There are as many types of catalytic converters and particulate traps as there are manufacturers
of these products. In this section, I will attempt merely to describe the functions of these emission- control accessories, quoting liberally from the advertised claims of their  manufacturers.

What are catalytic converters?

        Catalytic converters are designed to provide effective control of environmentally-regulated gases such as HC, CO, and NOx. They chemically convert engine emissions into environmentally- safe gases. This is done by dispersing catalytic materials over a large surface area at an operating temperature between 250 deg. C and 850 deg. C.

        Converters are essentially "afterburners" in that CO and HC are reburned with additional oxygen provided by an air pump or aspirator valve. This process of oxidation results in the conversion of CO and HC into carbon dioxide and water. A special type of converter, the three-way catalytic converter,  also break down oxides of nitrogen (NOx) into nitrogen and oxygen. Sometimes, a converter is integrated into a muffler and is known as a catalytic muffler and is claimed to reduce PM by a minimum of 25%.

        A converter produces a lot of extra heat at the exhaust. So, too much unburned fuel entering  the exhaust system due to a misfiring spark plug, rich fuel mixture, a gasket or valve leak, could cause temperatures to rise and melt the insides of the converter. This can cause partial or complete blockage of the exhaust smoke and damage to the engine. A converter may also overheat due to a dead oxygen sensor, a bad coolant sensor, a stuck-up thermostat, or an air pump/aspirator valve that's not working properly.

What are particulate traps?

        Particulate traps are sometimes known as "soot filters" in that exhaust smoke is passed through
a ceramic or metal filter in honeycomb design, trapping the carbon particles. The process of carbon
collection begins as soon as the engine is started and continues while the engine is operating. When
the temperature of the exhaust is over 375 deg. C for 20% of the duty cycle, the catalyst interacts
with the collected soot to burn the soot into carbon dioxide and water vapor which passes
through  the filter. This process is called regeneration and results in a clean filter. The regeneration process is dependent upon exhaust temperature. While the exhaust temperature does not have to be above 375 deg. C all the time, the more time above this temperature the cleaner the filter will be and the lower the back pressure.

        Most particulate filters on the market today are also  claimed by their manufacturers to reduce CO (up to 99%) and hydrocarbons (up to 96%), in addition to reducing smoke and particulates (up to 99%). Likewise, these filters are often catalysed (that is, contains catalyst chemicals).

        One must remember, though, that particulate filters and catalytic converters work best on diesels running on low-sulfur fuel.