Automotive Pollution and Control
Automotive Pollution and Control
Automotive Pollution and Control
com
UNIT – I INTRODUCTION
POLLUTION:
The mixing of unwanted and undesirable substances into our surroundings that
cause undesirable effects on both living and non living things is known as pollution.
AIR POLLUTION:
Air pollution is defined as the addition of unwanted and undesirable things to our
atmosphere that have harmful effect upon our planned life.
The amount of pollutants contributed by the above mentioned sources are as follows.
a.. Fuel tank evaporative loss 5 to 10 % of HC
b. Carburettor evaporative loss 5 % of HC
c. Crank case blow by 20 to 35 % of HC
d. Tail Pipe exhaust 50 to 60 % of HC and
almost all Co and NOx
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Emittant as a Pollutant:
An emittant is said to be a pollutant when it has some harmful effect upon our
surroundings.
The primary source of energy for our automotive vehicles is crude oil from
underground which typically contains varying amounts of sulphur. Much of the sulphur is
removed during refining of automotive fuels. Thus the final fuel is hydrocarbon with only
a small amount of sulphur. If we neglect sulphur and consider complete combustion, only
water and carbon dioxide would appear in the exhaust.
Water is not generally considered undesirable and therefore it is not considered as
a pollutant. Likewise carbon dioxide is also not considered as pollutant in earlier days. But
due to increase in global warming due to CO 2 which is a green house gas, now a days CO 2
is also considered as unwanted one.
Then apart from this we get sulphur dioxide a pollutant which is a product of
complete combustion. Apart from this all the compo unds currently considered as
pollutants are the result of imperfect or incomplete combustion.
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Smoke combines with fog and forms a dense invisible layer in the atmosphere
which is known as Smog. The effect of Smog is that it reduces visibility.
b. Carbon monoxide:
Carbon monoxide is formed during combustion in engine only when there is
insufficient supply of air. The main source is the engine exhaust.
The toxicity of carbon monoxide is well known. The hemoglobin the human blood
which carries oxygen to various parts of the body has great affinity towards carbon
monoxide than for oxygen. When a human is exposed to an atmosphere containing carbon
monoxide, the oxygen carrying capacity of the blood is reduced and results in the
formation of carboxy hemoglobin. Due to this the human is subjected to various ill effects
and ultimately leads to death.
The toxic effects of carbon monoxide are dependent both on time and
concentration as shown in the diagram.
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e. Particulates:
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Particulate matter comes from hydrocarbons, lead additives and sulphur dioxide. If
lead is used with the fuel to control combustion almost 70% of the lead is airborne with
the exhaust gasses. In that 30% of the particulates rapidly settle to the ground while
remaining remains in the atmosphere. Lead is well known toxic compound
.Particulates when inhaled or taken along with food leads to respiratory problems and
other infections.
Particulates when settle on the ground they spoil the nature of the object on which
they are settling. Lead, a particulate is a slow poison and ultimately leads to death.
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Because of the cooling, there is a cold zone next to the cooled combustion chamber
walls. This region is called the quench zone. Because of the low temperature, the fuel-air
mixture fails to burn and remains unburned.
Due to this, the exhaust gas shows a marked variation in HC emissio n. The first
gas that exits is from near the valve and is relatively cool. Due to this it is rich in HC. The
next part of gas that comes is from the hot combustion chamber and hence a low HC
concentration. The last part of the gas that exits is scrapped off the cool cylinder wall and
is relatively cool. Therefore it is also rich in HC emission.
b. Incomplete combustion:
Under operating conditions, where mixtures are extremely rich or lean, or exhaust
gas dilution is excessive, incomplete flame propagation occurs during combustion and
results in incomplete combustion of the charge.
Normally, the carburettor supplies air fuel mixture in the combustible range. Thus
incomplete combustion usually results from high exhaust gas dilution arising from high
vacuum operation such as idle or deceleration.
However during transient operation, especially during warm up and deceleration it
is possible that some times too rich or too lean mixture enters the combustion chamber
resulting in very high HC emission.
Factors which promote incomplete flame propagation and misfire include:
a. Poor condition of the ignition system, including spark plug
b. Low charge temperature
c. Poor charge homogeneity
d. Too rich or lean mixture in the cylinder
e. Large exhaust residual quantity
f. Poor distribution of residuals with cylinder
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Valve overlap, engine speed, spark timing, compression ratio, intake and exhaust
system back pressure affect the amount and composition of exhaust residual. Fuel
volatility of the fuel is also one of the main reasons.
c. Scavenging:
In 2-stroke engine a third source of HC emission results from scavenging of the
cylinder with fuel air mixture. Due to scavenging part of the air fuel mixture blows
through the cylinder directly into exhaust port and escapes combustion process
completely. HC emission from a 2-Stroke petrol engine is comparatively higher than 4-
Stroke petrol engine.
2. Carbon monoxide:
Carbon monoxide remains in the exhaust if the oxidation of CO to CO 2 is not
complete. This is because carbon monoxide is an intermediate product in the combustion
process. Generally this is due to lack of sufficient oxygen. The emission levels of CO from
gasoline engine are highly dependent on A/F ratio.
The amount of CO released reduces as the mixture is made leaner. The reason that
the CO concentration does not drop to zero when the mixture is chemically correct and
leaner arises from a combination of cycle to cycle and cylinder to cylinder mal distribution
and slow CO reaction kinetics. Better carburetion and fuel distribution are key to low CO
emission in addition to operating the engine at increased air- fuel ratio.
3. Oxides of Nitrogen:
Nitric oxide is formed within the combustion chamber at the peak combustion
temperature and persists during expansion and exhaust in non-equilibrium amount. Upon
exposure to additional oxygen in the atmosphere, nitrogen dioxide ( NO 2 ) and other oxides
may be formed.
It should be noted that although many oxides of nitrogen may be also formed in
low concentrations like, Nitrogen trioxide (N 2 O3 ), Nitrogen pent oxide (N 2 O 5 ) etc., they
are unstable compounds and may decompose spontaneously at ambient condition to
nitrogen dioxide.
A study of the equilibrium formation of the different nitrogen oxides showed that
No is the only compound having appreciable importance with respect to engine
combustion. In engine terminology an unknown mixture or nitrogen oxides usually NO
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Mechanism of NO formation:
The nitric oxide formation during the combustion process is the result of group of
elementary reaction involving the nitrogen and oxygen molecules. Different mechanism
proposed are discussed below.
a. Simple reaction between N 2 and O2
N2 + O2 2 NO
This mechanism proposed by Eyzat and Guibet predicts NO concentrations much
lower that those measured in I.C engines. According to this mechanism, the formation
process is too slow for NO to reach equilibrium at peak temperatures and pressures in the
cylinders.
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The maximum NO levels are formed with AFR about 10 percent above stoichiometric.
More air than this reduces the peak temperature, since excess air must be heated from
energy released during combustion and the NO concentration fall off even with additional
oxygen.
