The paper mainly focuses on applying the two-stage combustion system with a pre-chamber into the stationary internal combustion spark ignited engine. It especially concentrates on applying throttle less operation at partial load and reduction of the NOx emission. Considerations conducted in the paper are based on the in-cylinder combustion progress analysis. Additionally, analysis of tailpipe toxic emission, with particular focus on the NOx formation in the engine equipped with the pre-chamber, is also performed. The paper presents both results of 3-D combustion modeling in the SI engine and results conducted on a test SI engine. The 3-D modeling was performed in the KIVA-3V code. Next, results from modeling were compared with results obtained from tests. Finally, satisfactory good consistency between modeled and experimental courses of both pressure, temperature and NOx were obtained. Thus, the engine model with the proposed two-stage combustion system properly simulates engine working conditions on the test bed. Results from both analyses confirmed that the two-stage combustion system significantly shortens combustion duration of an ultra lean gasoline-air mixture and contributes to reduction in NOx. (C) 2013 Elsevier Ltd. All rights reserved.