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Reducing pollutant emissions of passenger car is one of the main task of manufacturers. One way to reduce emission is to operate SI engine in lean combustion with natural gas. The objectives of this work are, to investigate differences between natural gas and gasoline exhaust emissions and combustion, and then, to show the accuracy of a zero-dimensional two zones thermodynamic model for NOx predictions. Data are acquired in a 4 cylinders bi-fuel engine. With natural gasl, SI engine operates at lower ratio than gasoline leading to a reduce in NOx emission. The thermodynamic model calculates NOx emissions and time burning duration from the cylinder pressure. Typical difference between natural gas and gasoline have been found: With natural gas flame ignition duration decreases and flame propagation duration increases. An improvement of NOx formation model have been developed by locking into account cycle to cycle cylinder pressure in.