Measurements taken on NO concentrations at the exhaust valve indicate that the
concentration rises to a peak and then fall as the combustion gases exhaust from the
cylinder. This is consistent with the idea that NO is formed in the bulk gases. The first gas
exhausted is that near the exhaust valve followed by the bulk gases. The last gases out
should be those from near the cylinder wall and should e xhibit lower temperatures and
lower NO concentration.
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Flame Quenching:
The phenomenon of flame quenching at the engine walls and the resulting
unburned layer of combustible mixture play a significant role in the overall problem of air
pollution.
It has long been understood that a flame will not propagate through a narrow
passage. It has been found that the walls comprising the narrow passage quench the flame
by acting as a sink for energy. The minimum distance between two plates through
which a flame will propagate is defined as the quenching distance . The quenching
distance is found to be a function of pressure, temperature and reactant composition.
When a flame is quenched by a single wall as would be the case in the combustion
chamber of a S.I engine, the distance of the closest approach of the flame to the wall is
smaller than the quenching distance. This distance is called the dead space. In general, the
dead space has been assumed to range from 0.33 to1.0 of the quenching distance.
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Friedman and Johnson, Green and Agnes, Gottenbery and others have made
significant work on this area. The following points are drawn from their experiments.
1. Essentially the expression for quenching distance is of the form
1
qd = -------
Pα Tβ
Where the values of α and β depends on the stoichiometry of the combustible mixture.
2. Lean mixtures have significantly large quenching distance than stoichiometry or
rich mixture at any given pressure.
3. There exist a direct linear relationship between the total exhausted hydrocarbon
and surface to volume ratio, a direct linear relationship between the representatives
measured quench distance and the quantity of unburned hydrocarbons in the combustion
products.
4. The quenching distance of copper, mica, glass and platinum surfaces were the
same and hence they concluded that the quenching effect was indepe ndent of the surface
material.
5. As the temperature of the wall increases, the flame can propagate closer to it. If
high temperature materials could be used to make the cylinder walls in an engine capable
of withstanding 800 °C to 1200 °C temperature, the quench layer thickness can be reduced
to bring down the concentration of hydrocarbons.
Danial proposed that the unburned hydrocarbons that are exhausted during the
cruise and acceleration modes are due to the quenching of flames by the walls of the
combustion chamber piston.
He measured the thickness of the dark zone between the flame and the combustion
chamber wall in a single cylinder engine that was fitted with a single quartz head. The
dark zone or dead space was measured by taking stroboscopic picture of successive cycle
through the quartz cylinder head, and he showed that the quantity of fuel trapped in the
dead space was sufficient to account for the unburned hydrocarbons emitted from the
engine. He also reported that the thickness of the dark zone was a function of temperature
and pressure as referred by Friedman and Johnson.
Tabaczynski proposed that there are four separate quench regions in the cylinder of
a S.I engine. As shown in fig 3.1, these four quench layers may be expected to be
exhausted from the cylinder at different times during the exhaust stroke. Regions 1 and 2
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shown in the figure are the head and side wall quench layers respectively. Region 3
represents the piston face quench layer and region 4 corresponds to the quench volume
between the cylinder wall, piston crown and first compression ring.
It has been proposed that the head quench layer and part of the side wall quench
layer nearest the exhaust valve leave the cylinder when the exhaust valve opens. Due to
the low flow velocities near the piston face, the piston face quench layer will probably not
leave the cylinder at any time during the stroke. During the expansion stroke, the
hydrocarbons from the crevice between the piston crown and the first compression ring are
laid along the cylinder wall.
As the piston begins its upward stroke, it has been shown that a vortex is formed
which scraps up the hydrocarbons along the wall and forces them to be exhausted near the
end of the exhaust stroke.
The exhaust emission of hydrocarbons, carbon monoxide and nitric oxide can be
minimized by the control of several inter related engine design and operating parameter.
Fuel preparation, distribution and composition are also factors. In this section the effects
on emissions of factors which the engineer has under his control when designing and
tailoring his engine for minimum exhaust emissions are discussed. The factors include:
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temperature is over 650 °C and with oxygen available appreciable exhaust after reaction
does occur.
b. CO emission:
CO emissions are high at rich air fuel ratios and decreases as the mixture is leaned.
On the richer side, a change of only 1/3 air fuel ratio leads to a change of 1.0% in exhaust
CO. The reason that the CO concentration does not drop to zero when the mixture is
chemically correct and leaner arises from a combination of cycle to cycle and cylinder to
cylinder mal distribution and slow CO kinetics.
2. POWER OUTPUT:
a. Hydrocarbon emission:
Hydrocarbon concentration does not change as load is increased while speed and
mixture ratio are held constant and spark is adjusted to MBT. This result is to be viewed as
arising from effects of several factors some of which tend to reduce HC while others tend
to increase them, apparently counter balancing one another.
A factor which increases the HC formation as load increases is the reduced time
within the exhaust system. The residence time of the exhaust gas in the very hot section of
the exhaust system is very important for increased exhaust after-reaction.
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b. CO emission:
At a fixed air- fuel ratio there is no effect of power output on CO emission
concentration. However, as in the case of HC emissions, CO emission on mass basis will
increase directly with increasing output, giving advantage for a small light and efficient
car.
3. ENGINE SPEED:
a. Hydrocarbon emission:
HC emission is considerably reduced at higher engine speeds. This is because with
increase in engine speed, the combustion process within the cylinder is increased by
increasing turbulent mixing and eddy diffusion. In addition, increased exhaust port
turbulences at higher speeds promotes exhaust system oxidation reactions through better
mixing.
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b. CO emission:
Speed has no effect on CO concentration. This is because oxidation of CO in the
exhaust is kinetically limited rather than mixing limited at normal exhaust temperatures.
4. SPARK TIMING:
a. HC emission:
HC emission has huge impact on spark timing. As the timing is retarded, the HC
emissions are reduced. This is because, the exhaust gas temperature increases which
promotes CO and HC oxidation. This advantage is gained by compromising the fuel
economy.
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b. CO emission:
Spark timing has very little effect on CO concentration. But at very high retarded
timing, the CO emission increases. This is due to lack of time, to complete oxidation of
CO.
6. VALVE OVERLAP:
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a. HC emission:
Increasing valve overlap has an effect similar to increasing the back pressure. The
charge is further diluted with residual gases. A slight 2 overlap provided minimizes
emission due to re burning of exhaust tail gas which is rich in HC.
Combustion deteriorates with lean mixture as residual is increased. If the mixture
ratio is richened to provide stable idle and off- idle performance, then HC advantage will
be lost and CO will be increased.
In general, minimum HC emissions are obtained with moderate or low back
pressure with minimum overlap.
b. CO emission:
There is no effect of overlap on CO concentration at a constant mixture ratio.
However any increase in richness of the mixture for smooth idle or off idle will increase
the CO directly. This is due to lack of insufficient supply of oxygen for complete
oxidation of CO.
