A sensitivity analysis of NOx formation in micro-pilot ignited natural gas dual fuel engines is performed based on a phenomenological combustion model. The model’s NOx formation mechanism incorporates a super-extended Zel’dovich mechanism (up to 43 reactions). The sensitivity analysis compares the contribution of each major reaction to NOx formation, and identifies the rate controlling NOx formation reactions. The formation rates for reactions involving NOx are also investigated to reveal the primary NOx formation paths. Results show that there are two main NOx formation paths both in the packets zone and the burned zone. The rate limiting reactions for the packets zone are identified as:  
O+N2=NO+N
 
N2+HO2=NO+HNO
Rate limiting reactions for the burned zone are:  
N2O+M=N2+O+M
 
N2+HO2=NO+HNO
Since the aforementioned reaction significantly influence the net NOx prediction, it is important that the corresponding reaction rates be determined fairly accurately. Finally, because the quasi-steady-state assumption is commonly used for certain species in NOx modeling, a transient relative error is estimated to evaluate its use. The relative error in NOx prediction with and without this assumption is of the order of 2 percent. Clearly, sensitivity analysis can provide valuable insight into understanding the possible NOx formation pathways in engines and improve the status of current prediction tools to obtain better estimates.
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