The internal combustion engine of an automobile generates a lot of pollutants, but the major ones are CO, NOx and unburnt hydrocarbons.
The carbon-monoxide gas is present in the exhaust if the combustion (or burning) of the fuel is not complete, mainly due to insufficient air. It can be fixed in the 3-way catalytic converter, which oxidizes it to CO2. It can also be controlled by use of Exhaust Gas Oxygen Sensor or EGOS, which is located in the exhaust pipe and sends back a signal to the Electronic Control Unit (ECU). This alters the fuel:air ratio and other parameters like the ignition advance angle to let the combustion to be complete.
The second pollutant is the group of nitrogen oxides, which can lead to acid rains. It is also converted in the catalytic converter where it is reduced to Nitrogen gas. NOx emissions can also be curbed by use of what is called the exhaust gas re-circulation system, which pumps back some of the exhaust gas back into the engine. While this may seem to be counter-intuitive, but it does help. What it essentially does is that it acts as inert gas which does not participate in the burning and thus it reduces the temperature of the cylinder. It’s evident that NOx is created at high engine temperatures so, this reduces the formation of NOx.
The third pollutant, which is the unburnt hydrocarbon (UBHC), is the left over fuel, unutilized and is passed on to the exhaust and eventually leaks into the atmosphere. This causes loss of fuel efficiency and reduces the mileage considerably. Its control is very similar to that of CO, just that it is easy to oxidize. The same process is used, like the CO, in the catalytic converter and the end products are water-vapor and carbon-dioxide. Another approach is the use of after-burner, which is like a combustion chamber located just after the exhaust port. It burns up the remaining UBHC and completes the process.
The fourth important pollutant is the carbon particles. It’s the same thing that makes your exhaust gas black in color. It is also responsible for lung diseases and other health issues. It can be prevented from leaking out to the atmosphere by use of filters, which trap them into them and prevent the escape. Later, these need to cleaned or replaced. The filters may be self-cleaning type too, which burn the carbon particles to carbon-dioxide and the filter is ready to be used again.