This document summarizes a study that used computational fluid dynamics (CFD) to simulate diesel engine combustion and predict nitrogen oxide (NOx) emissions. The study modeled a direct injection diesel engine with a 4-hole injector. CFD simulations were performed to analyze in-cylinder flow, temperature distribution, and NOx concentration over time. Simulation results predicted NOx emissions that agreed with experimental engine test measurements to within 3-5%. The study demonstrated CFD can provide insight into combustion features and help optimize engine design to reduce NOx emissions.