OBJECTIVES
To make the students to realize the impact of
automobile emissions on the environment and expose student to factors affecting
the formation and control of automobile pollutants.
UNIT – I INTRODUCTION 9
Vehicle
population assessment in metropolitan cities and contribution to pollution,
effects on human health and environment, global warming, types of emission,
transient operational effects on pollution.
UNIT – II
POLLUTANT
FORMATION IN SI ENGINES 9
Pollutant
formation in SI Engines, mechanism of HC and CO formation in four stroke and
two stroke SI engines, NOx formation in SI engines, effects of design and
operating variables on emission formation, control of evaporative emission. Two
stroke engine pollution.
UNIT – III POLLUTANT FORMATION IN CI ENGINES 9
Pollutant
formation in CI engines, smoke and particulate emissions in CI engines, effects
of design and operating variables on CI engine emissions. Nox formation and
control. Noise pollution from automobiles, measurement and standards.
UNIT – IV CONTROL
OF EMISSIONS FROM SI AND CI ENGINES 9
Design
of engine, optimum selection of operating variables for control of emissions,
EGR, Thermal reactors, secondary air injection, catalytic converters,
catalysts, fuel modifications, fuel cells, Two stroke engine pollution control.
UNIT
– V MEASUREMENT
TECHNIQUES EMISSION STANDARDS AND TEST PROCEDURE 9
NDIR, FID, Chemiluminescent analyzers, Gas Chromatograph,
smoke meters, emission standards, driving cycles – USA, Japan, Euro and India.
Test procedures – ECE, FTP Tests. SHED
Test – chassis dynamometers, dilution tunnels.
TOTAL
: 45
TEXT BOOKS
1.
Paul
Degobert – Automobiles and Pollution – SAE International ISBN-1-56091-563-3,
1991.
2.
Ganesan,
V- “Internal Combustion Engines”- Tata McGraw-Hill Co.- 2003.
REFERENCES
1.
SAE
Transactions- “Vehicle Emission”- 1982 (3 volumes).
2.
Obert.E.F.- “Internal Combustion Engines”- 1988
3.
Marco Nute- “ Emissions from two stroke engines,
SAE Publication – 1998
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.
Major sources of Air pollution:
1. Automotive Engines
2. Electrical power generating
stations
3. Industrial and domestic fuel
consumption
4. Refuse burning of industrial
processing, wastes etc.,
Sources of Pollutants from Gasoline Engine:
There are four possible sources of
atmospheric pollution from a petrol engine powered vehicle. They are
1. Fuel Tank
2. Carburettor
3. Crank case
4. Engine
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
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 CO2 which is a green house gas,
now a days CO2 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 compounds currently considered as pollutants are the result of
imperfect or incomplete combustion.
Pollutants Pollutant
Effects
Unburned Hydro
Carbons (UBHC) Photochemical
Smog
Nitric Oxide Toxic
, Photochemical Smog
Carbon monoxide Toxic
Lead compounds Toxic
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.
Effect of Pollutants on Environment:
a. Unburned Hydro Carbons ( UBHC ):
The major sources of UBHC in an
automobile are the engine exhaust, evaporative losses from fuel system, blow by
loss and scavenging in case of 2-stroke petrol engines.
Unburned or partially burned
hydrocarbons in gaseous form combine with oxides of nitrogen in the presence of
sunlight to form photochemical smog.
UBHC + NOx à Photochemical smog
The products of photochemical smog
cause watering and burning of the eyes and affect the respiratory system,
especially when the respiratory system is marginal for other reasons.
Some of the high molecular weight
aromatic hydrocarbons have been shown to be carcinogenic in animals. Some of
the unburned hydrocarbons also serve as particulate matter in atmosphere.
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.
c. Oxides of Nitrogen ( NOx ) :
Oxides of nitrogen ( NO, NO2
, N2O2 etc) are formed at higher combustion temperature
present in engines and the engine exhaust is the major source.
Like carbon monoxide, oxides of
nitrogen also tend to settle on the hemoglobin in blood. Their most undesirable
effect is their tendency to join with moisture in the lungs to form dilute
nitric acid. Because the amounts formed are minute and dilute, their effect is
very small but over a long period of time cam be cumulatively undesirable,
especially when the respiratory problems for other reasons are found.
