EagleTribune.com, North Andover, MA

March 30, 2014

A primer on automotive emissions

Under the Hood
Brad Bergholdt

---- — I’ve heard you refer many times to the emissions produced by automobiles. Could you please elaborate on them? What emissions are produced, and what I can do to reduce them?

Great question. The four main automobile/light truck emissions are; hydrocarbons (HC), carbon monoxide (CO), oxides of nitrogen (NOx), and carbon dioxide (CO2).

Hydrocarbon emissions are basically unburnt fuel. These can be gasoline vapors escaping the fuel tank, and the results of incomplete combustion. Vehicles built since 1970 attempt to collect, store, and burn such vapors. Vehicles built since 1996 take things quite a bit further with tighter containment, leakage monitoring and on-board refueling vapor recovery. Exhaust hydrocarbons occur when the engine is too cold to insure complete combustion; the air/fuel mixture is incorrect; a compression or mechanical fault occurs; or the ignition system fails to deliver strong spark at exactly the right time. The catalytic converter cleans up HC, CO and NOx, but can’t be expected to work miracles when an upstream fault is severe. Catalytic converters also need to be hot to function properly.

Carbon monoxide is produced when there isn’t sufficient oxygen during the combustion process. This can be caused by something as simple as a dirty air filter, a fuel system fault, or a miscalculation by the engine management system. CO is a dangerous gas, as it is odorless and colorless. You should never operate an internal-combustion engine in an area without sufficient ventilation.

Oxides of nitrogen are produced when the combustion process reaches the upper end of desired temperature. Combustion can be cooled slightly can be done by reinserting inert exhaust gasses and carefully managing ignition timing and valve operation. Post-combustion treatment occurs within the first bed of the catalytic converter. In sunlight, HC and NOx combine to create ground-level ozone, the main component of air pollution. It should be noted that ozone in the upper atmosphere is a good thing, blocking harmful rays of the sun.

An engine that runs perfectly produces mainly CO2, water, unchanged nitrogen and traces of other things. We actually look for high CO2 production as a sign of a healthy engine. CO2 is a greenhouse gas, so we need to find other ways to propel our vehicles if we are to help reduce global warming.

The most important steps you can take to reduce emissions are performing periodic, recommended maintenance; consolidating trips, which cuts down on cold starts; maintaining recommended tire pressure, which ensures safer operation and saves fuel; properly handling and storing gasoline; and promptly resolving the cause of a “check engine” light.

Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose, Calif. Readers may send him email at under-the-hood@earthlink.net; he cannot make personal replies.

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