lower side of the engine block. Fresh
air came in through an open oil
filler cap or breather. The air passed through the crankcase
mixing with blow-by gases. The
motion of the vehicle and the air blowing past the open end of
the road draft tube caused a low
pressure area (vacuum) at the end of the tube. Crankcase
emissions were simply drawn out of the
road draft tube into the air.
To control the crankcase emission, the road draft tube was
deleted. A hose and/or tubing was
routed from the crankcase to the intake manifold so the blow-by
emission could be burned with
the air/fuel mixture. However, it was found that intake manifold
vacuum, used to draw the
crankcase emissions into the manifold, would vary in strength at
the wrong time and not allow
the proper emission flow. A regulating valve was needed to
control the flow of air through the
crankcase.
Testing, showed the removal of the blow-by gases from the
crankcase as quickly as possible, was
most important to the longevity of the engine. Should large
accumulations of blow-by gases
remain and condense, dilution of the engine oil would occur to
form water, soots, resins, acids
and lead salts, resulting in the formation of sludge and
varnishes. This condensation of the blow-
by gases occurs more frequently on vehicles used in numerous
starting and stopping conditions,
excessive idling and when the engine is not allowed to attain
normal operating temperature
through short runs.
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EVAPORATIVE EMISSIONS
Gasoline fuel is a major source of pollution, before and after
it is burned in the automobile
engine. From the time the fuel is refined, stored, pumped and
transported, again stored until it is
pumped into the fuel tank of the vehicle, the gasoline gives off
unburned hydrocarbons (HC) into
the atmosphere. Through the redesign of storage areas and
venting systems, the pollution factor
was diminished, but not eliminated, from the refinery
standpoint. However, the automobile still
remained the primary source of vaporized, unburned hydrocarbon
(HC) emissions.
Fuel pumped from an underground storage tank is cool but when
exposed to a warmer ambient
temperature, will expand. Before controls were mandated, an
owner might fill the fuel tank with
fuel from an underground storage tank and park the vehicle for
some time in warm area, such as
a parking lot. As the fuel would warm, it would expand and
should no provisions or area be
provided for the expansion, the fuel would spill out of the
filler neck and onto the ground,
causing hydrocarbon (HC) pollution and creating a severe fire
hazard. To correct this condition,
the vehicle manufacturers added overflow plumbing and/or
gasoline tanks with built in
expansion areas or domes.
However, this did not control the fuel vapor emission from the
fuel tank. It was determined that
most of the fuel evaporation occurred when the vehicle was
stationary and the engine not
operating. Most vehicles carry 5-25 gallons (19-95 liters) of
gasoline. Should a large
concentration of vehicles be parked in one area, such as a large
parking lot, excessive fuel vapor
emissions would take place, increasing as the temperature
increases.
To prevent the vapor emission from escaping into the atmosphere,
the fuel systems were
designed to trap the vapors while the vehicle is stationary, by
sealing the system from the
atmosphere. A storage system is used to collect and hold the
fuel vapors from the carburetor (if
equipped) and the fuel tank when the engine is not operating.
When the engine is started, the
storage system is then purged of the fuel vapors, which are
drawn into the engine and burned
with the air/fuel mixture.