Another effect is that, the oxides
of nitrogen are also one of the essential component for the formation of
photochemical smog.
d. Sulphur dioxide:
Much of the sulphur dioxide combines
with other materials in the atmosphere and forms sulphates which ultimately
form particulate matter.
e.
Particulates:
Particulate matter comes from
hydrocarbons, lead additives and sulphur dioxide. 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.
UNIT
- II POLLUTANT FORMATION IN SI ENGINES
CHEMISTRY OF SI ENGINE COMBUSTION:
In
a Spark ignition engine a perfectly mixed air fuel mixture enters the engine
during suction stroke. The charge is compressed well and at the end of end of
compression stroke, the charge is ignited by means of spark from spark plug.
The air fuel mixture is delivered to engine by means of carburettor.
The
quantity and quality of charge entering the engine is controlled according to
the engine speed and load conditions.
GASOLINE ENGINE EMISSIONS
The emissions
form gasoline powered automobiles are mainly
1. Unburned Hydro Carbons
2. Carbon monoxide
3. Oxides of nitrogen
4. Oxides of sulphur and
5. Particulates including smoke
Pollutant formation in Gasoline engine:
1.
Hydrocarbons:
Hydrocarbon exhaust emission may
arise from three sources as
a. Wall quenching
b. Incomplete combustion
of charge
c. Exhaust scavenging in
2-stroke engines
In an automotive
type 4-stroke cycle engine, wall quenching is the predominant source of exhaust
hydrocarbon under most operating conditions.
a. Wall quenching:
The
quenching of flame near the combustion chamber walls is known as wall
quenching. This is a combustion phenomenon which arises when the flame tries to
propagate in the vicinity of a wall. Normally the effect of the wall is a
slowing down or stopping of the reaction.
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 emission. 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
Carburetion
and mixture preparation, evaporation and mixing in the intake manifold,
atomization at the intake valve and swirl and turbulence in the combustion
chamber are some factors which influence gaseous mixture ration and degree of
charge homogeneity including residual mixing.
The
engine and intake system temperature resulting from prior operation of the
engine affect charge temperature and can also affect fuel distribution.
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 CO2 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 ( NO2) 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 (N2O3 ), Nitrogen pent oxide (N2O5
) 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 and NO2 is known as NOx. It is expected that higher temperature
and availability of oxygen would promote the formation of oxides of nitrogen.
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 N2
and O2
N2 + O2 à 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.
b.
Zeldovich Chai Reaction mechanism:
O2 à 2 O ------------- ( 1)
O + N2 à
NO + N ------( 2 )
N + O2 à
NO + O ------( 3 )
The chain reactions are initiated by
the equation ( 2 ) by the atomic oxygen, formed in equation ( 1 ) from the
dissociation of oxygen molecules at the high temperatures reached in the
combustion process. Oxygen atoms react with nitrogen molecules and produces NO
and nitrogen atoms. In the equation ( 3 ) the nitrogen atoms react with oxygen
molecule to form nitric oxide and atomic oxygen.
According to this mechanism nitrogen
atoms do not start the chain reaction because their equilibrium concentration
during the combustion process is relatively low compared to that of atomic
oxygen. Experiments have shown that equilibrium concentrations of both oxygen
atoms and nitric oxide molecules increase with temperature and with leaning of
mixtures. It has also been observed that NO formed at the maximum cycle
temperature does not decompose even during the expansion stroke when the gas
temperature decreases.
In general it can be expected that
higher temperature would promote the formation of NO by speeding the formation
reactions. Ample O2 supplies would also increase the formation of
NO. The NO levels would be low in fuel rich operations, i.e. A/F 15, since
there is little O2 left to react with N2 after the
hydrocarbons had reacted.
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 exhibit lower
temperatures and lower NO concentration.
4. Particulate matter and Partial Oxidation
Products:
Organic and inorganic compounds of
higher molecular weights and lead compounds resulting from the use of TEL are
exhausted in the form of very small size particles of the order of 0.02 to 0.06
microns. About 75% of the lead burned in the engine is exhausted into the
atmosphere in this form and rest is deposited on engine parts.
Some traces of products of partial
oxidation are also present in the exhaust gas of which formaldehyde and
acetaldehyde are important. Other constituents are phenolic acids, ketones,
ethers etc., These are essentially products of incomplete combustion of the
fuel.
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.
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 independent 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 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.
