Engine Emission and Control
ME 432 ICE
2
Learning Objectives
• Introduction
• Historical Perspective
• International Standards
• Engine Emission
• Emission Control Method
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Introduction
• SI Engine
▫
Carbon Monoxide (CO)
▫
Hydrocarbon (HC)
▫
Oxides of Nitrogen (NOx)
• CI Engine
▫
Carbon Monoxide (CO) (Much lower as compared to SI Engine)
▫
Hydrocarbon (HC)
▫
Oxides of Nitrogen (NOx)
▫
Solid Carbon Particulates
▫
Oxides of Sulphur (SO2 and SO3)
(Much lower as compared to SI Engine)
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Historical Perspective
• During the 1940s air pollution as a problem was first recognized in the Los Angeles
basin.
• Two causes of this were the large population density and the natural weather
conditions. Smoke and other pollutants combined with fog to form smog.
• In 1966 HC and CO emission limits were introduced in California.
• All of North America usually follows California’s lead (all US in 1968).
• By making more fuel efficient engines and with the use of exhaust after treatment,
emissions per vehicle of HC, CO, and NOx were reduced by about 95% during the
1970s and 1980s.
• Automobiles are more fuel efficient now (2x compared to 1970).
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International Standards
• US Standards
http://www.un.org/esa/gite/iandm/faizpaper.pdf
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International Standards
• Euro Standards
http://www.un.org/esa/gite/iandm/faizpaper.pdf
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Engine Emission
• Carbon Monoxide (CO)
• Hydrocarbon (HC)
• Oxides of Nitrogen (NOx)
• Solid Carbon Particulates
• Oxides of Sulphur (SO2 and SO3)
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Hydrocarbons (HC)
There are some unburnt hydrocarbon in the exhaust.
It
is
objectionable
because
of
its
odour,
its
photochemical smog, and its having a carcinogenic
effect.
Photochemical smog -- cause watering and burning of
the eyes, and affect the respiratory system.
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HC emissions from SI Engines
The most widely accepted causes for hydrocarbon emissions in exhaust gases
of SI engines are:
1.
Flame Quenching: Flame Quenching: Flame quenching at combustion
chamber walls, leaving a layer of unburned fuel-air mixture adjacent to
the walls.
2.
Crevices Volumes and Flow in Crevices:
Cervices filled with
unburned mixture during compression (as much as 3.5%) and remains
unburned after flame passages, since the flame cannot propagate into the
crevices. Main crevice regions are the spaces between the piston, the
piston rings and the cylinder walls. The other crevice regions are space
around the plug center electrode, crevices around the intake and exhaust
vales heads. Cervices volume is greatest when engine is cold and upto
80% of all HC emission from this source.
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HC emissions from SI Engines (Contd.)
3.
Valve Overlap: It is must to obtain satisfactory performance
from the engine. During the valve over lap, both exhaust and
intake valves are open, it provide the path where the fresh air-fuel
mixture can flow directly into the exhaust.
4.
Oil and Deposits on Walls: Oil film and deposits on the
cylinder walls absorb fuel during intake and compression, and the
fuel vapour is desorbed into the cylinder during expansion and
exhaust.
5.
Incomplete combustion: Either partial burning or complete
misfire, occurring when the combustion quality is poor, e.g.
during engine transients when air-fuel, exhaust gas recirculation,
and spark timing may not be adequately control.
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HC emissions from CI Engines
The CI engines operate with an overall fuel-lean equivalence ratio,
therefore they emit only about one-fifth of the hydrocarbon emissions of
an SI engine. Following are the major causes for hydrocarbon emissions in
the exhaust of CI engines:
1.
The diesel fuel contains components of higher molecular weights on
average than those in a gasoline fuel, resulting in higher boiling and
condensing temperatures. This causes hydrocarbon particles to
condense on the surface of the solid carbon soot generated during
combustion. Most of this is burned as mixing continues and the
combustion process proceeds but a small amount is exhausted out of
the cylinder.
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HC emissions from CI Engines
2. Incomplete combustion: The air-fuel mixture
in a CI engine is heterogeneous with fuel still being
added during combustion. It causes local spots to
range from rich to lean and many flame essentially
has one flame front. Incomplete combustion may
be caused by undermixing or overmixing. With
undermixing, the fuel rich zones some fuel particle
may not find the enough oxygen.
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HC emissions from CI Engines (Cont.)
3. A small amount of liquid fuel is often trapped on the
tip of the injector nozzle even when injection stops.
This small volume of fuel is called sac volume. This sac
volume of liquid fuel is surround by a fuel-rich
environment. Therefore,
it
causing
presence
of
hydrocarbon emission in the exhaust.
4. CI engines also have hydrocarbon emission for the
some reasons do.
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Carbon Monoxide (CO)
Carbon monoxide is toxic. The hemoglobin in the blood
which carries oxygen to different parts of the body has a
higher affinity for CO then for oxygen.
Generally, CO is generated in an engine when fuel-rich
equivalence ratio as there is not enough oxygen to convert
all carbon to carbon dioxide.
For fuel lean mixtures, CO concentration in the exhaust
are very low.
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Carbon Monoxide (CO) Contd.
Poor mixing and local rich regions will also be source
of CO emission.
A well-designed SI engine operating under ideal
conditions can have an exhaust mole fraction of CO as
low as 0.001.
CI engine that operate overall lean generally have very
low CO emissions.
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Oxides of Nitrogen (NOx)
The oxides of nitrogen tend to settle on the hemoglobin in
the blood. In lungs ---- dilute nitric acid. NOx --one of
primary cause of photochemical smog.
Ozone is harmful – lungs, others biological tissues, crops
and trees. It react with rubber, plastics and other materials
cause damage.
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Oxides of Nitrogen (NOx)
Most of the oxides of nitrogen – nitric oxide (NO) and small amount of NO2 and
others.
Nox is mostly from atmospheric nitrogen. There are a number of possible reactions
that form NO, some of are
N, O, OH are formed form the dissociation of N2, O2 and H2O vapour at high
temperatures that exist in the combustion chamber.
Higher the combustion reaction temperature --- more diatomic nitrogen (N2) will
dissociate to monoatomic nitrogen (N) and more NOx will be formed.
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Oxides of Nitrogen (NOx)
The flame temperature is maximum – Φ =1 but maximum NOx
is formed ---slightly lean (Φ =0.95) as high temperature with
excess oxygen help the formation of NOx.
Most important engine variable for NOx is Φ.
If ignition spark is advanced, cylinder temperature increase --more NOx.
CI Engine – divided combustion chambers and indirect
injection tend to generate higher levels of NOx.
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Particulates
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Effect of Operating Variable on SI
Engine Exhaust
• Equivalence Ratio
• Engine Speed ---(Increase)-- Reduce HC,
Increase NOx and no effect on CO.
• Spark Timing -- (Retard) – Reduce HC, Reduce
NOx and CO.
• Compression Ratio – (Decrease) -- Reduce HC
and NOx and no effect on CO.
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Control of Exhaust Emission
• Catalytic Converters
• Thermal Rectors
• Particulate Traps