The urea NOx selective catalytic reduction (SCR) is an effective technique for the reduction of NOx emitted from diesel engines. Urea spray quality has significant effect on NOx conversion efficiency. Air less injection is one of effective, less complex way of injecting urea spray into the Exhaust stream. Further with air less injection it become more challenging in an engine platform of ~3 to 4L where Exhaust mass flow and temperature are relatively less. The droplet diameter and velocity distribution of De-Nox system has taken as input along with Engine raw emission data for a numerical model. The atomization and evaporation of airless urea injection systems were modeled using computational fluid dynamics. The numerical model was validated by the experimental results. The numerical results under the different boundary conditions and injection positions of nozzle (includes Injection angle, offset from mixer and design of dozing adaptor) were compared in order to optimize the spray characteristics and avoid any chance of crystallization on mixer and adaptor housing. The complete system has validated on Engine test bed with specific test pertaining to crystallization. It also improved the overall Nox conversion efficiency of the system.