The effects of adding ethane or nitrogen on the ignition and combustion of a non-premixed high-pressure methane-air jet have been investigated using fundamental studies in a shock tube and advanced computational modeling. The results are then used to interpret the performance of a pilot-ignited natural gas engine fueled with similar fuels. The results show that the influence of the additives on the gaseous jet ignition process is relatively small, but that they have a greater effect on the research engine, where both fuels have similar influences on the spatial relationship between the gaseous jet and the pilot flame.