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holds good only in case of carbureted engine. At light loads and low manifold pressure,
additional HC emissions results from wall quenching accompanying rich mixtures
delivered from the carburetor and incomplete combustion at manifold pressures below
15cm of Hg.
9. SURFACE TEMPERATURE:
a. HC emission:
Combustion chamber surface temperature affects the unburned HC emissions by
changing the thickness of combustion chamber quench layer and degree of after burning.
Higher the combustion chamber surface temperature, the lower are the HC emissions.
In addition to changing quench distance and after-reaction, changing engine
temperature increases fuel evaporation and distribution, and result in a faster reaction and
hence reduced HC emission.
b. CO emission:
An increase in surface temperature of chamber increases the rate of oxidation of
CO and hence reduces CO emission. Further exhaust after reaction also increases resulting
in decrease in CO emission.
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a. HC emission:
Because hydrocarbon emissions arise primarily from quenching at the combustion
chamber wall surface, it is desirable to minimize the surface area of the chamber. The ratio
of surface area to volume of the combustion chamber (S / V) is useful for interpreting the
effects of many designs and operating variables on HC concentration. Lowering the S / V
ratio reduces HC emission concentration.
b. CO emission:
CO concentration has no effect on surface to volume ratio.
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Displacement pe r cyl. 41 41
Bore 4 3.62
Stroke 3.25 4
Stroke/bore .813 1.105
s/v 8 6.1
The engine with s/v 6.1 should provde good emission result. Unfortunately this
requirement is opposed to modern design practice of short stroke for reduced friction,
increased power and economy. Long stroke engines tend to be large, heavy and more
expensive and they have poor fuel economy and reduced peak power.
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On the other hand, as engine efficiency is lowered, mass flow is increased for a
given horse power level which increases mass emissions.
On the other hand with large reduction in compression ratio, the temperature in
chamber decreases and it increases both HC and CO emission.
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With very rich mixtures, low peak combustion temperatures and low oxygen
concentration lead to low NO. For mixtures leaner than 15.5:1 there is enough oxygen but
the temperature is very less and hence lower the NO formation. Thus NO concentration is
very low for very lean as well as very rich mixtures.
2. SPARK TIMING:
An advance in spark timing increases the maximum cycle temperature and
therefore results in increased NO concentration.
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3. MANIFOLD PRESSURE:
An increase in manifold vacuum decreases load and temperature. As a result the
ignition delay is increased and the flame speed is reduced. Both these factors increase the
time of combustion. This reduces the maximum cycle temperature and thus reducing NO
concentration in the exhaust.
4. ENGINE SPEED:
An increase in engine speed has little effect on ignition delay. Increase in engine
speed results in an increase in flame speed due to turbulence and reduces heat losses per
cycle which tends to raise compression and combustion temperature and pressure. If spark
timing is held constant, a greater portion of this combustion tends to occur during
expansion where temperature and pressure are relatively low.
This is most pronounced for the slowest burning mixture ratio of 19:1. For richer
mixtures which burn faster, the effect of reduced heat losses at higher speeds
predominates.
These are two opposing influences – an increase in the rate of NO formation due to
reduced heat losses opposed by a reduction in the rate of NO formation due to late
burning. For rich mixtures where combustion and NO formation are rapid, the former
predominates. For lean mixtures where combustion and NO formation are slow, the later
effect predominates.
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6. HUMIDITY:
The reduction in NO formation caused by an increase in mixture humidity is
mainly due to the drop in maximum flame temperature. Test on hydrogen-air, and
ethylene-air mixture indicates that 1% of water vapour reduces the flame temperature by
20C. This reduces the initial rate of NO production by about 25%.
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Engine changes which decrease surface to volume ratio reduce heat loss to the
coolant. As a result NO concentration may increase.
SL.NO. VARIABLE HC CO NO
INCREASED
1 Load _ _ Increase
2 Speed Decrease - Increase/Decrease
3 Spark retard Decrease - Decrease
4 Exhaust back pressure Decrease - Decrease
5 Valve overlap Decrease - Decrease
6 Intake manifold - - Increase
pressure
7 Combustion chamber Increases - Increases
deposit
8 S/V ratio Increase - -
9 Combustion chamber Increase - -
area
10 Stroke to bore ratio Decrease - -
11 Displacement per cyl. Decrease - -
12 Compression ratio Increase - Increase
13 Air Injection Decrease Decrease Increase
14 Fuel injection Decrease Decrease Increase
15 Coolant temperature Decrease Decrease Increase
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The pollutants from diesel engines can be categorized into two types:
1. Visible and 2. Invisible. The first one consists of smoke and metallic particulates.
Smoke being so conspicuous and odorous is objected to public and also reduces visibility
and has smudging character but is not harmful to health.
The second type consists of CO, un burnt hydrocarbons including poly nuclear
aromatics, oxides of N2, SO2 and partially oxidized organics (aldehydes, ketones etc.,)
Among these pollutants smoke, CO, UBHC and oxides of nitrogen are of most
immediate concern.
Carbon monoxide:
It is formed during combustion when there is insufficient oxygen to oxidize the
fuel fully. Compression ignition engines have long been known to produce low levels of
CO because of excess amount of air available for combustion. Theoretically it should not
emit any CO as it always operated with large amount of excess air. Nevertheless CO is
present in small quantities ( 0.1 to 0.75%) in the exhausts. This is possible because of the
fact that fuel injected in later part of the injection does not find enough oxygen due to local
depletion in certain parts of the combustion chamber.
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Unburnt Hydrocarbons:
The concentrations of hydrocarbons in diesel exhaust varies for a few parts per
million to several thousand parts per millions depending on engine speed and load.
Hydrocarbons in engine exhaust are composed of many individual hydrocarbons in the
fuel supplied to the engine as well as number of hydrocarbons partially unburnt produced
during the combustion process. In addition some unburnt hydrocarbons may be from
lubricating oils. Tests on engine with single component fuels shows that these engines
contained hydrocarbons of higher and lower molecular weights, than original fuel as well
as molecules with different structures. Aromatic compounds have been observed in
exhaust of engines operated on pure paraffins. Poly nuclear aromatics found in exhaust are
products of this synthesis.
During the normal operation the relatively cold walls “quench” the fuel air mixture
and inhibit combustion leaving a thick skin of unburnt air fuel mixture over the entire
envelope of the combustion chamber. The amount of unburnt fuel depends on the
thickness of quench zone and the effective combustion chamber area. The thick ness of
quench zone depends on many variables as combustion temperature, pressure, mixture
ratio, turbulence and residual gas dilution. Higher surface to volume ratio of combustion
chamber leads to greater fraction of unburnt hydrocarbon from the quench zo ne.
Partially oxidized hydrocarbons (aldehydes) have been associated with diesel
exhaust. They produce objectionable odor and are high when engine idles and under cold
starting indicating poor combustion.
