When hydrogen is added to a gasoline fueled spark ignition engine the lean limit of the engine can be extended. Lean running engines are inherently more efficient and have the potential for significantly lower NOx emissions. In the engine concept examined here, supplemental hydrogen is generated on-board the vehicle by diverting a fraction of the gasoline to a plasmatron where a partial oxidation reaction is initiated with an electrical discharge, producing a plasmatron gas containing primarily hydrogen, carbon monoxide, and nitrogen.Two different gas mixtures were used to simulate the plasmatron output. An ideal plasmatron gas (H2, CO, and N2) was used to represent the output of the theoretically best plasmatron. A typical plasmatron gas (H2, CO, N2, and CO2) was used to represent the current output of the plasmatron. A series of hydrogen addition experiments were also performed to quantify the impact of the non-hydrogen components in the plasmatron gas. Various amounts of plasmatron gas were used, ranging from the equivalent of 10%-30% of the gasoline being reformed in the plasmatron.All of the data was compared to a baseline case of the engine operating stoichiometrically on gasoline alone. It was found that the peak net indicated fuel conversion efficiency of the system was increased 12% over the baseline case. In addition, at this peak efficiency point the engine out NOx emissions decreased by 94% (165ppm vs. 2800ppm) while the hydrocarbon emissions decreased by 6%.In the data analysis, the relative air/fuel ratio was found to be an inadequate measure of mixture dilution. Two dilution parameters were defined and used. The Volumetric Dilution Parameter, VDP, represents the heating value per unit volume of the air/fuel mixture. Pumping work reductions due to mixture dilution correlate with VDP. The Thermal Dilution Parameter, TDP, represents the heating value per unit heat capacity of the air/fuel mixture. Combustion and emissions parameters correlate with TDP.