Out of the alternative fuels for spark ignition (SI) engines, natural gas (NG) is one of the best substitutes for gasoline. To show the full ecologic potential of NG, based on a current turbocharged diesel engine, a so-called "monovalent" SI engine was developed at the University of Pitești. In other words, to properly make use of the NG high-octane number, the prototype engine features an 18.25 compression ratio. Therefore, the compromise usually made to allow the SI engine to run in bi-fuel mode (either fueled with gasoline or with NG) is not present in this case. This being the context, the paper presents experimental results obtained at the engine test bench when using NG. As it is a highly compressed SI engine, gasoline could not be used because of the knocking issues. So, to be able to perform a comparison of the NG effects, liquified petroleum gas (LPG) was also used. The operating point used is 2000 rpm and 2 bar of BMEP. Since during engine testing, the exhaust gas analysis was not available, to obtain information about the chemical species that resulted while burning NG and LPG, a 1D model was developed in AVL Boost, which was calibrated and validated based on the available experimental data issued from the test bed. Thus, this paper presents an analysis of the engine-out emissions while using NG and LPG, showing a significant decrease in NOx and HC for the NG case. [ABSTRACT FROM AUTHOR]