emission control system
emission control system, in automobiles, means employed to limit the discharge of noxious gases from the internal-combustion engine and other components. There are three main sources of these gases: the engine exhaust, the crankcase, and the fuel tank and carburetor. The exhaust pipe discharges burned and unburned hydrocarbons, carbon monoxide, oxides of nitrogen and sulfur, and traces of various acids, alcohols, and phenols. The crankcase is a secondary source of unburned hydrocarbons and, to a lesser extent, carbon monoxide. In the fuel tank and (in older automobiles) the carburetor, hydrocarbons that are continually evaporating from gasoline constitute a minor but not insignificant contributing factor in pollution. A variety of systems for controlling emissions from all these sources have been developed.
In the crankcase—the portion of the engine block below the cylinders where the crankshaft is located—leaked combustion gases are combined with ventilating air and returned to the intake manifold for reburning in the combustion chamber. The device that performs this function is known as the positive crankcase ventilation valve, or PCV valve.
To control exhaust emissions, which are responsible for two-thirds of the total engine pollutants, two types of systems are used: the air-injection system and the exhaust gas recirculation (EGR) system. In EGR a certain portion of exhaust gases are directed back to the cylinder head, where they are combined with the fuel-air mixture and enter the combustion chamber. The recirculated exhaust gases serve to lower the temperature of combustion, a condition that favours lower production of nitrogen oxides as combustion products (though at some loss of engine efficiency). In a typical air-injection system, an engine-driven pump injects air into the exhaust manifold, where the air combines with unburned hydrocarbons and carbon monoxide at a high temperature and, in effect, continues the combustion process. In this way a large percentage of the pollutants that were formerly discharged through the exhaust system are burned (though with no additional generation of power).
Another area for additional combustion is the catalytic converter, consisting of an insulated chamber containing ceramic pellets or a ceramic honeycomb structure coated with a thin layer of metals such as platinum and palladium. As the exhaust gases are passed through the packed beads or the honeycomb, the metals act as catalysts to induce the hydrocarbons, carbon monoxide, and nitrogen oxides in the exhaust to convert to water vapour, carbon dioxide, and nitrogen. These systems are not completely effective: during warm-up the temperatures are so low that emissions cannot be catalyzed. Preheating the catalytic converter is a possible solution to this problem; the high-voltage batteries in hybrid cars, for example, can provide enough power to heat up the converter very quickly.
In the past, gasoline fumes evaporating from the fuel tank and carburetor were vented directly into the atmosphere. Today those emissions are greatly reduced by sealed fuel-tank caps and the so-called evaporative control system, the heart of which is a canister of activated charcoal capable of holding up to 35 percent of its own weight in fuel vapour. In operation, fuel-tank vapours flow from the sealed fuel tank to a vapour separator, which returns raw fuel to the tank and channels fuel vapour through a purge valve to the canister. The canister acts as a storehouse; when the engine is running, the vapours are drawn by the resultant vacuum from the canister, through a filter, and into the combustion chamber, where they are burned.
Improvements in combustion efficiency are effected by computerized control over the whole process of combustion. This control ensures the most efficient operation of the systems described above. In addition, computer-controlled fuel-injection systems ensure more precise air-fuel mixtures, creating greater efficiency in combustion and lower generation of pollutants.