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When most people think of automotive-related pollution, they think of only what comes out of the tailpipe.But vehicles can actually emit pollutants three different ways:
  • Gasoline Vapors from the fuel tank and carburetor, or "Evaporative Emissions."
  • Combustion Byproducts and vapors from the engine's crankcase, or "Blowby" emissions.
  • Exhaust Gases produced by combustion or "Tailpipe" emissions. These include unburned hydrocarbons (HC), carbon monoxide (CO), carbon dioxide (CO2), oxides of nitrogen (NOX), water vapor (H2O), particulates (soot or chunks of carbon), various sulfur compounds and other substances.
:The most important factors to exhaust emissions are the Air/Fuel ratio, Ignition timing and advance,and Mechanical considerations.Such as the design of the combustion chamber, camshaft timing, valve duration and overlap, intake manifold design and tempature, engine compression, piston to cylinder wall clearances, the type of valve seals used and their condition.
:When gasoline is burned inside an engine, combustion is never totally complete. There is always a tiny amount of fuel that fails to burn, or is only partly burned. When gasoline is drawn through the carburetor or sprayed out of a fuel injector, it is broken up into tiny droplets.To burn properly, the tiny droplets of fuel must be mixed with oxygen in the ideal ratio, 14.7:1(14.7 pounds of air to 1 gallon of fuel).But air contains only about 21% of oxygen, so it takes a lot of air to provide enough oxygen for combustion.
:The air/fuel mixture is drawn into the combustion chamber, compressed, and ignited by a spark.The air/fuel mixture must be mixed in the correct proportions for the combustion process to be totally complete. Then all the oxygen would combine with all the gasoline to produce heat energy, water vapor, and carbon dioxide.
Too Rich
:An engine needs a richer than usual air/fuel ratio when it is fist started because the fuel does not vaporize very easily in a cold engine.A richer mixture is initially provided by the the choke on a carbureted engine, or a coldstart valve, or increased injector duration on engines with electronic fuel injection. As the engine warms up the mixture is gradually leaned until it is balanced. On late model engines with oxygen sensors, and either feedback carburetion or electronic fuel injection, the engine computer keeps the fuel mixture balanced.
But sometimes things get out of whack.A choke may be stuck or misadjusted. The fuel level inside the carburetor bowl may be to high due to a misadjusted,leaky,or fuel saturated float.There may be excessive fuel pressure or a leaky needle valve that's flooding the carb with fuel.Someone may have replaced the main metering jets in the carb with ones that are to large, or screwed up the idle mixture adjustment screws.A plugged PCV valve or hose can contribute to a rich mixture.On late model engines with electronic feedback carburetors, the carburetor mixture control solenoid may be defective or misadjusted.Or the oxygen sensor may be bad.On a fuel injected engine, a leaky cold start valve can leak extra fuel in to the intake manifold.A bad fuel pressure regulator may be raising the fuel pressure too much.A computer or sensor problem may be throwing the mixture off.Any of these conditions can make the fuel mixture run rich.
When the air/fuel mixture is too rich, it means there is too much fuel and not enough oxygen, or the air/fuel ratio is lower than 14.7:1.There will not be enough oxygen to burn the fuel completely.This results in incomplete combustionand the formation of carbon monoxide(co) in the exhaust.
Too Lean
Some times the air/fuel mixture may be too lean.Any number of things might cause this: a vacuum leak in a engine's vacuum hoses, carburetor, or an intake manifold wich is a common cause.Others include a float level in the carburetor that's too low, jets that are dirty or to small, misadjusted idle mixture screws, and low fuel pressure in a fuel injected engine.Also, obstructions in the fuel line and clogged fuel injectors, or injectors with insufficient flow capacity for the application (wrong injectors).
A lean mixture,too much oxygen and not enough fuel, may create an emissions problem if the air fuel ratio is leaner than about 18:1, or too lean for what the engine needs to handle its present load. Under these circumstances, the mixture may fail to ignite. This condition is known as "lean misfire" and typically cause a rough idle. It can also cause misfiring at high speed cause there's not enough fuel to keep up with the incoming air.If lean misfiring occurs momentarily while accelerating, it can cause a hesitation, stumble or bog, when the throttle opens. This kind of driveability problem is often caused by a weak or plugged accelerator pump in a carburetor. Dirty fuel injectors, a faulty throttle position sensor, or an accumulation of heavy carbon deposits on the intake valves are problems as well.
Lean mixtures may also cause other problems because they elevate combustion temperatures.If things get too hot, the engine may go into preignition and/or detonation and suffer expensive damage, like a burned piston.
Too Hot
Excessively high combustion temperature inside an engine is not a desirable situation. Air is almost 80% nitrogen. Normally, nitrogen is inert and does not doe much of anything. But at temperatures above 2500 degrees F(in the combustion chamber),nitrogen and oxygen combine to form oxides of nitrogen. the abbreviation"NOx" is used to describe the various nitrogen compounds that they form.
The higher the combustion temperature, the greater the tendancy to form NOx. In an engine without emission controls, combustion temperatures can easily exceed 2500 F. Therefore, some means of lowering the temperatures must be used to minimize the formation of NOx. This system is the "Exhaust Gas Recirculation" (EGR) system.