OXIDES OF NITROGEN:
This is more significant. The formation of Nitric oxide, the major component of
oxides of nitrogen depends on number of operating conditions of diesel engine. The main
factors that control this formation are amounts of oxygen available and the peak
temperature in the zones with sufficient oxygen and residence times at temperatures above
2000K.
Both open and pre-combustion chamber produce small amount of oxides of
nitrogen when air fuel ratio is about 0.01 to maximum near air fuel ratio of about 0.035
ratios. Additional fuel tends to lower air fuel ratio; the charge temperature also reduces
which consequently reduces oxides of nitrogen.
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retarded timing is the reduced rat of formation due to decrease in the temperature of the
diffusion flames as most of these flames occur during the expansion stroke.
2. Rate of Injection:
Higher initial rates of injection have been found to be effective in reducing the
exhaust smoke.
3. Injection nozzle:
The size of the nozzle holes and the ratio of the hole length to its diameter have an
effect on smoke concentration. A larger hole diameter results in less atomization and
increased smoke. An increase in the length/diameter ratio beyond a certain limit also
results in increased smoke.
4. Maintenance:
The engine condition plays a very important role in deciding the smoke levels. The
maintenance affects the injection characteristics and the quantity of lubricating oil which
passes across the piston rings and thus a profound effect on smoke generation tendency of
the engine. Good maintenance is a must for lower smoke levels.
5. Fuel:
Higher cetane number fuels have a tendency to produce more smoke. It is believed
to be due to lower stability of these fuels. For a given cetane number less smoke is
produced with more volatile fuels.
6. Load:
A rich fuel-air mixture results in higher smoke because the amount of oxygen
available is less. Hence any over loading of the engine will result in a very black smoke.
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The smoke level rises from no load to full load. During the first part, the smoke level is
more or less constant as there is always excess air present. However in the higher load
range there is an abrupt rise in smoke level due to less available oxygen.
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Fumigation rate of about 15% gives best smoke improvement. However this
improvement varies greatly with engine speed. At low engine speeds 50 to 80% smoke
reduction is obtained. This decrease as speed increases until a speed at which there is no
effect of fumigation.
DIESEL ODOUR:
Ever since the first diesel engine was developed, the odor from its exhaust has been
recognized as undesirable. Determination of the cause of this odor has been difficult
because of the complexity of the heterogeneous combustion process and the lack of
chemical instruments available. In practice the human nose plays a significant role in odor
measurement.
The members of the aldehydes family are supposed to be responsible for the
pungent odors of diesel exhaust. Though the amo unt of aldehydes is small being less than
30ppm, the concentration as low as 1ppm are irritating the human eyes and nose.
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cyclo paraffins some with olefinic and or paraffinic side changing were reported as major
contributors to the burnt odor note of the exhaust.
Various non aromatic hydrocarbons with more than one double or triple bond were
also reported to contribute to the burnt odor note. Furan aldehydes, aromatic benzene and
paraffinic aldehydes from ethanol to n-octanol were found be important odor contributors
and have individual odors that varied from pleasant to pungent. Some heterocyclic sulfur
compounds, thiophene and benzothiophene derivatives were also reported to be odor
contributors.
FACTORS AFFECTING ODOR PRODUCTION:
1. Fuel air ratio:
The fact that very lean mixtures result in odorous diesel exhaust has already been
discussed.
2. Engine operation mode:
It has been found that the mode of operation of the engine significantly affects the
exhaust odor. Maximum odor occurs while accelerating from idle and minimum odor
results when the engine is running at medium sped and or at part loads.
Effect of engine operating mode on odor production ( 4-stroke normally aspirated
medium speed diesel engine)
Engine operation mode Odor intensity ( Turk number)
Idle 3.6
Acceleration 4.1
Part load 3.0
Full load 3.5
3. Engine type:
The odor intensity does not vary with the engine type as can be seen from the table.
The odor intensity from all the engines is more or less the same.
Engine type Odor intensity ( Turk number)
Two stroke 3.5
Four stroke normally aspirated 3.3
( medium speed)
Four stroke normally aspirated 3.5
( high speed)
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4. Fuel composition:
It is really surprising to find that the composition of the fuel has no effect on
exhaust odor intensity. The changes in fuel composition result in different second stage
combustion time in diesel combustion and it is expected that this will affect the degree of
oxidation if quenching is taking place. However the results contradict this expectation.
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asked to determine whether or not any odor is detectable. The odor intensity is assumed to
be proportional to the dilution ratio at which the odor is just detectable to the panel.
The natural dilution technique was developed in order to determine whether diesel
powered vehicles could meet the motor vehicle exhaust odor on standards set by the state
of California. During the course of these tests, a panel is seated at varying distance from a
vehicle. Both the vehicle and the panel are located inside a large municipal hanger, in
order to minimize the effects due to winds and the panelists are asked to determine
whether or not they can detect any odor from the vehicle. Their responses are utilized to
evaluate threshold response distances.
Variations of the direct method have been used to rate the quality and intensity of
diesel odor and hence thy fall into the second category of odor detection methods. When
applying this method, the exhaust from the diesel engine is usually diluted with odor free
air at the engine exhaust pipe and the resulting mixture of gases which consists of raw
diesel exhaust mixed with odor free air in ratios ranging from 1 to 200 flows dynamically
through a presentation system to the panelists. The panelists who have been previously
trained to evaluate both quality and intensity as determined by the Turk kit are asked to
record their response to test gases as a function of dilution ratio and experimental
parameters.
Design changes:
The effects of engine design and operating variables on exhaust emission were
discussed in a detailed manner already. Based on the discussions made already the engine
design modifications approaches to control the pollutants are discussed below.
1. NOx is decreased by
A. Decreasing the combustion chamber temperature
The combustion chamber temperature can be decreased by
1. Decreasing compression ratio
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3. CO can be decreased by
1. Lean air fuel ratio
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The exhaust gas for recirculation is passed through the control valve for regulation
of the rate and inducted down to the intake p[ort, The recycle rate control valve is
connected to the throttle shaft by means of appropriated linkage and the amount of valve
opening is regulated by throttle position. The link is designed so that recycled exhaust is
normally shut off during idle to prevent rough engine operation. This is also shut off
during full throttle, acceleration to prevent loss of power when maximum performance is
needed.
The NOx concentration will vary with the amount of recycling of gas at various air
fuel ratios. About 15% recycle will reduce NOx emission by about 80%. The maximum
percentage which can be circulated is limited by rough engine operation and loss of power.
The above figure shows a vacuum controlled EGR valve used to control the
recycle rate. A special passage connects the exhaust manifold with the intake manifold.
This passage is opened or closed by a vacuum controlled EGR valve. The upper part of the
valve is sealed. It is connected by a vacuum line to a vacuum port in the carburettor. When
there is no vacuum the port, there is no vacuum applied to the diaphragm in the EGR
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valve. Therefore, the spring holds the valve closed. No exhaust gas recirculates. This is the
situation during engine idling when little NOx is formed.