EFFECT OF DESIGN AND OPERATING VARIABLES ON
GASOLINE ENGINE EXHAUST EMISSIONS
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:
·
Air fuel ratio
·
Load or power level
·
Speed
·
Spark timing
·
Exhaust back pressure
·
Valve overlap
·
Intake manifold pressure
·
Combustion chamber deposit build up
·
Surface temperature
·
Surface to volume ratio
·
Combustion chamber design
·
Displacement per cylinder
·
Compression ratio
·
Stroke to bore ratio
In
the following discussions, the hydrocarbons and CO emissions are treated
together; because once they are formed they both can be reduced by chemical
oxidation process in either the cylinder or the exhaust system. On the other
hand nitric oxide, once formed must be reduced by a chemical reduction process.
In
the first case for HC and CO reduction, excess O2 is required where
as in the second case for NO reduction a deficiency of O2 is
desirable.
EFFECTS ON UNBURNED HYDROCARBONS AND CARBON
MONOXIDE
1. AIR – FUEL RATIO:
a.
Hydrocarbon emission:
Hydrocarbon
emissions are high at rich air fuel ratios and decrease as the mixture is
leaned up to about 17:1. When operation leaner than 17 or 18:1 is attempted,
emissions increases because of incomplete flame propagation and the engine
begin to misfire.
The basic factor contributing to the
shape of the curve for HC emissions are the effect of mixture ratio on quench
layer thickness and on fuel concentration within that quench layer, and the
effect of mixture ratio on the availability of excess oxygen in the exhaust to
complete the combustion and on the exhaust system temperature. When the
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.
Factors tending to reduce HC
concentration include decreased quench thickness and increased exhaust
temperature. Quench layer thickness decreases inversely as pressure increases
and the mean cylinder pressure increases linearly with increase in load.
Increased temperature with increasing load tends to increase exhaust
after-reaction.
However, an almost linear increase
in HC mass emissions is observed as load is increased. A light car with low
power is better than a large car on mass emission basis.
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.
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.
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.
5. EXHAUST BACK PRESSURE:
a.
HC emission:
Increasing
exhaust back pressure increases the amount of residual exhaust gas left in the
cylinder at the end of the exhaust. If this increase in dilution does not
affect the combustion process adversely, the HC emissions would be marginally
reduced. The reduction arises from leaving the tail end of the exhaust, which
is rich in HC, in the cylinder. This tail will be subsequently burned in the
next cycle. If the back pressure is increased more and more, HC emission would
rise sharply because of the effect of excessive dilution on combustion.
On the other hand, increased
dilution at idle increases HC emission concentration. At idle, dilution is
already quirt high and combustion is marginal and the engine cannot tolerate
much more exhaust dilution.
6. VALVE OVERLAP:
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.
7. INTAKE MANIFOLD PRESSURE:
a.
HC emission:
The
intake manifold pressure variation reflects the variation in power output of an
engine. Between 22cm and 60cm of Hg manifold pressure, the A/F ratio is lean
which minimizes HC and CO emissions. Above 60cm of Hg, the engine power
increases and the carburettor switch to rich mode. The rich mixture increases
HC and CO emissions. This holds good only in case of carbureted engine.
8. COMBUSTION CHAMBER DEPOSITS:
a.
HC emission:
It
is well known that in a normal engine the major source of combustion chamber
deposit is TEL, a fuel additive used to suppress combustion knock. The deposits
act to increase the surface area of the chamber because of their irregular
porous nature.
As
a result, the mass of quenched HC increases. Deposits act as a sponge to trap
raw fuel which remains unburned and adds to exhaust. All these tend to increase
the HC emission.
Tests have indicated that removal of
deposits, depending on the extent of deposit build up, would reduce about 15%
in HC emissions. Addition of fuel additives to reduce deposit build up may be
helpful. Ethylene dibromide is commonly added to motor fuel to reduce lead
deposits from TEL. Any modification to both fuels and lubricants can indirectly
reduce HC emissions through deposit modification.
b.
CO emission:
There is no effect of deposit build
up on CO emission.
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.
10. SURFACE TO VOLUME RATIO:
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.
11. COMBUSTION CHAMBER DESIGN:
One
of the most important factors that the emission engineer has under his control
is the combustion chamber design. For a given clearance volume, reducing the
surface area is an important way of reducing HC emission. Designing a
combustion chamber to create better turbulence will reduce both HC and CO
emission.
