EMISSION CONTROL SYSTEM
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Unit 11
Title :Emission Control System
General Objective:
To understand the characteristics of emission control system
Specific Objectives:
At the end of this unit you should be able to:
1. define and compare the various types of air pollution
produced by automobile.
2. explain the emission control system.
3. explain the catalytic converter system.
4. explain the PCV system.
5. explain the EGR valve system.
6. explain the Air Injection system.
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Input
This section introduces the subject matter that you are going to learn.
11.0
Introduction
In this unit we are to discuss the various types of pollution such as smog,
hydrocarbon (HC), Carbon Monoxide (CO), Nitrogen Oxide (NO x),and how
pollution is being controlled through emission control system.
11.1
Types of Pollutions
11.1.1 Smog
Smog is a term for the brownish-yellow haze that hangs in the
warm, still air. It is produced from industrial pollutants and automobiles.
Hydrocarbon (HC), Nitrogen Oxide (NOx) and volatile organic compounds
(VOCs) are the pollutants that produce smog.
11.1.2 Hydrocarbon (HC)
It is another type of pollution produced by automobile combination
process. Fossil fuels are made of various hydrogen and carbon
molecules. Unburned hydrocarbons emitted by the automobile are largely
unburned fuel. Any fuel that is partially burned contains hydrocarbons
and this is one of the main ingredients in the production of smog.
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11.1.3 Carbon Monoxide (CO)
Carbon Monoxide is considered a deadly poisonous gas that is
colourless and odourless. It causes headaches and vision difficulties if a
person inhales a small quantities. But in larger quantities, it may cause
sleepiness and death. Carbon Monoxide emissions are increased as the
combustion process becomes less efficient. It forms in the engine exhaust
when there is insufficient oxygen to form the carbon dioxide. As a result,
(refer to picture 11.1) whenever the engine operates, rich air-fuel mixture
increases CO.
11.1.4 Nitrogen Oxide (NOx)
Nitrogen Oxide is form freely under extreme heat condition. As the
combustion process becomes leaner, combustion temperatures typically
increase. Higher temperature causes nitrogen oxides to be produced.
When the combustion temperatures reach 2,200 to 2,500 degrees
Fahrenheit, the nitrogen and oxygen in the air-fuel mixture combine to
form large quantities of nitrogen oxide.
Figure 11.1: Situation of NOx, CO and HC
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11.2
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Catalytic converter
Catalytic converters provide another method of treating exhaust gases. It
is located in the exhaust system between the engine and the muffler. They are
used to convert harmful pollutants such as HC, CO and NO x into harmless
gases. A catalyst is a material that causes a chemical reaction without becoming
part of the reaction process. The catalyst is not chemically changed in the
process.
The catalyst which is used on the catalytic converter depends on the type
of the pollutant being removed. When the exhaust gases are passed through a
coated honeycomb core, the HC and CO react with the oxygen in the air. The
result is a formation of water and carbon dioxide. The metal rhodium is to reduce
NOx into nitrogen and oxygen.
The reaction within the catalyst produces additional heat in the exhaust
system. This additional heat (>1,600 Fahrenheit) is necessary for the catalyst to
operate correctly. Because of these high temperatures, catalytic converters are
made of stainless steel. This shield is used to protect the underbody from
excessive heat. It is important that only unleaded fuel is to be used with a
catalytic converter. Leaded gasoline will destroy the effectiveness of the catalyst
as an emission control device.
Figure 11.2 : Catalytic Converter
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11.2.1 Types of catalytic converter
There are two types of catalytic converter:
a)
Two-way converter
b)
Three-way converter
Both types can employ either a monolith or a pellet design. The pellet
converter consists of two louvered sheet-metal retainers which they called
beads. The monolith converter can have a catalyst made of either ceramic
of metal. The designs are shown in diagrams respectively ( Figure 11.2 )
11.2.1.1
a) Two-way converter
The two-way catalytic converter reduces carbon monoxide and
hydrocarbon particles. It does not reduce any nitrogen oxide emissions.
Here, only platinum and palladium are used as catalysts to reduce
hydrocarbon and carbon monoxide.
11.2.1.2
b) Three-way converter
The three-way converter is designed to reduce nitrogen oxide emissions.
Additional catalyst bed which is coated with platinum and rhodium is used.
The bed not only helps to reduce HC and CO but also lowers the level of
nitrogen oxide emissions. Below is the diagram of a three-way converter.
Figure 11.3 : Converter of emission control
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Activity 11A
This section tests your understanding of the subject matter. You are to
answer the following questions.
11.1
What types of pollution are produced by automobiles ?
11.2
Describe the operation of a catalytic converter
11.3
Explain the purpose and the operation of a catalyst
11.4
What are the two types of converters ?
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Input
This section introduces the subject matter that you are going to learn.
11.3
Positive Crankcase Ventilation (PCV) System
During normal engine operation, a considerable amount of dirty air passes
through the engine crankcase. This air is the result of a process called blow-by.
Blow-by is a product of the combustion process which produces a small
crankcase pressure. The gases from blow-by are very acidic and will erode the
lubricant and metal within the engine.
Picture 11.4:(PCV) valve
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11.3.1 Purpose of PCV System
The purpose of a positive crankcase ventilation (PCV)
system, is to take the vapour produced in the crankcase during the
normal combustion process, and redirecting them into the air/fuel
intake system to be burned during combustion. These vapours
dilute the air/fuel mixture, they have to be carefully controlled and
metered so as not to affect the performance of the engine. This is
the job of the positive crankcase ventilation (PCV) valve.