As the throttle valve opens it passes the vacuum port in the carburettor. This allows
intake manifold vacuum to operate the EGR valve. Then vacuum raises the diaphragm,
which lifts the attached valve off its seat. Now exhaust gas flows into the intake manifold.
There the exhaust gas mixes with the air fuel mixture and enter the engine cylinders.
At wide open throttle, there is little vacuum in the intake manifold. This produces a
denser mixture which burns cooler during the combustion process. Therefore at wide open
throttle there is less need for exhaust gas recirculation. Due to low vacuum, the EGR valve
is nearly closed.
A thermal vacuum switch on many cars prevents exhaust gas recirculation until the
engine temperature reaches about 100 F 0r 37.8C. The thermal vacuum switch is also
called a coolant temperature override switch (CTO switch). It is mounted in a cooling
system water jacket, so it senses coolant temperature. If this temperature is below 100F,
the switch remains closed. This prevents the vacuum from reaching the EGR valve, so the
exhaust gas does not recirculate. Cold engine performance immediately after starting is
improved. After the engine warms up it can tolerate exhaust gas recirculation. Then the
CTO valve opens. Now vacuum can get to the EGR valve, so that exhaust gas can
recirculate.
EGR invariably results in drop in power, increased fuel consumption and rough
combustion. In addition excessive intake system deposit buildup and increased oil
sludging occur.
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Fumigation technique:
This method consists of introducing a small amount of fuel into the intake
manifold. This starts precombustion reactions before and during the compression stroke
resulting in reduced chemical delay, because the intermediate products such as peroxides
and aldehydes react more rapidly with oxygen than original hydrocarbons. The shortening
of delay period curbs thermal cracking which is responsible for soot formation.
Fumigation rate of about 15% gives best smoke improvement. However this
improvement varies greatly with engine speed. At low engine speeds 50 to 80% smoke
reduction is obtained. This decrease as speed increases until a speed at which there is no
effect of fumigation.
During the compression and combustion strokes, highly corrosive blowby gases
are forced past the piston rings into the crankcase. The amount of blowby entering the
crankcase generally increases with engine speed. The amount of blowby also depends on
other conditions including piston, ring and cylinder wear. The actual amount of wear may
be small, perhaps only a few thousands of an inch. But almost any wear is enough to
weaker the sealing effect of the rings and permit blowby to increase. Blowby gases contain
burned and unburned fuel, carbon and water vapour from the combustion chamber. When
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the engine is cold, some of the water vapour of the blowby condenses on the cylinder
walls and crankcase. It forms into droplets and runs down into the oil pan. Gasoline
vapour also condenses on cold engine parts and drips down into the oil pan. This gasoline
dilutes and thins the oil, reducing its lubricating ability.
The churning action of the rotating crank shaft can whip the water and engine oil
into thick, gummy substance called sludge. The acid compounds from the blowby can get
into the sludge and cause corrosion and faster wear of engine parts. Sludge can also clog
oil passages and prevent normal engine lubrication, thereby leading to early engine fa ilure.
Blowby causes pressure in the crankcase. If this pressure is allowed to build up,
engine oil is forced past the oil seals and gaskets and out of the engine. To help to control
the effect of blowby, there must be a way to relieve the crankcase press ure caused by blow
by gases.
CRANKCASE VENTILATION
To avoid the above said problems, the unburned and partly burned gasoline and the
combustion gases and water vapour must be cleared out of the crankcase by providing
crankcase ventilation systems.
In early engines, the crankcase ventilation system was very simple. It provided
crankcase breathing by passing fresh oil through the crankcase. On almost all American
made automobile engines built prior to 1961, the fresh air entered through an air inlet at
the top front of the engine. The fresh air is mixed with the blowby fumes and other
vapours in the crankcase. These vapours were routed out of the crankcase through a large
hollow tube called the road draft tube, which discharged under the car into the atmosphere.
The fresh air inlet was usually the crankcase breather cap. On most engines it also
served as the cap for the crankcase oil filler tube. The cap was open, or vented with holes
on both sides to let fresh air to pass through. The cap was filled with oil soaked steel wool
or similar material to serve as an air filter. The filter prevented dust particle in the air from
getting into the crankcase oil and causing engine wear.
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For the engine to operate properly under all conditions of speed and load, a flow
control valve is required. Without a flow control valve, excessive ventilation air passes
from the crankcase into the intake manifold during idling and low speed. This upsets the
engine air fuel ratio and results in poor idling with frequent stalling.
The PCV valve is installed in a tube from the crankcase vent to the intake
manifold. The PCV valve is a variable orifice valve. A variable orifice is a hole that acts as
a valve by changing the size to vary the flow rate through it. This valve is also called a
metering valve, a modulator valve and a regulator valve.
A typical PCV valve consists of a coil spring, a valve and a two piece outer body
which is usually crimped together. At idle or low speed, high intake manifold vacuum
tends to pull the valve closed or into its minimum flow condition. As the valve tries to
close it compresses the valve spring. The smaller opening now allows a much smaller
volume of blow by gas to pass through. At high engine speeds, the compressed spring
overcomes the pull of the vacuum on the valve. The spring begins to force the valve open
towards the maximum flow condition. As the valve moves open, the flow capacity
increases. This is to handle the greater volume of blowby that results from an increase in
engine load and speed.
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CLOSED PCV:
The crankcase emission control system described above is not completely effective
in controlling crankcase emissions. In open type system, blowby in excess of the PCV
valve flow rate escapes to the atmosphere through the open oil filler cap. To overcome this
problem, a closed positive crankcase ventilation system was developed. All cars
manufactured in California in 1963 and later used a closed type of positive crankcase
ventilation system.
The blowby gases are turned to the engine cylinder through the intake manifold
and under appropriate conditions, through the carburettor air cleaner. The PCV valve
described earlier is generally used as the flow control valve. A closed oil filler cap is used.
Other possible outlets for blow by gases, such as dipstick tube are sealed.
All cars are now being equipped with such closed PCV system wherever there are
air pollution regulations. These systems have completely eliminated the crankcase as a
source of atmospheric contamination and no additional control in future is required in this
direction.
EVAPORATIVE EMISSIONS AND CONTROL
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Carburettor hydrocarbon vapour losses arise from distillation of fuel from the float
bowl. Carburettor fuel temperature often reaches 55°C during warm weather engine
operation and may rise up to 80°C during a hot soak. Hot soak is a condition when a
running car is stopped and its engine turned off. During the soak a significant fraction of
the fuel will boil off and a large portion of the loss finds its way into the atmosphere.
There is a considerable rise in fuel system temperature following shut down after a hard
run.
The basic factors governing the mass of fuel distilled from carburettor during a hot
soak period are
maximum fuel bowl temperature
amount of fuel in the bowl
amount of after-fill and
distillation curve of the fuel
Tests have indicated that a less volatile fuel would reduce the evaporative losses
considerably. A fuel with 20% distilled at 72 °C would give 22% less losses as compared
to a fuel which distilled 25% at 72 °C .
