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.

A variety of systems for controlling emissions from all sources have been developed. Though the systems vary in detail, they are similar in principle. Crankcase emission controls provide positive crankcase ventilation by recirculating blowby (the leakage of combustion gases), combined with ventilating air, to the intake manifold for reburning in the combustion chamber.

To control exhaust emissions, which are responsible for two-thirds of the total engine pollutants, two types of system are used: the air-injection system and the improved combustion system. In a typical injection system, an engine-driven pump draws air through the carburetor air cleaner and pumps it into the exhaust ports of the cylinder head, in which it combines with the unburned hydrocarbons and carbon monoxide at 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 eliminated. In some cases a specially designed exhaust manifold known as a thermal reactor is used to promote more complete combustion of the gases.

Improvements in combustion efficiency are effected by modifications in the components that control the whole process of combustion. These modifications have involved changes in the shape of the combustion chamber formed by the end of the piston and the cylinder head; a computer-controlled carburetor to ensure more precise and leaner (lower gasoline content) air–fuel mixtures; spark-timing changes to provide a retarded spark (late ignition) when idling; and increases in the idling speed.

Emissions from the fuel tank and carburetor are reduced by the 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, the fuel-tank vapours flow from the sealed fuel tank to a vapour separator that returns the raw fuel to the tank and channels the fuel vapour to the canister. Fuel vapours from the carburetor bowl, which are consumed when the engine is running, flow to the canister via the air cleaner when the engine is stopped. The canister acts as a storehouse; when the engine is running, the vapours are purged from the canister through the air cleaner into the combustion chamber, where they are burned.

Other emission controls include the catalytic converter, consisting of an insulated chamber containing pellets of a variety of metal oxides through which the exhaust gases are passed. The hydrocarbons and carbon monoxide in the exhaust are converted to water vapour and carbon dioxide. These systems are not completely effective: during warmup the temperatures are so low that emissions cannot be catalyzed, and the catalysts are rendered ineffective by the lead compounds sometimes added to gasoline.