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IN00521A.gif (1899 bytes)Pollution Control Systems



Controlling the menace:

There is now a growing realization in the society at large about the need to curb the automobile related pollution and though catalytic converter has captured the popular imagination as the panacea for controlling pollution, in reality there are several methods and devices that help control pollution in the automobile. Some of these act independently while others function in tandem with other systems.

Proper maintenance of the vehicle does not mean regularly topping the air, oil and water coolant levels alone. It means getting to understand the function of various components and ensuring they perform the intended purpose. Many a times inexperienced and unqualified mechanics simply disconnect the system or component without realizing its function as it does not interfere with the normal running of the vehicle thereby exposing it to slow but sure damage in the long run. Most of the devices are meant to ensure the correct air/fuel mixture and controlled exhaust emissions in the fall in this category.

In view of this it is pertinent to know the various devices and systems that are incorporated in the automobile and the role they perform. Maintaining them in working condition will help ensure that the vehicle does not exceed its emission limits

Evaporative Emission Control System:

This system is to control non-exhaust pollution originating from evaporation from the fuel tank and the float chamber of the carburetor. Evaporative emissions from the carburetor are higher due to its proximity to heat generated by the engine. The system includes a positive seal fuel tank cap, vapour vent line, a canister containing, canister purge line, vacuum single line and a purge control valve. Fuel vapours from the fuel tank and carburetor float bowl are absorbed in the carbon in the canister. When there is high vacuum in inlet manifold, vacuum single line forces purge-control valve to open and admit fuel vapours from the canister into the intake manifold, to be burnt with incoming charge.

Crankcase ventilation system:

In older model engines, vapour from the crankcase were allowed to escape to the atmosphere through a venting pipe installed on the tappet/rocker-cover/engine-block/crankcase. On all new generation engines a close crank-case ventilation system is adopted. Vapours from the crankcase are routed to the air intake hose through a metering positive crankcase ventilation valve installed on the hose to be burned along with air/fuel mixture. This PCB valve requires replacement at regular intervals of 40-50000 kms.



Exhaust Gas Re-circulation System:

Conditions of high-pressure and high-temperature exist in an engine which are conductive for the formation of oxides of nitrogen. Reduction is one of the conditions i.e. high-pressure or high-temperature, reduces formation of oxides of nitrogen. Exhaust gas recirculation helps reduction in combustion temperature in an engine. Exhaust gas are drawn into intake manifold from the exhaust manifold through an EGR valve. This valve remains closed during idle , as exhaust gases cause rough idling. At full throttle also the valve remains closed as exhaust gas recirculation is not required. Sometimes the device known as back-pressure transducer (BPT valve) is also incorporated in the system. All valves are basically to regulate the EGR to match the varying operating conditions besides bringing down the temperature to reduce the formation of oxides of nitrogen, hydro-carbons which escape through exhaust gases are also burnt with EGR

Spark Timings Control System:

The system is designed to retard full spark-advance except when the car is in high gear. In conventional engines the vacuum in the inlet manifold through a hose is utilised to operate the mechanism in the distributor for the purpose. However, lately the system has been improved to match load conditions as well as the operating temperature, for better results.

Full spark timing is retarded except when the car is in high gear and the engine is at normal operating temperatures. At all other times, the spark advance is retarded to one degree or another. This is achieved by a thermal vacuum valve, a high gear detecting switch and number of hoses. This system is called ' Transmission Controlled System ' (TCS)

In some later model cars, a system which works solely from engine coolant temperature changes has been incorporated. System includes a thermal vacuum valve, a vacuum delay valve and attendant hoses. This system is called 'Spark Timing Control System' (STC). This system performs the same function as TCS i.e., to retard full advance at times when high levels of pollutants would otherwise escape into exhaust gases.

Generally both the systems are trouble free unless wiring or hoses are mishandled or disconnected.

Catalytic Converters:

Catalytic converters are being widely used the world over This system is designed for the oxidation of pollutant gases escaping primary combustion in the engine within the exhaust system. The catalytic converter is a muffler shaped device installed into the exhaust system. The temperature for bulk gas oxidation and reduction of hydro0-carbon gases, carbon mono-oxide and nitric oxide gas is about 600 to 700 C. The temperature of exhaust gases in the exhaust system is lower. The catalytic converter oxidizes and reduces all the three pollutant gases at lower temperature because of catalytic chemical reaction. Catalytic converter becomes effective in the temperature range between 250 to 300 C. The use of catalytic converter has become wide spread as it is an effective means of controlling pollution.

The converter is filled with a monolithic substrate coated with small amounts of platinum and palladium through a catalytic action, a chemical change converts carbon monoxide and hydrocarbon into carbon monoxide and water. Such a converter is called a two-way catalytic converter.

A three-way catalytic converter is installed on cars to check pollution. Such a converter uses thin coating of platinum, palladium and rhodium over a support metal and acts on all three major constituents of exhaust pollution: hydro-carbon, carbon monoxide and oxides of nitrogen, oxidising these to water, carbon-dioxide and free hydrogen and nitrogen respectively.

Necessary oxygen required for catalytic reaction is provide to the converter by air induction system. Air is lead into the through an air-induction pump or by pulsations in the exhaust which cause air section. No regular maintenance is required for the catalytic converter system, except for periodic replacement of the air filters of induction system, is provided. Catalytic converters may also require replacements at about 80,000 kms or more. The catalysts durability is affected by engine durability. Any engine malfunction that will expose the catalysts to excessive amounts of unburnt fuel will severely overheat the catalyst and impair its efficiency and its life.