The carburettor bowl volume has a significant effect on evaporative losses.
Increase in the volume of the bowl increases the losses linearly. If an insulated spacer is
placed between the carburettor and the inlet manifold, almost 50% reduction can be
observed.
Filling of the carburettor (after- fill) to the original liquid level is similar to an
increase in the bowl volume and the distillation losses would increase by about 15%.
FUEL TANK EVAPORATIVE LOSSES:
Fuel tank losses occur by displacement of vapour during filling of petrol tank, or
by vaporization of fuel in the tank, forcing the vapour through a breather vent to the
atmosphere. When the temperature is low, the fuel tank breathes in air. When the
temperature goes high it breathes out air, loaded with petrol vapours. Fuel tank losses
occur because the tank temperature is increased during the vehicle operation which causes
an increase in the vapour pressure and thermal expansion of tank vapour.
The mechanism of tank loss is as follows: When a partially filled fuel tank is open
to atmosphere, the partial pressure of vapour phase hydrocarbons and vapour pressure of
the liquid phase are equal and they are in equilibrium. If the temperature of the liquid is
increased, say by engine operation, the vapour pressure of the liquid will increase and it
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will vaporize in an attempt to restore equilibrium. As additional liquid vaporize the total
pressure of the tank increases and since the tank is open to atmosphere, the vapour will
flow out of the tank. This outflow to the vapour will increase if in addition to liquid
temperature rise, the vapour temperature is also increased.
The evaporation from the tank is affected by a large number of variables of which
the ambient and fuel tank temperature, the mode of vehicle operation, the amount of fuel
in the tank and the capacity, design and location of the fuel tank with reference to exhaust
system and the flow pattern of the heated air underneath the vehicle.
Less the tank fill, greater is the evaporative loss. The effect of the tank fill and the
temperature are shown in the table. This reflects the difference in the tank vapour space.
Also when a car is parked in a hot location, the evaporation of gasoline in the tank
accelerates and so the evaporation loss is greater.
The operational modes substantially affect the evaporation loss. When the tank
temperature rises, the loss increases. The fuel composition also affects the tank losses.
About 75% of the HC losses from the tank are C4 and C5 hydrocarbons.
Design factors that affect the evaporative losses include the peak tank temperature,
the area of the liquid vapour surface, and the amount of agitation. It is obvious that nay
design change which reduces the peak tank temperature will reduce the tank loss. Such
modifications include tank insulation, lower surface to volume ratio of tank, better tank
orientation or location for reduced heat pick up from solar radiation or other heat sources
such as the exhaust system.
The surface area for evaporation and tank agitation are factor which influence the
speed with which equilibrium is achieved. Baffles in the tank can reduce losses by
maintaining concentration gradients.
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During hot soaks, vapours from the fuel tank are routed to a storage device.
Carburettor vapours may be vented to the storage system or retained internally in the
carburettor or induction system volume. A schematic diagram of this arrangement is
shown in the figure.
Upon restart, filtered air is drawn through the stored vapours and the mixture is
metered into the intake system and burned in the engine. In this manner the storage device
is purged (removed off the retained vapour). The operations of the purge control valve are
controlled by the exhaust back pressure.
The storage system consists of a canister containing activated charcoal located in
the engine compartment. Activated charcoal has an affinity for HC and on a recycle basis
can store 30-35 grams of fuel per 100 grams of charcoal without breakthrough. Typically
700-800 grams of charcoal are used in a vehicle system.
One problem with any storage system is the possibility of liquid fuel entering the storage
device. Ball check valves or vapour liquid separators assure than only fuel vapours reach
the storage device. In addition, a dead volume in the tank allow for thermal expansions of
a full fuel tank. About 10% of the tank volume is partially walled off from the remainder
of the tank. When the tank is filled, this volume remains nearly empty. After a period of
time, the fuel fills the additional volume thereby leaving room for expansion in the rest of
the tank. Otherwise expansion could force the liquid fuel into the charcoal canister or the
crankcase.
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THERMAL REACTORS:
Thermal reactor is a chamber in the exhaust system designed to provide sufficient
residence time to allow appreciable homogeneous oxidation of HC and CO to occur. In
order to improve CO conversion efficiency, the exhaust temperature is increased by
retarding spark timing. This however results in fuel economy loss.
The air is supplied from an engine driven pump through a tube to a place very near
to the exhaust valve. To achieve a high degree of exhaust system oxidation of HC and CO,
a high exhaust temperature coupled with sufficient oxygen and residence time to complete
the combustion is needed. Oxides of nitrogen are not reduced. In fact, they may be
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increased if sufficiently high exhaust temperature results from the combustion of CO and
HC with the added air or if the injected air enters the cylinder during the overlap period,
thereby leaning the mixture in the cylinder.
Warren has derived the following equation for the concentration of hydrocarbons
leaving the exhaust system.
Co = Ci * exp Kr O2 P2 V
K3 T2 W
Where,
Co = Concentration of HC leaving the thermal reactor
Ci = Concentration of HC leaving cylinders and entering the thermal reactor
Kr = Specific reaction rate ft3 /lbm – mole/sec
K3 = constant
O2 = oxygen concentration in exhaust gases. Volume percent
P = exhaust pressure ( Psi)
V = thermal reactor volume available for reaction Cu.ft
T = Absolute temperature C
W = Mass flow rate of air ( lb/sec)
Note the importance of pressure term. Increasing exhaust system back pressure
promote after reaction. However commercially, the possible back pressure increase is
small.
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The graph shows the effect of temperature on specific reaction rate Kr, calculated
from the above equation by warren from his experimental data. The nearness of his curve
to a straight line suggests the equation is a good approximation for the overall reactions
occurring. Note that a decrease in exhaust temperatures from 1100C to 1000F decrease the
reaction rate by a factor of 10.
The graph shows the effect of temperature and reactor volume on exhaust
hydrocarbon concentration at an oxygen input concentration of 3%. Reactor volume may
be viewed as the volume of the exhaust system which is insulated and ant the high
temperature needed for reaction.
Note that if the exhaust temperature were 1400F, only twice the convention system
volume is required for virtually complete elimination of the hydrocarbons. On the other
hand, if the temperature were only 1200F, eight times the volume would achieve only a
76% reduction. A pair of conventional exhaust manifolds has about 0.09 ft3 of volume.
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Increasing the exhaust system volume increases the residence time d uring which
reactions can occur. This is a benefit, providing the added surface area does not result in
excessive cooling. Thus when large volume exhaust manifolds are to be used, they must
be well insulated.
Brownson and stebar have studied thermal reactor performance for a reactor
coupled to a single cylinder CFR engine. In their work an insulated exhaust mixing tank of
150 cubic inch was used for some tests. They determined that the basic factors governing
the combustion of CO and hydrocarbons in the exhaust system are composition of the
reacting mixture, temperature and pressure of the mixture, and residence time of the
mixture or time available for reaction.
