A SIMPLIFIED CONTROL INTRODUCTION Further Nitrogen oxides from motor vehicle exhaust are undesirable in the atmosphere. By absorbing sunlight and initiating the photochemical reaction they become a key component in the formation of photochemical smog. more, one of the oxides, nitrogen dioxide, is inherently toxic at a few parts per million concentration. Nitrogen oxides in vehicle exhaust are a product of the combustion process. Formation occurs during periods of peak flame temperature but decomposition, which would normally occur immediately after, is substantially prevented by the rapid decline of temperature during the expansion and exhaust cycles. Atlantic Richfield has developed a simplified system for reducing the concentration of nitrogen oxides in the exhaust of gasoline powered motor vehicles. This system, designated the Nitric Oxide Reduction (N-O-R) System, represents the culmination of work started in 1960. The reduction of nitrogen oxides is achieved by reducing peak combustion temperatures coupled with a reduction of the oxygen concentration in the air-fuel mixture. This brochure summarizes information concerning the occurrence of nitrogen oxides in motor vehicle exhaust, the control system developed, and the results achieved. NITROGEN OXIDES IN VEHICLE EXHAUST The concentration of nitrogen oxides in vehicle exhaust is extremely variable. Concentrations may be as low as 600 or 700 parts per million (ppm) for some vehicles or as high as 3,000 or 3,500 ppm for others with the average concentration falling somewhere in between. As a matter of information, the average concentration in the exhaust from 124 late model passenger cars approached 2,700 ppm when these vehicles were operated at 50 miles per hour (mph). The concentration in the exhaust from another group of 43 selected vehicles was approximately 2,100 ppm when they were tested using the California cycle. These emission figures are not comparable but they are indicative of the magnitude of nitrogen oxides emissions requiring control. As a point of reference, the California standard for nitrogen oxides in vehicle exhaust is 350 ppm when tested with a defined cycle. The standard was developed by starting with a base line emission rate of 1,000 ppm for uncontrolled vehicles and reducing this figure by 65%. The reason for the variability in the concentration of nitrogen oxides in the exhaust from different vehicles is not clear. It is known that the concentration is a function of carburetion and ignition. For example, enrichening the air-fuel mixture or retarding the ignition system spark advance reduces the concentration. Conversely, leaning the air-fuel mixture or advancing the spark increases emissions. Furthermore, emissions of nitrogen oxides are influenced by the mode of vehicle operation. Emissions are near maximum at high cruising speeds and may be as low as 100 or 150 ppm during idle and deceleration. |