Zero-dimensional simulation models are often used to interpret experimental pressure data to better understand combustion processes of internal combustion (IC) engines. This study attempts to improve upon the single-zone model, a type of zero-dimensional model, by using a two-stage model rigorously described in the literature, because computed single-zone temperatures are generally too low for accurate modeling of pollutant gases that are very sensitive to temperature, such as nitric oxide. For example, the thermal kinetic of nitric oxide (NO) formation requires temperatures above 1800 K. Upon implementation of the so-called two-stage model, this study shows calculation improvements compared to a single-zone model. Specifically, NO emission is calculated and compared to experimental results taken from a four-cylinder medium duty diesel engine. By using the two-stage model, soot emissivity is also calculated and its relation with NO formation is discussed.