Utah County Emissions Practice Test

Oxides of nitrogen (NOx) are typically generated in higher quantities at lean air-fuel ratios, which means that there is more air compared to fuel in the combustion process. As the air-fuel ratio increases, especially beyond the stoichiometric point (approximately 14.7:1 for gasoline), combustion temperatures rise. At a leaner mixture like a ratio of 17:1, the combustion efficiency increases, and the peak combustion temperature also rises, leading to an increased formation of NOx. Hence, a ratio of 17:1 is often associated with the highest production of NOx emissions due to these factors. In contrast, at ratios closer to 12:1 (rich mixture), combustion temperatures are lower, which generally results in lower NOx emissions as there is insufficient oxygen to facilitate the formation of these pollutants. As the ratio increases to 15:1 and beyond, there are reductions in fuel efficiency and increased NOx emissions, but the peak is typically reached around the leaner ratios like 17:1 before diminishing again at even leaner mixtures, such as 20:1.