Combustion behavior of a spark ignition engine fueled with synthetic gases derived from biogas

Combustion results obtained from a spark ignition engine fueled with two synthetic gases obtained from catalytic decomposition of biogas are presented in this paper. These results are compared with those obtained when the engine was fueled with gasoline, methane and with the biogas from which synthetic gases are extracted. Experimental tests were performed under a wide range of speeds and at three equivalence ratios. Results showed that fractions of hydrogen in synthetic gases increased maximum pressures inside cylinder. Moreover, peak pressures were detected closer to top dead center than methane and biogas. Despite the fraction of diluents in the composition of synthetic gases, high speeds and lean conditions resulted in higher indicated efficiencies than those obtained with gasoline. Moreover, combustion speed and heat release rate were strongly influenced by the proportion of diluents and hydrogen in gaseous blends. CO and CO2 content in the composition of synthetic gases contributed to increase the exhaust concentrations of these pollutants compared with the other fuels, while HC decreased because of the small fraction of methane which remained unburned. Although NOx emissions were mitigated by diluents, like CO2 and air excess, high hydrogen fraction in composition of syngas involved elevated NOx emissions due to the increase in flame temperature that hydrogen produces.