Understanding and modeling nitrogen oxide emissions from automobiles during hot operation

Modeling tailpipe NOx emissions is difficult due to the complexity of combustion and the catalytic conversion of the exhaust. Additionally, most emissions modeling and testing is from driving conditions that are not necessarily representative of real-world driving. We present an algebraic model for NOx emissions before and after catalytic conversion for an average modern car, utilizing data that resembles real-world driving conditions, including heavy accelerations and air conditioner operation. The key independent variable is fuel rate, which is in turn modeled using the second-by-second speed and other vehicle characteristics. Under conditions when the air conditioner is running, the added accessory load is also modeled.; The air conditioner is a significant load on the engine and affects both fuel economy and emissions. The load is modeled as a function of ambient temperature, and its on-off cycling patterns are characterized. From this, the total fuel rate is calculated which is the basis for the emissions model.; NOx emissions out of the engine is physically modeled under low and moderate power driving conditions. With a 25% increase in fuel rate, there is an unexpected 70% increase in NOx emissions when the air conditioner is engaged. It is likely that the significant low power excess emissions are due to the air added to the combustion without a corresponding increase in engine speed.; The catalyst is modeled under moderate and high power conditions where much of the real-world emissions occur. We find that a measurement of precise combustion conditions is not necessary to predict emissions from these driving modes. Rather, trends are observed based on vehicle accelerations. Under other conditions, it is found that the catalyst performance is related to the rate of exhaust flowing through the converter.; Although tailored for an average car, individual automobiles can be modeled with reasonable success by adjusting the model parameters. However, they can have very different emissions profiles due to many uncontrollable factors. Suggestions are offered for vehicle designers and drivers that may decrease emissions and improve fuel economy by employing existing automobile air conditioners and emissions control technologies in a more efficient manner.