Understanding and Modeling NOx Emissions from Air Conditioned Automobiles

The emission of excessive quantities of NOx when the automobile air conditioner is turned on has received a fair amount of attention in recent years. Since NOx is a smog precursor, it is important to understand the reasons for this jump in emissions especially on hot sunny days when air conditioner usage is at a maximum. A simple thermodynamic model is used to demonstrate how the torque from a typical air conditioner compressor is mainly related to the ambient temperature. The compressor's on-off cycling patterns are also characterized. Since the compressor significantly loads the engine, it affects fuel economy and emissions. The key independent variable that we employ to represent engine load is fuel rate. The correlations between engine-out NOx emissions and fuel rate are shown for a number of light duty vehicles and trucks. From these, a physical model for engine-out NOx emissions (with and without air conditioning) is presented. The non-linear relationship between engine-out NOx and load does not by itself explain how a 25% increase in fuel rate results in an observed 70% increase in NOx emissions. It is likely that the significant idle and low load excess emissions are due to the added air (and fuel) to the combustion without a corresponding increase in engine speed when the air conditioner is engaged. With slight modifications, the model is simple enough to be used in conjunction with other modal emissions models. Comparisons are made with other modeling efforts such as MOBILE6. Finally, suggestions are offered to vehicle designers which may decrease emissions and improve fuel economy by employing existing automobile air conditioner technology in a more efficient manner.