Inert injection is an often-used technique to reduce NOx emissions from engines. Here the effects of a new Mitsubishi water injection system for a direct injection (DI) Diesel engine on exhaust emissions are examined. Stoichiometric flame temperature correlations of thermal NOx emissions for conventional gas turbine combustors provide an activation energy to form NO of approximately 135 kcal/g-mol, the value for the Zeldovich mechanism with O/O2 equilibrium. Two theoretical limiting temperatures determined to bracket NOx emissions data for gas turbines are computed for the Diesel engine considered here. At low water to fuel ratios, the reductions of NOx for the DI Diesel engine are less than predicted for uniform distribution of an inert throughout the charge, but as the water to fuel ratio is increased the reductions are bounded successfully by the limiting temperatures. The measured reductions are also compared with those in another DI Diesel utilizing split injections; water injection at greater than 30% water to fuel ratio by mass via the hybrid injector is found to be the more effective technique to lower NOx emissions.