12. STROKE / BORE RATIO:
Another
design factor is stroke to bore ratio. Engines with small bore and long stroke
have lower S / V ratio. Engines with low surface to volume ratio provide a good
emission reduction compared to the engine with higher surface to volume ratio.
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.
13. DISPLACEMENT PER CYLINDER:
For a given displacement, engines
with larger cylinders have smaller surface to volume ratio. This result
suggests that for an engine of given displacement, hydrocarbon emissions can be
reduced by decreasing the number of cylinders and increasing the displacement
per cylinder.
14. COMPRESSION RATIO:
A
decrease in compression ratio decreases surface to volume ratio. Decrease in
compression ratio increases the clearance volume greatly with little increase
in surface area. Due to this decrease in surface to volume ratio the HC
emission is reduced.
A decrease in compression ratio
decreases the HC emission on a second way also. With reduced compression ratio,
thermal efficiency is lowered and as a result exhaust gas temperature is
increased. This improves exhaust system after-recirculation and lowers the HC
emission even more.
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.
EFFECT OF NITRIC OXIDE:
The concentration of NO in the
exhaust gases depends upon the difference between the rate of its formation at
the highest temperature in the cycle and the rate of its decomposition as the
temperature decreases during the expansion stroke. A study of the decomposition
rate of NO indicates that the amount decomposed is negligible because of the
short time available during the expansion stroke.
1. EQUIVALENCE RATIO:
The
equivalence ratio affects both the gas temperature and the available oxygen
during combustion. Theoretically an increase in the equivalence ratio form 1.0
to 1.1 results in an increase of maximum cycle temperature by about 55C while
oxygen concentration is reduced by 50%. At equivalence ratio of 1.1, NO in the
exhaust is very low. Maximum NO concentration occurs at an equivalence ratio of
0.8. The maximum cycle temperature with this lean mixture is lower than with a
rich mixture but available oxygen concentration is much higher.
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.
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.
5. COOLANT TEMPERATURE AND DESPOSIT:
An increase in the coolant
temperature results in a reduction of heat losses to the cylinder walls and an
increase in the maximum gas temperature. This results in an increase in NO
concentration.
An increase in deposit thickness
causes an increase in compression ratio, reduction in heat losses to the
coolant and an increase in NO concentration.
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%.
7. EXHAUST GAS RECIRCULATION:
Recycling of a portion of exhaust
gas to inlet charge increases dilution. This reduces peak combustion
temperature, since the inert exhaust gas re circulated will act as a heat sink.
This also reduces the oxygen availability.
About
15% recycle will reduce NOx emission by about 80%. The maximum percentage which
can be re circulated is limited by rough engine operation and loss of power.
8. SURFACE TO VOLUME RATIO:
Engine changes which decrease
surface to volume ratio reduce heat loss to the coolant. As a result NO
concentration may increase.
EFFECT OF DESIGN AND OPERATING VARIABLES ON
EXHAUST EMISSIONS
|
SL.NO.
|
VARIABLE INCREASED
|
HC
|
CO
|
NO
|
|
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 pressure
|
-
|
-
|
Increase
|
|
7
|
Combustion
chamber deposit
|
Increases
|
-
|
Increases
|
|
8
|
S/V ratio
|
Increase
|
-
|
-
|
|
9
|
Combustion
chamber area
|
Increase
|
-
|
-
|
|
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
|
UNIT-
III POLLUTANT FORMATION IN CI
ENGINES
DIESEL ENGIEN
EXHAUST EMISSIONS:
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.
FORMATION OF
POLLUTANTS IN DIESEL ENGINES:
Unlike a gasoline engine, where fuel
and air are premixed into a homogenous form before entering the cylinder, in
the diesel engine fuel is injected into the compressed air charge inside the
cylinder. As the mixing of air and fuel has to take place entirely in the
combustion chamber, complete mixing is virtually impossible and infinite
variations in air-fuel mixture ratio takes place within the same cylinder. Also
as the load requirement is met through variation in the quantity of fuel
injected, the overall air fuel ratio varies within wide limits, about 20:1 to
60:1.
A normally rated and well maintained
engine emits negligible amount of CO and unburnt hydrocarbons, through
considerable amount of oxides of nitrogen and smoke are emitted.
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.
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 thickness 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 zone.
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.
