Although alternative NOx control schemes, such as catalysis, are promising means of reducing emissions from Diesel engines, many such methods have yet to be developed into reliable and cost-effective solutions. Consequently, NOx reduction through in-cylinder techniques remains the most widely used approach in meeting current and future emissions standards. One such common technique is the use of an inert diluent, such as water/steam or exhaust gas recirculation (EGR), introduced into the combustion chamber to reduce the peak flame temperatures associated with NO formation. Here the role of water/steam in reducing NOx emissions is analyzed in depth. In particular, two methods of water injection are studied: stratified fuel-water-fuel injection and intake manifold fumigation.In each case, the NOx emissions are modeled using a two-zone characteristic time model (CTM) based on the dominant physical and chemical subprocesses occurring in the cylinder. The formation of NO occurs in the first zone, which is characterized by the adiabatic stoichiometric flame temperature. The NO formed decomposes to some extent in the second zone, which is defined by the end of combustion temperature and equilibrium composition. The resulting model is algebraically simple and, thus, well suited to both the quantitative and qualitative analyses presented here.The comparison of this model to measured NOx emissions provides insight into the role of inert diluents in reducing NOx emissions from DI Diesel engines. Specifically, the CTM is used to compare the effectiveness of different water distribution techniques. In addition, this same analysis yields a methodology for estimating the fraction of NO decomposed within an engine. Finally, further investigation with the CTM will provide information regarding the fluid mechanic relationship between injection timing and NOx emissions.