The graph shows the hydrocarbon and CO emissions as a function of air- fuel ratio
and injected air flow rate. The emission concentration results were corrected for the added
air. Injected air flow rate is indicated as a percentage of the engine air volume flow rate.
An insulated 150 cubic inch exhaust mixing tank was used.
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The minimum HC concentrations occurred at rich mixtures. When too much air
was injected, especially at lean mixtures, excessive cooling of the exhaust increased HC
concentrations above those with no air. Thus the normal oxidation process was apparently
inhibited by this cooling. A small increase in CO occurred slightly richer than
stoichiometric. At stoichiometric mixtures and leaner, CO was very low. Best results
occurred for rich mixtures with air injection at 20-30% of inlet air flow. The air- fuel ratio
for best emission reduction was 13.5:1. Normally engine operation at such a rich mixture
would reduce fuel economy by 10%.
At each air- fuel ratio there exists one minimum air injection rate that provides
maximum emission reduction. Minimum air flow is desired in order to reduce pump
power requirement, size and cost. Graph shows the optimum air injection rate for both HC
and CO emissions.
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CATALYTIC CONVERTERS:
Catalytic converters provide another way to treat the exhaust gas. These devices
located in the exhaust system, convert the harmful pollutants into harmless gases.
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The figure shows a single bed catalytic converter. The exhaust gas and air are
passed through a bed of platinum coated pellets or honeycomb core. HC and CO react
with the oxygen in the air. Harmless ware and carbon dioxide are formed. The catalyst
platinum act on the exhaust gas in two ways, converting HC and CO to carbon dioxide and
water. So it is called a two way catalyst.
Figure show a dual bed catalytic converter. The exhaust gas first passes through
the upper bed. The upper bed contains a reducing catalyst ( example rhodium). NOx is
reduced to nitrogen and oxygen in the upper bed. Then secondary air is mixed with the
exhaust gas. The mixture of exhaust gas and secondary air flows to the lower bed. The
lower bed contains an oxidizing catalyst ( example platinum). HC and CO are oxidized to
water vapour and carbon dioxide in the lower bed. Here the catalyst rhodium is a one way
catalyst since it acts o NOx only. Platinum is a two way catalyst since it acts on HC and
CO.
A three way catalyst is a mixture of platinum and rhodium. It acts on all three of
the regulated pollutants ( HC, CO and NOx) but only when the air- fuel ratio is precisely
controlled. If the engine is operated with the ideal or stoichiometric air-fuel ratio of 14.7:1.
The three way catalyst is very effective. It strips oxygen away from the NOx to form
harmless water, carbon dioxide and nitrogen. However the air- fuel ratio must be precisely
controlled, otherwise the three way catalyst does not work.
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Figure shows a three way catalytic converter. The front section( in the direction of
gas flow) handles NOx and partly handles HC and CO. The partly treated exhaust gas is
mixed with secondary air. The mixture of partly treated exhaust gas and secondary air
flows into the rear section of the chamber. The two way catalyst present in the rear section
takes care of HC and CO.
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NO-Reduction Catalysts:
From the literature, it is seen that the following materials have been tried
successfully as reduction catalysts in the vehicle emission control
1. Copper oxide-chromia
2. Copper oxide – Vanadia
3. Iron oxide – Chromia
4. Nickel oxide pelleted on monolithic ceramic and metallic supports
5. Monel metal
6. Rare earth oxides
HC/CO oxidation catalysts:
1. Noble metal catalysts such as activated carbon, palladium or platinum
2. Transition metal oxide catalysts such as copper, cobalt, nickel and iron chro mate
as well as vanadium or manganese promoted versions of these metals.
3. Copper chromite-alumina and platinum oxide –alumina catalysts were
developed with sufficient activity, stability and mechanical strength.
The catalysts chosen for vehicle emission control should satisfy the following:
1. High conversion efficiency under transient conditions
2. Effective for wide range of temperature ( for ambient to 1600 F)
3. Must withstand the poisoning action of additives in the gasoline that are emitted in the
exhaust
4. Must be able to withstand thermal shock
5. Be attrition resistant to highly turbulent flows through the converter
6. Vehicle operation for 50,000 miles
7. Convert into harmless products
8. Cheap and readily available.
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1. Wate r injection:
In this a small amount of water is injected into the combustion chamber. Due to
this the peak combustion temperature is reduced and thus NOx emission is reduced.
Graph shows nitric oxide reduction as a function of water rate. The spark advance
was kept constant and the power loss was balanced by leaning the A/F ratio of the mixture.
The specific fuel consumption as clear from the graph, decreases a few percent at medium
water injection ratio. So for no attempts have been done to use water as a deice for
controlling the NOx, perhaps because of complexity varying the amount of injection rate
in relation to engine requirements.
2. Direct air Injection:
In this compressed air is introduced into the combustion chamber in addition to air
fuel charge from the carburettor. This gives better combustion and hence reduced
hydrocarbon and CO emission. This will also give tremendous power boost with some
saving in fuel. But extra equipment in the form of air compressor and air valves will raise
the cost very much. Also, exhaust gas recirculation will still be needed to curb NOx
emissions.
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3. Ammonia Injection:
In this ammonia is injected into the exhaust gas. Ammonia reacts with NOx in
exhaust and forms nitrogen and water. Thus NOx emission is reduced.
As a fuel, ammonia does not hold much promise, but if used as an exhaust additive
it can give excellent control for NOx emission. Ammonia and nitric oxide interact to form
nitrogen and water. Ford motor company has been doing investigations with injecting
Ammonia-water in the exhaust manifold, downstream from the port.
For an effective utilization of Ammonia injection, the exhaust gas temperature has
to be kept within strict limits and the injecting device has to be put sufficiently down to
bring the gas temperature to 165C. This also demands a very close tolerance in air- fuel
ratio supplied by the carburettor. The present carburettors are incapable of this and it
might be necessary to adopt electronic injection system to keep it.
4. Electronic Injection:
It is possible to develop an electronic injection system with sensors for air
temperature, manifold pressure and speed which will precisely regulate the fuel supply
giving only such air fuel ratio that will give no hydrocarbon or CO emissions.
Since the injection can be affected in individual intake ports, the problem of fuel
distribution among various cylinders will automatically be avoided.
The emissions on deceleration can be completely removed by shutting off the fuel
supply when the throttle is closed. But this system will still not be able to control the HC
emission. Combination of electronic injection and ammonia as an exhaust additive has an
attractive future.
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Federal exhaust emission test procedures for light duty vehicles under 6000 lb
GVW covering the period 1972 to 1975 assess hydrocarbon, carbon monoxide and nitr ic
oxide emissions in terms of mass of emission emitted over a 7.5 mile chassis
dynamometer driving cycle. Results are expressed as grams of pollutant emitted per mile.