Formation of
oxides of nitrogen:
Since nitrogen is a high temperature
species its formation is influenced by combustion temperature and time
available for combustion. Hence NO tends to increase with advanced injection
timing. Also NO produced increase with fuel supply. Notable exception is
prechamber. In direct injection engines
NO reaches maximum value at stoichiometric air fuel ratio, as lean and rich mixtures
tend to reduce combustion temperatures. Increase in compression ratio leads to
increase in combustion temperature and hence higher NO formation.
Valve overlap has significant effect
on NO formation. Higher valve overlap dilutes the incoming air more and more
leading to increasing in fuel/air ratio. This in turn reduces combustion
temperatures and hence lowers NO formation.
Earlier inlet valve opening before
TDC leads to increased dilution of incoming air and hence lower NO.
Extended inlet valve opening up to
20 has no effect on NO formation as it does not vary manifold pressure.
Extended exhaust valve opening
before bottom dead centre results in marginal increase in NO due to better
scavenging, conversely later exhaust valve opening leads to delayed scavenging
and higher dilution.
Exhaust valve closing determine
effect of scavenging and pronounced effect on dilution and hence Nitrogen
formation.
DIESEL ENGINE
SMOKE EMISSION:
Engine exhaust smoke is a visible
indicator of the combustion process in the engine. Smoke is due to incomplete
combustion. Smoke in diesel engine can be divided into three categories: blue,
white and black.
Blue smoke:
It results from the burning of
engine lubricating oil that reaches combustion chamber due to worn piston rings,
cylinder liners and valve guides.
White or cold
smoke:
It is made up of droplets of unburnt
or partially burnt fuel droplets and is usually associated with the engine
running at less than normal operating temperature after starting, long period
of idling, operating under very light load, operating with leaking injectors
and water leakage in combustion chamber. This smoke normally fades away as
engine is warmed up and brought to normal stage.
Black or hot
smoke:
It consists of unburnt carbon
particles ( 0.5 – 1 microns in diameter) and other solid products of
combustion. This smoke appears after engine is warmed up and is accelerating or
pulling under load.
Formation of
smoke in Diesel engines:
The main cause of smoke formation is
known to be inadequate mixing of fuel and air. Smoke is formed when the local
temperature is high enough to decompose fuel in a region where there is
insufficient oxygen to burn the carbon that is formed. The formation of
over-rich fuel air mixtures either generally or in localized regions will
result in smoke. Large amounts of carbons will be formed during the early stage
of combustion. This carbon appears as smoke if there is insufficient air, if
there is insufficient mixing or if local temperatures fall below the carbon reaction
temperatures (approximately 1000C) before the mixing occurs.
Acceptable performance of diesel
engine is critically influenced by exhaust some emissions. Failure of engine to
meet smoke legislation requirement prevents sale and particularly for military
use, possible visibility by smoke is useful to enemy force. Diesel emissions
gives information on effectiveness of combustion, general performance and
condition of engine.
FACTORS
AFFECTING SMOKE FORMATION:
The smoke intensity in the diesel
exhaust is generally affected by many parameters. By controlling them, smoke
intensity may be reduced.
1. Injection
timing:
Advancing the injection timing in
diesel engines with all other parameters kept constant results in longer delay
periods, more fuel injected before ignition, higher temperatures in the cycle
and earlier ending of the combustion process.
The
residence time is therefore increased. All these factors have been fond to
reduce the smoke intensity in the exhaust. However earlier injection results in
more combustion noise, higher mechanical and thermal stresses, and high NO
concentration.
In a recent study, khan reported
that a very late injection reduces the smoke. The timing after which this
reduction occurs is that at which the minimum ignition delay occurs. He
suggested that one of the factor that contributes to the reduction in smoke at
the 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. 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.
7. Engine
type and speed:
Naturally aspirated engines have
higher smoke levels at higher loads than turbo charged engines, because the
later have sufficient oxygen even at full loads. The smoke is worse at low as
well as at high speeds. This follows the volumetric efficiency curve of the
engine in some measure as it drops at the extremes of speed.
8. Fuel air
ratio:
The smoke increases with richening
the mixture. The increase in smoke occurs even with as much as 25% excess air
in cylinder, cleanly indicating that the diesel engine has a mixing problem.
CONTROL OF
DIESEL ENGINE SMOKE:
Smoke can be
reduced by some of the following methods:
1. Derating:
Derating is nothing but making the
engine to run at lower loads. At lower loads more excess air is present in the
combustion chamber and hence the smoke developed is less as already discussed.
