Effects of CO<sub>2</sub> Concentration on Combustion Characteristics of Compressed Biomethane Gas

<div class="section abstract"><div class="htmlview paragraph">One way to decarbonize spark-ignition (SI) engines is to use alternative fuels to improve thermal efficiency. Compressed biomethane gas (CBG), mainly composed of methane and carbon dioxide produced from food waste, has attracted attention as an alternative fuel, but its carbon dioxide content is indeterminate. This study investigates the effects of carbon dioxide content on engine performance (thermal efficiency, etc.), emission characteristics, and turbulent burning velocity using a CBG surrogate fuel mixed with methane and carbon dioxide. A single-cylinder SI engine is used as the test engine, and experiments are conducted under different load conditions with a constant crank angle of 50% mass fraction burned (CA50). Engine performance is analyzed based on heat balance from in-cylinder pressure analysis. Emission characteristics are measured using an emission gas analyzer. Using the boundary conditions obtained from the experiments, parameters such as unstretched laminar burning velocity, flame stretch, and turbulent intensity are calculated numerically to predict the turbulent burning velocity, which is compared with the combustion duration obtained from the experiments. Experimental results show that thermal efficiency increased with an increasing carbon dioxide blending ratio of 0-10%. As for nitrogen oxides in the exhaust gas, the amount of nitrogen oxides emitted is suppressed by a decrease in the cylinder temperature with an increase in the carbon dioxide blending ratio. In addition, the turbulent burning velocity decreased with increasing carbon dioxide blending ratio due to a decrease in the unstretched laminar burning velocity.</div></div>