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in automotive engineering, means employed to limit the discharge of noxious gases from the internal-combustion engine. There are four main sources of these gases: the engine exhaust, the crankcase, the fuel tank, and the carburetor. The exhaust pipe discharges burned and unburned hydrocarbons, carbon monoxide, oxides of nitrogen and sulfur, and traces of various acids, alcohols, phenols, and heavy metals such as lead. The crankcase is a secondary source of unburned hydrocarbons and, to a lesser extent, carbon monoxide. In the fuel tank and the carburetor, the hydrocarbons that are continually evaporating from the gasoline constitute a minor but not insignificant contributing factor in pollution