However this means a loss of output.
2. Proper
maintenance of the engine:
Maintaining the engine properly,
especially the injection system, will not only result in reducing smoke but
also keep the performance of the engine at its best.
3. Proper
choice of combustion chamber design and operating conditions:
A proper choice of combustion
chamber design results in better mixing of fuel and air in the chamber and
hence reduces the smoke level to a considerable level.
4. Use of
smoke suppression additives:
Some barium compounds if used in
fuel reduce the temperature of combustion, thus avoiding the soot formation.
Even if formed they break it into fine particles, thus appreciably reducing
smoke. However, the use of barium salts increases the deposit formation tendencies
of engine and reduces the fuel filter life.
5. Adopting
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.
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 amount of aldehydes is small being less than 30ppm, the concentration as
low as 1ppm are irritating the human eyes and nose.
Mechanism of
odour production:
Some experimental results indicate
that the products of partial oxidation are the main cause of odor in diesel exhaust.
This partial oxidation may be because of either very lean mixture or due to
quenching effect.
In diesel combustion there are most
probably regions in which the fuel/oxygen/inert mixture is outside the
flammability limits.
The
fuel in these regions which are too lean to burn might only partially oxidize
resulting in odors. This is most likely to occur during idling and or part load
operation of the engine. Also the fact that chemical reactions take place
during the second stage of diesel combustion suggest that if the reactions are
quenched during this period, partial oxidation products will result in odour in
the diesel exhaust.
Barns concluded in his research that
diesel odor resulted from partial oxidation reaction in the fuel lean regions
which are almost inevitably formed in heterogeneous combustion. Graph shows the
relative odor producing capabilities of different air fuel regions. The data
shown in graph were obtained using CFR engine and varying air-fuel ratios and
the inlet air temperatures.
Odor relevant
compounds:
Until recently very little was known
about the compound or compounds that contribute to the odorous qualities of
diesel exhaust. Rounds and Pearsall correlate odor with the aldehydes in diesel
exhaust gas. However Vogh says that aldehydes are not significant contributors
to the overall odor problem. Vogh also says that neither SO2 nor particulate
contribute significantly to diesel odor emissions.
Research work at the Illinois
Institute of Technology, Research Institute ( IITRI ) has contributed
significantly to the development of a better understanding of the chemical
nature of the odor contributors in diesel exhaust. Based on the IITRI work,
various high molecular weight cyclic and aromatic hydrocarbons including
naphthalene, tetra ling and 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 ( medium speed)
|
3.3
|
|
Four stroke
normally aspirated ( high speed)
|
3.5
|
|
Four stroke –
Turbo charged
|
3.4
|
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.
5. Odor
suppressant additives:
It has been claimed from time to
time, by different manufactures of odor suppressant additive compounds that
they reduce the odor. However small and rather insignificant effects upon the
odor has been found in comparison of exhausts from treated and untreated fuels.
No predictable and reliable correspondence between the additives and odor is
found.
Odor
Measurement Techniques:
Effective chemical or physical
methods for the measurement of odor have not been developed, and therefore the
human nose plays a significant role in all odor studies. In practice odor is
measured by a specially selected, specially trained human panel.
When the nose is subjected to an
odor, the physiological response to the odor can be classified by either
intensity or intensity and quality. The Turk kit contains a number of different
standard odors that are classified as a) burnt/smoky b) oily
c) pungent/acid
and d) aldehydic/aromatic. It has been generally accepted as a standard for
rating the intensity and quality of an unknown odorous sample.
Odor-detection methods that have
been developed todate may be placed in two general categories. The first
category includes methods that only rate the over all odor intensity, while the
second group is employed to classify odors by quality and intensity. The
threshold dilution technique and natural dilution technique that are described
below fall into the first category.
The threshold dilution technique
consists of presenting raw diesel exhaust synthetically diluted with variable
quantities of odor free air to a panel of “Sniffers” person who smells. A
series of diluted samples, both above and below the threshold dilution ratio,
are presented to members of the panel and the individual panel members are
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
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.
