The effect of auxiliary gas injection on diesel engine combustion and emissions was studied using KIVA, a multidimensional computational fluid dynamics code. Auxiliary gas injection (AGI) is the injection of gas, in addition to the fuel injection, directly into the combustion chamber of a diesel engine. The objective of AGI is to influence the diesel combustion via mixing to reduce the emissions of pollutants (soot and NOx). In this study, the accuracy of modeling high-speed gas jets on very coarse computational grids was addressed. KIVA was found to inaccurately resolve jet flows near walls. The cause of this inaccuracy was traced to the RNG k - ϵ turbulence model with the law-of-the-wall boundary condition used by KIVA. By prescribing the length scale near the nozzle exit, excellent agreement between computed and theoretical jet penetration was obtained for a transient jet into a quiescent chamber at various operating conditions. The effect of AGI on diesel engine combustion and emissions was then studied by incorporating the coarse grid gas jet model into a detailed multidimensional simulation of a Caterpillar 3401 heavy-duty diesel engine. The effects of AGI timing, composition, amount, orientation, and location were investigated. The effects of AGI and split fuel injection were also investigated. AGI was found to be effective at reducing soot emissions by increasing mixing within the combustion chamber. AGI of inert gas was found effective at reducing emissions of NOx by depressing the combustion peak temperatures. The use of split fuel injection with AGI was found to be very beneficial for reducing the rapid pressure rise rates seen with single injection, and led to simultaneous NOx and soot reductions. Finally, comparison of AGI simulations with available experimental data were conducted for a TACOM-Labeco engine. The results showed that AGI improved soot oxidation throughout the engine cycle, with a slight NOx penalty.