The goal of the emission control system is simple, minimize pollution.
The first emission control device was the positive crankcase ventilation valve, also known as the PCV valve. It was federally mandated in 1963. During the compression stroke a small percentage of the raw air/fuel mixture squeezes between the pitons and the cylinder walls and goes into the crankcase. This raw mixture of air and fuel is very polluting. Before 1963 these gasses were vented into the atmosphere. With a PCV valve, these raw gases are put back into the engine where they are burned. This reduces pollution while marginally increasing gas mileage.
In 1972 the government mandated “Exhaust Gas Recirculation” valves, also know as EGR valves. This valve opens when the engine is warmed-up and turning faster than the idle speed. It allows a predetermined amount of burnt gases to be recycled back into the intake. Over 98% of the exhaust gases are inert since they were ‘burnt’ in a previous combustion event. Nitrous oxide is the main component of “Smog” and is caused by high combustion temperatures. Mixing raw air/fuel with inert gas from the EGR valve decreases the combustion temperatures, preventing (limiting) the production of nitrous oxides.
In 1975 catalytic converters were introduced. Catalytic converters are very effective means to reduce air pollution. When you burn gas, you produce H20 (water), carbon dioxide (C02) and Carbon Monoxide (C0) and some gasoline remains unburnt (HC). H20 and CO2 are desirable results of combustion, but HC and C0 are very polluting and therefore undesirable. The catalytic converter converts CO into CO2 and converts HC into H2O and C02. Catalytic converters can be destroyed instantly with leaded gas. When they were introduced, the gasoline manufacturers also introduced lead free gas.
Oxygen sensors are used to fine tune the air fuel mixture. The ideal mixture is 14.7 parts of air to one part of gasoline. This ratio is called stochiometric. A rich running car means there is too much fuel for the amount of air, and a lean running condition means there is too much air for the amount of gasoline. A very rich or very lean running engine will ruin a catalytic converter. To keep the engine close to stochiometric, manufacturers added sensors to measure the amount of air going into the engine. Knowing how much air is going into the engine, the computer can add the proper amount of gasoline. The Oxygen sensor is a feedback device which tells the computer if the mixture is too lean or too rich. Since the oxygen sensor helps the computer maintain stochiometric mixture it reduces pollution and increases gas mileage.
The EVAP system was introduced around 1996. This system is designed to prevent gas vapors from escaping into the atmosphere. Gasoline in your gas tank naturally vaporizes. In older cars this vapor was vented into the atmosphere. Since 1996 these vapors are collected in a canister filer. When you are moving above a predetermined speed, two valves open up allowing the vapors in the canister filter to vent into the engine where they are burnt. This devise increases gas mileage and decreases pollution.