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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.