New and upcoming exhaust emissions regulations and fuel consumption reduction requirements are forcing the development of innovative and particularly complex intake-engine-exhaust layouts. Especially in the case of Compression Ignition (CI) engines, the HC-CO-NOx-PM after-treatment system is becoming extremely expensive and sophisticated, and the necessity to further reduce engine-out emission levels, without significantly penalizing fuel consumption figures, may lead to the adoption of intricate and challenging intake-exhaust systems configurations. The adoption of both long- and short-route Exhaust Gas Recirculation (EGR) systems is one example of such situation, and the need to precisely measure (or estimate) mass flow rates in the various elements of the gas exchange circuit is one of the consequences. Within this context, the paper presents an innovative solution for real-time estimation of the exhaust gas mass flow rate of a modern Turbo-Diesel Engine, equipped with Variable Geometry Turbine (VGT), Diesel Particulate Filter (DPF), and EGR.The proposed methodology is based on the measurement of standard operating parameters of the UEGO sensor incorporated heater, such as the applied voltage and the sensing tip temperature, which are normally available to the Engine Control Unit (ECU), and on the measurement (or estimation) of exhaust gas pressure and temperature levels, in the same location where the linear oxygen sensor is installed.The model for mass flow rate estimation has been developed starting from the consideration that the UEGO sensing element must be kept at constant temperature for optimal operation, thus allowing the development of physical laws similar to those used for anemometers. As shown in the paper, this approach leads to the determination of physical correlations between calculated convection coefficients and estimated velocity of the gas inside the probe. Dimensional analysis and similarity concepts may then be applied to directly estimate exhaust gas mass flow rate. Finally, the intake air mass flow rate may be evaluated via direct AFR measurement, taking also into consideration dynamic transport delays.Such a model may be identified for any oxygen sensor type and installation layout, and it may be applied both to Spark Ignition (SI) and CI engines, the latter being the test case used for demonstrating the feasibility of the proposed approach.Several experimental tests have been conducted to identify unknown parameters, and to evaluate the model performance. The results presented in this work show that a satisfying accuracy level may be reached on a wide range of flow rate values, and alternative approaches (such as Mass Air Flow sensors based or Speed-Density based) have been considered as benchmarks to evaluate the proposed methodology accuracy.