The Selective Catalytic reduction (SCR) using urea as reducing agent is currently regarded as the most promising after treatment technology in order to comply with strict RDE targets for NOX and particulate in Diesel application. Model-based control strategies are promising to satisfy the demands of high NOX conversion efficiency and low tailpipe ammonia slip. This paper deals with the development of a control oriented model of a Cu-zeolite urea-SCR system for automotive diesel engines. The model is intended to be used for the real-time urea-SCR management, depending on engine NOx emissions and urea storage. In order to ensure suitable computational demand for the on-board implementation, a reduced order one-state model of urea storage has been derived from a quasi-dimensional four-state model of the urea-SCR plant. The model has been developed with the aim of investigating the essential behavior of the system, such as the ammonia coverage ratio dynamics, to realize emission control objectives. Parameters identification and model validation have been performed vs. experimental data measured on engine test bench. The accuracy of the reduced-order model is demonstrated by comparing NO, NO2 and NH3 concentrations with those measured during typical maneuver used to characterize the maps-based models in ECU. It is observed that the model reproduces validation experiments with good accuracy.