There are two procedures in using the same test equipment which assess vehicle
emissions. One, which is termed as CVS-1 (constant volume sampling), employs a single
bag to collect a representative portion of the exhaust for subsequent analysis. This single
bag system applied to testing of 1972, 1973 and 1974 vehicles. Based on this test,
emission standards for vehicles have been set at
Hydrocarbons 3.4 g/mile ( 1972 to 1974 )
Carbon monoxide 3.9 g/mile ( 1972 to 1974 )
Oxide of nitrogen 3.0 g/mile ( 1973 to 1974 )
The second test procedure, termed CVS-3 uses three sampling bags and is designed
to give a reduced and more realistic weighing to cold start portion of the test. This three
bag system applies to testing of 1975 to 1976 vehicles. Exhaust emission standards based
on this test are
Hydrocarbons 0.41 g/mile ( 1975 to 1976 )
Carbon monoxide 3.4 g/mile ( 1975 to 1976)
Oxide of nitrogen 3.0 g/mile ( 1975 )
One of the latest U.S standards ( 1982) for passenger cars and equivalents are
Hydrocarbons 0.41 g/mile
Carbon monoxide 3.4 g/mile
Oxide of nitrogen 1.5 g/mile
These are measured by following a prescribed test procedure.
Driving Cycle:
The driving cycle for both CVS-1 and CVS-3 cycles is identical. It involves
various accelerations, decelerations and cruise modes of operation. The car is started after
soaking for 12 hours in a 60-80 F ambient. A trace of the driving cycle is shown in figure.
Miles per hour versus time in seconds are plotted on the scale. Top speed is 56.7 mph.
Shown for comparison is the FTP or California test cycle. For many advanced fast war m-
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up emission control systems, the end of the cold portion on the CVS test is the second idle
at 125 seconds. This occurs at 0.68 miles. In the CVS tests, emissions are measured during
cranking, start-up and for five seconds after ignition is turned off fo llowing the last
deceleration. Consequently high emissions from excessive cranking are included. Details
of operation for manual transmission vehicles as well as restart procedures and permissible
test tolerance are included in the Federal Registers.
CVS-1 system:
The CVS-1 system, sometimes termed variable dilution sampling, is designed to
measure the true mass of emissions. The system is shown in figure. A large positive
displacement pump draws a constant volume flow of gas through the system. The exhaust
of the vehicle is mixed with filtered room air and the mixture is then drawn through the
pump. Sufficient air is used to dilute the exhaust in order to avoid vapour condensation,
which could dissolve some pollutants and reduce measured values. Excessive dilution on
the other hand, results in very low concentration with attendant measurement problems. A
pump with capacity of 30-350 cfm provides sufficient dilution for most vehicles.
Before the exhaust-air mixture enters the pump, its temperature is controlled to
within +or – 10F by the heat exchanger. Thus constant density is maintained in the
sampling system and pump. A fraction of the diluted exhaust stream is drawn off by a
pump P2 and ejected into an initially evacuated plastic bag. Preferably, the bag should be
opaque and manufactured of Teflon or Teldar. A single bag is used for the entire test
sample in the CVS-1 system.
Because of high dilution, ambient traces of HC, CO or NOx can significantly
increase concentrations in the sample bag. A charcoal filter is employed for leveling
ambient HC measurement. To correct for ambient contamination a bag of dilution air is
taken simultaneously with the filling of the exhaust bag.
HC, CO and NOx measurements are made on a wet basis using FID, NDIR and
chemiluminescent detectors respectively. Instruments must be constructed to accurately
measure the relatively low concentrations of diluted exhaust.
Bags should be analyzed as quickly as possible preferably within ten minutes after
the test because reactions such as those between NO, NO2 and HC can occur within the
bag quite quickly and change the test results.
CVS-3 SYSTEM:
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The CVS-3 system is identical to the CVS-1 system except that three exhaust
sample bags are used. The normal test is run from a cold start just like the CVS-1 test.
After deceleration ends at 505 seconds, the diluted exhaust flow is switched from the
transient bag to the stabilized bag and revolution counter number 1 is switched off and
number 2 is activated. The transient bag is analyzed immediately. The rest of the test is
completed in the normal fashion and the stabilized bag analyzed. However in the CVS-3
test ten minutes after the test ends the cycle is begun and again run until the end of
deceleration at 505 seconds. This second run is termed the hot start run. A fresh bag
collects what is termed the hot transient sample. It is assumed that the second half of the
hot start run is the same as the second half of the cold start run and is not repeated. In all,
three exhaust sample bags are filled. An ambient air sample bag is also filled
simultaneously.
STANDARDS IN INDIA:
The Bureau of Indian Standards ( BIS ) is one of the pioneering organizations to
initiate work on air pollution control in India. At present only the standards for t he
emission of carbon monoxide are being suggested by BIS given in IS:9057-1986. These
are based on the size of the vehicle and to be measured under idling conditions. The CO
emission values are 5.5 percent for 2 or 3 wheeler vehicles with engine displace ment of
75cc or less, 4.5 percent for higher sizes and 3.5 percent for four wheeled vehicles.
IS: 8118-1976 Smoke Emission Levels for Diesel vehicles prescribes the smoke
limit for diesel engine as 75 Hatridge units or 5.2 Bosch units at full load and 60-70
percent rated speed or 65 Hatridge units under free acceleration conditions.
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the burner assembly and the mixture burned in a diffusion flame. Ions that are produced in
the flame move to the negatively polarized collector under the influence of an electrical
potential applied between the collector plates. At the negative collector, the ions receive,
via a current network, electrons that are collected from the flame zone at the positive
collector. Thus a small current proportional to the amount of hydrocarbon entering the
flame flows between the collector plates. This small current is amplified using a high
impedance direct current amplifier, the output of which becomes an indication of
hydrocarbon present.
The detector responds to carbon that is linked with hydrogen as in equation 1 and
the response is largely independent of the molecular configuration, i.e hydrocarbon
species. Thus the detector is essentially a carbon atom counter.
The output of the FID depends on the number of carbon atoms passing through the
flame in a unit time. Doubling the flow velocity would also double the output. Hexane (
C6H14) would give double the output of propane ( C3H8). Therefore FID output is
usually referred to a standard hydrocarbon usually as PPM of normal hexane.
Characteristics of the FID are improved with most burned designs if instead of
using pure hydrogen fuel, the hydrogen is mixed with inert gas to decrease flame
temperature. This mixture of hydrogen and inert gas is referred to as fuel gas or fuel.
The FID responds directly to the amount of hydrocarbon entering the flame.
Therefore close control of sample flow is required. In general, the sample flow rate is
specified at the minimum amount that will give the required sensitivity in any given
instrument. Fuel and air flow rates also influence the response characteristics of the
detector. Response typically first rises and then fall with increased fuel rate, as shown in
the figure. Typical volume rates of instrument gases are sample 3-5 ml/min and fuel gas
mixture 75ml/min and air 200ml/min.
Presence of CO, CO2, NOx, water and nitrogen in the exhaust have no effect on
the FID reading.
FID analyzer is rapid, continuous and accurate method of measuring HC in the
exhaust gas concentrations as low as 1ppb can be measured.
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