UNIT - IV CONTROL OF EMISSIONS FROM SI
AND CI ENGINES
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
2. Retarding spark timing
3. Decreasing charge temperature
4. Decreasing engine speed
5. Decreasing inlet charge pressure
6. Exhaust gas recirculation
7. Increasing humidity
8. Water injection
9. Operating the engine with very
lean or very rich air fuel ratio
10. Decreasing the coolant
temperature
11. Decreasing the deposits
12. Increasing S/V ratio
B. By
decreasing oxygen available in the flame front
The amount of
oxygen available in the chamber can be controlled by
1. Rich mixture
2. Stratified charge engine
3. Divided combustion chamber
2.
Hydrocarbon emission can be decreased by
1. Decreasing the compression ratio
2. Retarding the spark
3. Increasing charge temperature
4. Increasing coolant temperature
5. Insulating exhaust manifold
6. Increasing engine speed
7. Lean mixture
8. Adding oxygen in the exhaust
9. Decreasing S/V ratio
10. Increasing turbulence
11. Decreasing the deposits
12. Increasing exhaust manifold
volume
13. Increasing exhaust back pressure
3. CO can be
decreased by
1. Lean air fuel ratio
2. Adding oxygen in the exhaust
3. Increasing coolant temperature.
EXHAUST GAS
RECIRCULATION:
In exhaust gas recirculation a portion
of the exhaust gas is recirculated to the cylinder intake charge. This reduces
the peak combustion temperature, since the inert gas serves as a heat sink.
This also reduces the quantity of oxygen available for combustion.
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 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.
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.
CRANKCASE EMISSION AND CONTROL
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 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
failure.
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 pressure 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.
ROAD DRAFT
TUBE EMISSIONS:
The road draft tube system worked
well to keep the crankcase free of fumes and pressure build up. However it
discharged all the crankcase pollutants into the atmosphere. This discharge
through the road draft tube represented about 20% of the total HC emissions
from an automobile. Therefore controlling blow by was the first step in
eliminating atmospheric pollution from the automobile.
OPEN PCV
SYSTEM:
An early system that partially
controlled crankcase emission was installed on cars built for sale in California
In this system a tube is connected
between a crank case vent and the intake manifold. While the engine is running,
intake manifold vacuum is used to pull vapour from the crankcase through the
tube into the intake manifold. Fresh ventilating air is drawn into the
crankcase through an open oil filler cap. In the intake manifold, the crankcase
vapours are mixed with the incoming air-fuel mixture and sent to the cylinders
for burning.
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.
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
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
Hydrocarbon
evaporative emissions from a vehicle arise from two sources as evaporation of
fuel in the carburettor float bowl ( 5-10 percent ) of fuel in the fuel tank (
about 5 percent ).
CARBURETTOR
EVAPORATIVE LOSSES:
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 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.
EFFECT OF
FUEL TANK FILL ON EVAPORATIVE LOSS:
|
Tank fill
|
Ambient
temperature C
|
Temp. rise
during test in °C
|
Loss during
operations %
|
|
¼
|
19
|
7
|
5.7
|
|
½
|
16
|
4
|
1.2
|
|
¾
|
18
|
2
|
0.1
|
|
Full
|
22
|
3
|
0.0
|
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.
EVAPORATIVE
EMISSION CONTROL DEVICES:
Evaporative emission control devices
are designed to virtually eliminate the hydrocarbon vapours emitted by the carburettor
and fuel tank during both running and hot soak. During running, fuel tank
vapours are inducted and burned in the engine. Carburettor losses are vented to
intake system. Vehicles without evaporative controls are estimated to 10se 30
g/day of HC from fuel tank filling and breathing. Another 40 g/day is lost by
evaporation from the carburettor ( hot soak loss ) when the vehicle is parked
after being operated. ON this basis, evaporative losses are estimated to be 23%
of total HC emissions.
The device as shown in the figure
consists of an absorbent chamber, the pressure balance valve and the purge
control valve. The absorbent chamber which consists of a charcoal bed or foamed
poly-urethane holds the hydrocarbon vapour before they can escape to atmosphere.
The carburettor bowl and the fuel tank are directly connected to the absorbent
chamber.
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.
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
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.
Co = Ci
* exp Kr O2
P2V
K3 T2W
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.
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.
Increasing the exhaust system volume
increases the residence time during 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.
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.
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.
In contrast to thermal reactors
efficient catalytic oxidation catalysts can control CO and HC emissions almost
completely at temperature equivalent to normal exhaust gas temperatures. Thus
the fuel economy loss necessary to increase the exhaust temperature is avoided.
