Motor vehicles produce many different pollutants. The principle pollutants of concern — those that have been demonstrated to have significant effects on human, animal, plant, and environmental health and welfare — include:

• Hydrocarbons: this class is made up of unburned or partially burned fuel, and is a major contributor to urban smog, as well as being toxic. They can cause liver damage and even cancer. The regulations regarding hydrocarbons vary according to the engine regulated, as well as the jurisdiction. In some cases, "non-methane hydrocarbons" are regulated, while in other cases, "total hydrocarbons" are regulated. Technology for one application (to meet a non-methane hydrocarbon standard) may not be suitable for use in an application that has to meet a total hydrocarbon standard. Methane is not toxic, but is more difficult to break down in a catalytic converter, so in effect a "non-methane hydrocarbon" standard can be considered to be looser. Since methane is a greenhouse gas, interest is rising in how to eliminate emissions of it.
• Carbon monoxide (CO): a product of incomplete combustion, carbon monoxide reduces the blood's ability to carry oxygen and is dangerous to people with heart disease.
• Nitrogen oxides (NO_x): These are generated when nitrogen in the air reacts with oxygen at the high temperature and pressure inside the engine. NO_x is a precursor to smog and acid rain.
• Carbon dioxide (CO₂): CO₂ is not a pollutant per se, but is a greenhouse gas and so plays a role in global warming. The only way to reduce CO₂ emission is to burn less fuel.
• Particulates — soot or smoke made up of particles in the micrometre size range. Particulate matter causes respiratory health effects in humans and animals.
• Sulphur oxide (SO_x) General term for oxides of sulphur, which are emitted from motor vehicles burning fuel containing a high concentration of sulphur.

Engine efficiency has been steadily improved with improved engine design, more precise ignition timing and electronic ignition, More precise fuel metering, and Computerised engine management.

Advances in engine and vehicle technology continually reduce the toxicity of exhaust leaving the engine, but these alone have generally been proved insufficient to meet emissions goals. Therefore, technologies to detoxify the exhaust are an essential part of emissions control.

Air injection

One of the first exhaust emission control systems is secondary air injection. Originally, this system was used to inject air into the engine's exhaust ports, providing oxygen to burn unburned hydrocarbons in the engine exhaust.

Exhaust Gas Recirculation

Many engines produced after the 1973 model year have an exhaust gas recirculation (EGR) valve between the exhaust and intake manifolds. The valve opens under certain conditions to admit exhaust into the intake tract. Exhaust is largely inert — it neither burns nor supports combustion — so it dilutes the air/fuel charge to reduce peak combustion chamber temperatures. This, in turn, reduces the formation of NO_x.

Catalytic converters

The catalytic converter is a device placed in the exhaust pipe, which converts hydrocarbons, carbon monoxide, and NO_x into less harmful gases using a combination of platinum, palladium and rhodium as catalysts.

In 1966, the first emission test cycle was enacted in the State of California measuring tailpipe emissions in PPM (parts per million). The Environmental Working Group used California ASM emissions data to create an Auto Asthma Index that rates vehicle models based on emissions of hydrocarbons and nitrogen oxides, the chemicals that create smog.

Some cities are also using a technology developed by Dr. Donald Stedman of the University of Denver which uses lasers to detect emissions while vehicles pass by on public roads, thus eliminating the need for owners to go to a test center. Stedman's laser detection of exhaust gases is commonly used in metropolitan areas.

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Automobile Emissions Control"