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OXIDES OF NITROGEN** |
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| From 1967 through 1972, the emphasis on emission inspection was on hydrocarbons (HC) and carbon monoxide (CO) in exhaust. These gases, the result of incomplete combustion, were identified as pollutants long before the automobile manufacturers were required to take action. The best way to remove them is to make them burn. This was accomplished by raising the temperatures involved and increasing the air intake. This had a detrimental effect, in that Nitrogen (N2), which comprises 78% of air and is usually considered inert, will oxidize at 2,500 degrees Fahrenheit. The combustion temperature in vehicles was raised over this level, and more of these NOx were generated. When HC, NOx, and ultraviolet light from the sun are combined, photochemical smog is created. Two parts HC and one part NOx combined to form three parts smog. If the HC is reduced to one part, but NOx is raised to two parts, the result is still three parts smog. With more cars entering the market the problem gets worse each year. The two major pollutants created are nitrogen dioxide (NO2) and nitric oxide (NO). Nitrogen dioxide is a reddish-brown gas with a sharp pungent odor. NO2 is considered a severe hemotoxin. It combines with the hemoglobin in the blood and causes rapid central nervous system paralysis. It causes pronounced irritation of the respiratory system and lung damage. Nitrogen dioxide is also major cause of acid rain. NO2 combines with the water vapor in the air forming nitric acid. Nitric oxide is a colorless gas that oxidizes to NO2 in the air. The oxidation of nitric oxide to nitrogen dioxide leads to the generation of ozone in the lower atmosphere. Whereas ozone in the upper atmosphere is beneficial since it absorbs the harmful ultraviolet rays from the sun, it is poisonous and, as a result, undesirable in the lower atmosphere. Therefore, the EPA has targeted reducing NOx as the means to reduce ozone in the air around us. Reducing NOx Emissions There are two ways to control NOx production from today’s vehicles: Pre-combustion and post-combustion. Pre-combustion NOx control is the primary method of reducing NOx emissions. Simply stated, it is necessary to keep combustion temperatures low. Methods of doing this include lowering compression, retarding timing, enriching the fuel mixture and EGR flow. Post-combustion NOx control occurs in the catalytic converter. However, not all converters reduce NOx emissions. Only converters with a reducing bed control NOx. This includes dual-bed converters and three-way converters. Post-combustion NOx control is only a secondary method of controlling NOx. Even if the converter is working perfectly, it won’t overcome an engine that’s creating too much NOx. Your primary concern for controlling NOx levels is in controlling how much NOx the engine produces. Several systems can be at the root of elevated NOx production. Some of the systems can be checked rather quickly; others require involved diagnosis. To diagnose and repair a NOx problem cost effectively, perform the following system checks in the order listed: * Ignition Timing - Check the base timing and the timing advance curve. Advanced timing for a given RPM and load can cause pinging. On engines with spark detonation circuits (knock sensors), perform a functional test of the circuit. If the circuit is not working properly, computer timing may be advancing too far. * EGR System - An inoperative EGR system, by not allowing exhaust gas to recirculate into the fresh intake charge, can cause elevated NOx production. Without the recycled exhaust, combustion occurs too rapidly in the cylinder, resulting in NOx. The exhaust gas reduces the temperature of combustion because the exhaust gas is low in oxygen, slowing combustion, and reducing combustion temperature. Check for clogged EGR passages, proper valve operation and EGR valve signal. * Fuel Octane - Using too low of an octane fuel grade can also cause NOx. Low octane fuel burns faster than high octane fuel. In a high compression engine, low octane fuel explodes instead of producing the desired controlled burn. This explosion causes NOx. Try a higher octane blend or a different brand of fuel. * Air Charge Temperature - If the initial charge temperature of the air/fuel mixture in the cylinder is too high, increased NOx emissions will result. During the compression stroke the air/fuel is compressed and the temperature rises to almost the auto-ignition point. If the mixture exceeds the auto-ignition temperature, the mixture self-ignites. Check for high intake air temperature or an overheating engine block. Look for heated air blend door, thermostat and air duct problems. Inspect the cooling system fan, fan clutch, thermostat, and coolant level. * Carbon Buildup - If you have checked for all other causes of NOx, suspect carbon buildup in the combustion chamber. Carbon buildup reduces combustion chamber volume, resulting in higher compression pressure, which can allow the air/fuel mixture to exceed the auto-ignition temperature during compression. Tests for carbon buildup are not always conclusive, so if you suspect carbon buildup, perform a carbon treatment and determine if that cures the NOx problem. Severe carbon buildup can show up during diagnosis as high cranking compression pressure, high per cylinder amperage draw or high starter amperage draw. One of the simple rules of thumb is an engine that pings produces NOx. If you correct the pinging, you correct the problem. Rich fuel mixtures mask an NOx problem. When you lean the mixture to correct high CO emissions, check for an NOx problem. An engine that runs rich often develops carbon buildup, so perform a carbon treatment to solve the problem. The Automotive Technician Training Program (ATTP) offers education on emission control and other topics of interest. To learn more about how ATTP can provide your business with better trained technicians, contact your association office. **Repair Shop and Gasoline Dealers Association News December 1998 |
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