Aim
To study and prepare report on the construction details, working principles and operation of the automotive emission / pollution control systems.
Theory
Labeled diagram, construction details, working principle and operation of the automotive emission / pollution control systems
Introduction
There are four areas in the automobile, which can emit pollutants into the atmosphere. These are, the fuel tank, carburetor, the crank case and the exhaust system. The fuel tank and the carburetor emit fuel vapours, the crankcase gives out the partly burnt-air-fuel mixture blown off through the piston rings, while the emissions from the exhaust system include unburnt hydrocarbons, carbon monoxide, nitrogen oxides and sulphur oxides. These exhaust constituents cause many serious diseases as given in table 17.1 Obviously, the atmospheric pollution can be decreased by controlling these areas of the automobile. To do this, two different approaches have been followed:
1 To reduce the formation of pollutants in the emissions by redesigning the engine ventilating system, carburetor and fuel tank. The combustion chamber, alongwith fuel system cooling system, ignition system and the exhaust system are also redesigned thus improving upon the combustion efficiency which reduces the emissions.
2. To destroy the pollutants after these have been formed. The details of both these approaches have been discussed in the following articles.
|
S No |
Constitutent |
Disease |
|
1 |
Carbon Monoxide (CO) |
Heart/Blood circulatory problems |
|
2 |
Hydrocarbons (HC) |
Lung diseases |
|
3 |
Nitrogen oxides (NOx) |
Asthma / Bronchitis |
|
4 |
Lead (Pb) |
Blood Cancer |
|
5 |
Suspended particulate matter (PM) |
Asthma / Bronchitis |
REDUCTION OF FORMATION OF POLLUTANTS
This is achieved by the following:
1. Closed crankcase ventilation
2. Fuel tank and carburetor ventilation
3. Redesigning the engine:
i) combustion chamber
ii) cooling system
iii) fuel supply system
iv) ignition system
CLOSED CRANCKCASE VENTILATION
There are two variations of this system viz. the positive crankcase ventilation and the fixed orifice system. In the former, filtered air from the carburetor air cleaner is introduced in the crankcase from where it carries away the blow by gases and the gasoline vapours enter the engine combustion chamber alongwith fresh charge and are burnt there.
In the fixed orifice system, a fixed orifice in the base of the carburetor takes the place of the PCV valve. The blow by gases are diverted to the inlet manifold through this orifice.
In both these types of closed crankcase ventilation systems, the sludge and the other similar elements during the course of time, clog the PCV valve or the orifice which may decrease and ultimately stop the flow of the blow by gases altogether, which would affect the engine performance very seriously. A coating of oil on the air cleaner filter, apart from various engine troubles, indicates that service of the system is needed urgently, which mainly requires the cleaning of the PCV valve or the orifice, the air cleaner filter hoses etc.
REDESIGNING THE COMBUSTION CHAMBER
To avoid the addition of lead is gasoline, the engine combustion chambers have been redesigned with lower compression ratios (from approximately 10.5 : 1 to 8.5 : 1) This loss of efficiency has been made good by increasing the temperature in the combustion chamber which results in improved combustion of the air fuel mixture and consequently in the decrease of pollutants in the exhaust.
CHANGES IN FUEL SUPPLY SYSTEM
Suitable modifications have been made in the intake manifolds so that rapid vaporization of fuel takes place during warm up of the engine. Further, cross over of the inlet and the exhaust manifolds is also redesigned so as to reduce the time required for the transfer of heat from the exhaust gases to the inlet gases. Moreover, in case of engines with petrol injection in particular, a tuned intake manifold is employed, wherein all runners are of equal length with very little restriction so that equal air flow takes place to all the cylinders. This combination of petrol injection and tuned intake manifold helps reduce emission of HC and NOx.
Apart from increasing the valve overlap discussed above, a better method of diluting the intake charge of the engine with exhaust gas is the exhaust gas recirculation (EGR) system, which reduces NOx emission. Due to absence of oxygen, exhaust gas itself will not burn; it only lowers the combustion temperature thereby reducing the formation of NOx The amount of exhaust gas admitted is regulated by a vacuum controlled valve, called the EGR valve.
The exhaust gases from the engine are passed through catalytic converters. A catalytic converter is a cylindrical unit about the size of a small silencer and is installed into the exhaust system of a vehicle such as a car, scooter, moped, motor cycle or auto rickshaw. It is placed between the exhaust manifold and the silencer. Inside the converter there is a honeycomb structure of a ceramic or metal, which is coated with alumina base materials and thereafter a second coating of precious metals platinum, palladium or rhodium or combinations of the same. This second coating serves as a catalyst. A catalyst is a substance which causes a chemical reaction that normally does not happen in the given condition. As a result of catalytic reactions, as the exhaust gases pass over the converter subtrate, toxic gases such as CO, HC and NOx are converted into harmless CO2 H2 and N2.
There are two types of catalytic converters.
1. A two way converter, which is used to control only CO and HC emissions by oxidation.
2. A three way converter, which is used almost in all cars. It controls CO and HC by oxidation as well as NOx by reduction.
Three way converters (TWC) are now commonly being used for petrol engines and operate in two stages. The first converter stage uses rhodium to reduce the NOx in the exhaust into nitrogen and oxygen. In the second stage, converter platinum or palla dium acts as oxidation catalyst to change HC and CO into harmless water and CO2.
Reference Books
1) Automotive Mechanics – Crouse / Anglin
2) Automobile Engineering – Dr Kirpal Singh
Viva Questions
1 How does an automobile contribute towards pollution of the atmosphere?
2 What is closed crankcase ventilation? Explain.
3 Discuss in details various changes in the fuel supply system of engines for controlling emission?
4 Describe clearly how the engine exhaust gases to convert the pollutants there in into harmless constituents.
5 Name the areas of automobile emitting pollutants and also name the main pollutants in the engine exhaust.
6 What is PCV, and what purpose it serve?
7 What are the two basic approaches to decrease pollution from automobiles?
8 How does the shape of combustion chamber effect the combustion efficiency?
9 What is the advantage of reducing compression ratio of the engine?
10 What is a “Three way converter”? Explain clearly how it is used to convert the pollutants.
11 Explain the function of catalytic converter?
12 How does the fuel injection help to reduce automobile pollution?
13 What is the effect of heating the supply air on the engine emission?
14 What pollutants in the engine exhaust are controlled by the air injection system?
15 What is catalytic converter?
16 What is catalyst?
17 What are the advantages and drawback of catalytic converter method compared to blowing of air only into exhaust manifold.
18 Which catalylist is used for reduction of noise?
19 Which catalyst is employed for oxidizing of HC and Co?
20 What air fuel ratio gives best fuel economy?