The amount of Diesel (DI) that can be replaced by Natural Gas (NG) in turbocharged Diesel vehicles converted to dual operation and under urban traffic conditions is determined by the rapid changes of engine loads, which also limits the thermodynamic performance of turbochargers. Turbochargers control the air flow that enters to the engine at every single moment of its operation, and therefore supplies the Oxygen (O 2 ) required for burning the fuels involved in the combustion process. This investigation models the energy consumption of a diesel engine operating in dual fuel mode in urban traffic conditions of Barranquilla, Colombia. This model is based on experimental studies of transient states of Turbocharged Diesel Engines and on recent research relating to the conversion of diesel engines to dual mode. Due to the absence of a standard test cycle for the city, this investigation uses a common driving behavior profile registered in 2006 with an urban bus Chevrolet B-70 with a Caterpillar 3126 Engine. It was determined that the greater replacement percentage was about 85% at maximum load and at cruising speeds, due to the air flow supplied by the compressor. The opposite effect was found at transient states; the absence of air is because of the turbocharger performance when the vehicle is leaving the stand-by condition.