Inside the catalytic converter the
exhaust gases pass over a large surface area coated with a catalyst. A catalyst
is a material that causes a chemical reaction without actually becoming a part
of the reaction process.
Catalytic reaction of NO can be
represented as follows:
NO + CO à CO2 + ½ N2
NO + H2 à H2O + ½ N2
10 NO + 4HC à 2H2O + 4CO2
+ 5 N2
HC / CO
oxidation is represented by
CO + ½ O2 à CO2
4HC + 5 O2 à
2H2O + 4CO2
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
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.
Generally
catalysts are classified as:
1. Supported
catalysts based on
a. Noble metals b. Transition metals
- Unsupported metallic alloys
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 chromate 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.
Converter
Design:
Converter volume is fixed, based on
the space velocity and exhaust flow rate
Space velocity = gas flow rate in
cm^3 /hr / converter volume in cm^3
The reciprocal of this expression is
the residence time. As the exhaust flow rate varies under different modes of
vehicle operation, an average gas flow rate of 0.85m^3/min and a space velocity
of 15000/hr are normally selected for the preliminary design of the converter.
This will give a converter volume of 3,540 cm^3 in each stage.
Draw backs of
the catalytic converter system:
1. Generally catalysts are active
only at relatively high temperature. Emissions during warm-up cannot be
catalysed and this period has particularly heavy emissions.
2. Catalysts operate over a wide but
not unlimited temperature range. A temperature control is required to avoid burnout
temperature at high speeds and loads.
3. Catalysts are poisoned by exhaust
constituents in particular lead compounds. Hence conversion efficiency
decreases with use.
4. The catalytic bed offers
considerable back pressure which increases with use.
5. Catalytic
converters are expensive.
Importance of
unleaded Petrol:
Vehicles
equipped with catalytic converters must use only un-leaded gasoline. If the
gasoline contains lead, the lead will coat the catalyst and the converter will
stop working.
OTHER EMISSIN
CONTROL DEVICES:
1. Water
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.
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.
UNIT – V
MEASUREMENT TECHNIQUES EMISSION STANDARDS
AND TEST PROCEDURE
VEHICLE EMISSION STANDARDS:
Federal exhaust emission test
procedures for light duty vehicles under 6000 lb GVW covering the period 1972
to 1975 assess hydrocarbon, carbon monoxide and nitric 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
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
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:
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
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.
EMISSION MEASURING INSTRUMENTS:
Terms of
expressions:
In emission measurement, volume
concentrations of the several components are characteristically expressed in
the following terms
1. Carbon dioxide and carbon monoxide
are expressed as percent of the sample volume.
2. Nitric oxide and nitrogen dioxide
are expressed as volume parts of NO or NO2 per million parts of the sample (
ppm). The total of NO and NO2 is designated as NOx.
3. Hydrocarbon is expressed as i.
Parts of hydrocarbon per million parts of the sample or ii. Parts of carbon per
million parts of the sample ( ppmc). The latter term is defined as the volume
concentration of hydrocarbon in the sample multiplied by the average number of
carbon atoms per molecule of that hydrocarbon. Thus 1ppm propane ( C3H8) is the
equivalent of 3ppm C hydrocarbon. In the early days of emission measurement
hydrocarbon emissions were measured in terms of the carbon equivalent of hexane
or ppm hexane. Thus in early usage ppm values were often assumed to be ppm
hexane even though not designated as hexane, this usage is ambiguous and should
be avoided.
Flame Ionisation Detector ( FID ):
The unburned hydrocarbons in the
exhaust consist of about 200 different compounds, each with different
composition and different number of carbon and hydrogen atoms. It is impossible
to detect each of these hydrocarbons separately. The over all concentration of
the unburned hydrocarbons may be found by measuring the equivalent
concentration of n-hexane ( C6H14). An accurate method of measuring the
unburned hydrocarbon emissions is to use the Flame Ionisation Detector ( FID ).
The working principle of FID is as
follows: A hydrogen-air flame contains a negligible amount of ions but if few
hydrocarbon molecules are introduced into the flame a larger number of ions are
produced. The ion yield is proportional to the amount of hydrocarbon introduced
into the flame.
The basic elements of a Flame
Ionisation Detector are as shown in the figure, a burner and ion collector
assembly. In practice, a sample of gas is mixed with hydrogen in 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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