Carbon Monoxide (CO%)

Carbon monoxide is formed when the fuel mixture is rich and there is insufficient oxygen to completely burn all the fuel. The richer the fuel mixture the greater the quantity of CO produced. That makes CO a good diagnostic indicator of incomplete combustion, carburetor maladjustments, clogged air filter, sticking choke, defective heated air intake system, and leaky fuel injectors.
Carbon monoxide emmisions are highest when the engine is first started. The fuel mixture is richer than normal during this time, and the catalytic converter has not yet reached operating temperature. On 1994 and newer cars, the converters are designed to reach operating temperature more quickly to reduce carbon monoxide emissions while the engine is warming up.
Carbon monoxide formation is minimized in the engine by leaning out the mixture as quickly as possible as the engine warms up, then a balanced fuel mixture (wich depends heavily on a good O2 sensor) must be maintained. The carbon monoxide that is formed during combustion is changed into carbon dioxide (CO2) in the catalytic converter. Carbon dioxide is not considered a pollutant, but it is a greenhouse gas that may contribute to global warming.
A well-tuned engine with a good catalytic converter will produce CO levelsin the exhaust that are practically zero % compared to as much as 2% in an engine without a converter.

Hydrocarbons (HCppm)

Hydrocarbon emissions are unburned gasoline and oil vapors. Though not directly harmful, hydrocarbons are a major contributor to the formation of atmospheric smog and ozone.
Elevated HC emissions, are measured in "Parts Per Million" (PPM) with an exhaust analyzer. The elevated HC emissions can be caused by ignition problems (fouled sparkplug or bad plug wire), lean misfire, loss of compression (burned valve orleaky head gasket), or engine wear that causes the engine to burn oil (worn valve guides,seals, and piston rings).
Hydrocarbon formation is minimized by maintaining a balanced air/fuel ratio, making sure the engine has reliable ignition (proper plug gap, clean plugs, good wires and distributor cap/roter). You must also have good compression and close tolerances in the engine piston rings that are fulley seated and seal properly, valve guides and seals that dont leak oil.
The hydrocarbons that do pass into the exhaust are "reburned" in the catalytic converter and transformed into water vapor and carbon dioxide. A late model engine in good running condition, properly tuned with a good converter should produce HC exhaust readings of less than 50 ppm. Compare this to several hundred ppm HC for an engine without a converter.

Oxides Of Nitrogen (NOx)

Nitrogen makes up almost 80% of the atmosphere. Though normally inert and not directly involved in the combustion process itself, flame temeratures above 2500 degrees F cause nitrogen and oxygen to combine. This combination forms various compounds called "oxides of nitrogen" or NOx. This typically occurs when the engine is under load and combustion temperatures soar.
Most of the NOx that comes out of the tailpipe is in the form of nitric oxide (NO), a colorless poisonous gas. It then combines with oxygen in the atmosphere to form nitrogen dioxide (NO2), wich creates a brownish haze in badly polluted areas.
To minimize the formation of NOx in the engine, Exhaust gas recirculation (EGR) is used. Recirculating a small amount of exhaust gas back into the intake manifold to dilute the air/fuel mixture has a "cooling" affect on combustion. This process keeps temperatures below the NOx formation threshold.
On many 1981, and later, engines with computerized engine controls, a special Three way catalytic converter is also used to further reduce NOx in the exhaust. The first chamber of the converter contains a special "reduction" catalyst that breaks NOx down into Oxygen and Nitrogen. The second chamber contains the "oxidation" catalyst that reburns CO and HC.