Lean-burn characteristics of a gasoline engine enriched with hydrogen from a plasmatron fuel reformer

Abstract

If a small amount of 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. Hydrogen addition reduces the combustion variability. In this engine concept supplemental hydrogen is generated on-board the vehicle by diverting a small fraction of the gasoline to a plasmatron where it is partially oxidized into a stream containing hydrogen, carbon monoxide, nitrogen, and carbon dioxide. It is then mixed in the intake port with the main fuel/air charge to provide hydrogen enhanced lean operation A series of experiments were performed to study the feasibility of this engine concept. Since the plasmatron is still under development the final composition of the plasmatron gas is not yet known. Therefore, two different bottled gases 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. In addition, a typical plasmatron gas (H2 , CO, N2 , and C0 2) was used to represent the current output of the plasmatron. In addition, a series of hydrogen only addition experiments were 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 converted in the plasmatron. At each of these fractions a sweep of the relative air/fuel ratio was performed, starting at stoichiometic and slowly increasing lambda until the engine began to misfire. At each operating point data was collected to quantify efficiency, emissions, and combustion stability. All of the data was compared to a baseline case of the engine operating stoichiometrically on gasoline only. It was found that the peak net indicated fuel conversion efficiency of the system increased 12% over the baseline case. In addition, at this peak efficiency point the engine out NOx emissions decrease by 94% (165ppm vs. 2800ppm) while the hydrocarbon emissions decreased by 6% (2210ppm vs. 2350ppm). NOx emissions reductions of 99% were possible although they occured at slightly lower overall efficiency points. In the analysis the relative air/fuel ratio was found to be an inadequate measure of mixture dilution. Two new dilution parameters were defined. The Volumetric Dilution Parameter, VDP, represents the heating value per unit volume of the air/fuel mixture. Pumping work reductions due to dilution correlate with VDP. The Thermal Dilution Parameter, TDP, represents the heating value per unit heat capacity of the fuel/air mixture. Combustion and emissions parameters correlate with TDP.