Vehicle emissions are pollutants produced by motor vehicles. There are four basic types of vehicle emissions: hydrocarbons, carbon monoxide, oxides of nitrogen, and particulates. These are important terms that should be understood by the automotive mechanic.


Hydrocarbons (HC)

Hydrocarbons, abbreviated HC, is a form of emission resulting from the release of UNBURNED FUEL into the atmosphere. All petroleum (crude oil) products are made of hydrocarbons (hydrogen and carbon compounds). This includes petrol, diesel fuel, LP-gas, and motor oil.

Hydrocarbon emission can be caused by incomplete combustion or by fuel evaporation. For example, HC is produced when unburned fuel blows out of an “out of tune” engine’s exhaust system.

It can also be caused by fuel vapors escaping from a car’s fuel system.

Hydrocarbon emissions are a hazardous form of air pollution. They can contribute to eye, throat, and lung irritation, other illnesses, and possibly cancer.


Carbon monoxide (CO)

Carbon monoxide, abbreviated CO, is an extremely toxic emission resulting from the release of PARTIALLY BURNED FUEL. It is due to the incomplete combustion of petroleum-based fuel.

Carbon monoxide is a colorless, odorless, but deadly gas. It can cause headaches, nausea, respiratory (breathing) problems, and even death if inhaled in large quantities. CO prevents human blood cells from carrying oxygen to body tissues. Any factor that reduces the amount of oxygen presents during combustion increases carbon monoxide emissions. For example, a rich air-fuel mixture (high ratio of fuel to air) would increase CO. As the mixture is leaned (more air, less fuel), CO emissions

are reduced. See Fig. 31-3.


Oxides of nitrogen (NOx)

Oxides of nitrogen, abbreviated NOx, are emissions produced by extremely HIGH TEMPERATURES during combustion. Air consists of about 80 percent nitrogen and 20 percent oxygen.

With enough heat (above 1000°C) nitrogen and oxygen in the air-fuel mixture combine to form NOx emissions.

Oxides of nitrogen produce the dirty brown color in smog. They also produce ozone in smog, which causes an unpleasant smell and an irritant to your eyes and lungs. NOx is also harmful to many types of plants and rubber products.

Basically, an engine with a high compression ratio, lean fuel mixture, high temperature thermostat, and resulting high combustion heat emits high levels of NOx. This poses a problem. These same factors tend to Improve fuel consumption and reduce HC and CO exhaust emissions. As a result, emission control systems must interact to lower each form of pollution.


Particulates are the solid particles of carbon soot and fuel additives that blowout of a car’s tailpipe.

Carbon particles make up the largest percentage of these emissions. The rest of the particulates consist of lead and other additives sometimes used to make petrol and diesel fuel.

Particulate emissions are a serious problem with diesel engines. You have probably seen a diesel truck or car blowing BLACK SMOKE (particulates) out and NOx emissions.

About 30 percent of all particulate emissions are heavy enough to settle out of the air. The other 70 percent, however, can float in the air for extended periods causing possible health hazards.

Sources of vehicle emissions

Vehicle emissions, as shown in Fig. 31-4, come from three basic sources:

  1. ENGINE CRANKCASE BLOWBY FUMES (20 percent of emissions).
  2. FUEL VAPOURS (20 percent of emissions).
  3. ENGINE EXHAUST GASES (60 percent of emissions).

Various engine modifications and emission control systems are used to reduce air pollution from these sources.


Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.