At idle, when the air/fuel mixture is very critical, just a little of
the vapours are allowed into the intake system. At high speed
when the mixture is less critical and the pressures in the engine are
greater, more of the vapours are allowed into the intake system.
When the valve or the system is clogged, vapours will back up into
the air filter housing or at worst, the excess pressure will push past
seals and create engine oil leaks. If the wrong valve is used or the
system has air leaks, the engine will idle rough, or at worst engine
oil will be sucked out of the engine.
Figure 11.5:A closed PCV system
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11.3.2 Positive Crankcase Ventilation (PCV) System operation
In this system, (Figure 11.5) , any crankcase vapours produced
are directed back into the base of the carburetor to be reburned. This
system is called a closed system, air is drawn through the carburetor air
cleaner assembly, into the engine valve compartment and crankcase.
These vapours are then drawn up through a vacuum-and-springcontrolled ventilating valve (PCV valve) and into the intake manifold. The
vapours are then mixed with the air-fuel mixture
combustion process.
and burned in the
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Activity 11B
This section tests your understanding of the subject matter. You are to
answer the following questions.
11.5
Explain the meaning of “ blow-by” in the engine.
11.6
The PCV system is part of the emission control system, explain the purpose of
this PCV System.
11.7
Label the components in Figure A.11.1
Figure A11.1
11.8
Explain the operation of the PVC System in Figure A11.1.
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Input
This section introduces the subject matter that you are going to learn.
11.4
Exhaust Gas Recirculation System (EGR)
When combustion temperatures are in the range 2,200 to 2,500
o
F,
nitrogen mixes with oxygen and produces oxides of nitrogen (NO x). This type of
emission has a detrimental effect to environment. The method used to reduce
oxides of nitrogen is to cool down the combustion process. This is done by using
an Exhaust Gas Recirculation (EGR) valve. The EGR valve is controlled either
by an engine vacuum or a coolant temperature valve, whereas, ECM unit in
modern automobiles is controlled by a computer.
Picture 11.6: EGR Coolant Temperuture Control
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11.4.1 Purpose of (EGR) Valve
The purpose of the exhaust gas recirculation valve (EGR) valve is
`to meter a small amount of exhaust gas into the intake system, this
dilutes the air/fuel mixture so as to lower the combustion chamber
temperature. Excessive combustion chamber temperature creates oxides
of nitrogen, which is a major pollutant. While the EGR valve is the most
effective method of controlling oxides of nitrogen, its design adversely
affects engine performance.
The engine is not designed to run on exhaust gas. For this reason
the amount of exhaust entering the intake system has to be carefully
monitored and controlled. This is accomplished through a series of
electrical and vacuum switches and the vehicle computer. Since EGR
action reduces performance by diluting the air /fuel mixture, the system
does not allow EGR action when the engine is cold or when the engine
needs full power.
Picture 11.7: EGR system by ECM controlled
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11.4.2 Computer-Controlled EGR.
This system typically consists of EGR valve , EGR valve position
sensor, EGR vacuum valve control solinoid, exhaust back pressure
transducer and computer ( Figure 11.7).
The exhaust back pressure
transducer generates a voltage signal that is sent to the computer. The
EGR valve position sensor also sends a signal to the computer. The
computer controls the EGR vacuum valve control solenoid to regulate the
vacuum applied to the EGR valve. This will control how far the EGR valve
is opened and how much exhaust is allowed to enter the engine. In
operation, part of exhaust gas (usually less than 10%) is sent back
through the intake manifold. The exhaust gases, which considerably
cooler than the combustion temperature, cool down the process of
combustion.
11.5
Air Injection System
The computer system is being used to inject air into the exhaust port of
the cylinder head, exhaust manifold, or the catalytic converter. The system
operates at all times and will bypass air during high speeds and loads an ECM
command. The air management valve performs the bypass or divert function,
and the check valve protects the air pump from damage by preventing a
backflow of exhaust gas.
The air injection system helps to reduce hydrocarbons (HC) and carbon
monoxide (CO) contained in the exhaust gases by injecting air into the exhaust
manifold during cold engine operation ( Figure 11.8). This air injection helps the
catalytic converter to reach a proper operating temperature quickly during
warming-up. When the engine is warm or is in closed-loop mode, the air system
injects air between the beds of a dual-bed catalytic converter and diverts air on
three-way converter to lower the HC and CO in the exhaust.
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Figure 11.8: Air Injection System in Closed-loop operation.
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Activity 11C
This section tests your understanding of the subject matter. You have to
complete the sentences below.
11.9
The EGR system reduces the formation of
in the engine.
11.10 The air injection system adds air to the emission control system to reduce
and
.
11.11 The modern emission control system uses the
EGR valve.
11.12 Label the components in Figure A11.2 below.
Figure A 11.2
.
to control the
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Self –Assessment
Self-assessment evaluates your understanding of each unit.
Question 11-1
Describe the air pollution below which are produced by automobiles
a.
smog
b.
hidrocarbon
c.
Carbon Monoxide
d.
Nitrogen Oxide
Question 11-2
Give two types of designs in the converter and draw a diagram to support your
explanation.
Question 11-3
Explain the purpose of a three-way catalytic converter.
Question 11-4
Explain the operation of PCV System.
Question 11-5
Explain how the EGR System can help to control emission from the engine.
Question 11-6
Explain the operation of Air Injection System.
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