Experimental Investigations of Methane-Hydrogen Blended Combustion in a Heavy-Duty Optical Diesel Engine Converted to Spark Ignition Operation

Abstract

The global need for de-carbonization and stringent emission regulations are pushing the current engine research toward alternative fuels. Previous studies have shown that the uHC, CO, and CO2 emissions are greatly reduced and brake thermal efficiency increases with an increase in hydrogen concentration in methane-hydrogen blends for the richer mixture compositions. However, the combustion suffers from high NOx emissions. While these trends are well established, there is limited information on a detailed optical study on the effect of air-excess ratio for different methane-hydrogen mixtures. In the present study, experimental investigations of different methane-hydrogen blends between 0 and 100% hydrogen concentration by volume for the air-excess ratio of 1, 1.4, 1.8, and 2.2 were conducted in a heavy-duty optical diesel engine converted to spark-ignition operation. The engine was equipped with a flat-shaped optical piston to allow bottom-view imaging of the combustion chamber. High-speed natural combustion luminosity images were recorded at a frame rate of 7.2 kHz for all cases, together with in-cylinder pressure measurements. Results showed that the increase in hydrogen concentration has shifted the CA50 towards TDC thus increasing the peak combustion pressure. Methane combustion shows the lean limit at lambda 1.4 and extension of the lean limit requires at least 20% of hydrogen addition while maintaining the COV of IMEP below 5%. However, at lambda 1.8 case, 60% of hydrogen enhancement was needed to achieve stable combustion. Overall, with higher hydrogen concentration, there is an improvement in the combustion stability irrespective of the air-excess ratio. Image analysis was performed on the high-speed natural combustion luminosity images to obtain quantitative information on the flame front propagation speed for the tested methane-hydrogen blends. Hydrogen addition results in an increase in flame front propagation speed. When the hydrogen concentration in methane-hydrogen blends is about 50% by volume and more, the flame kernel propagates rapidly at the onset of combustion and decreases, resulting in a shorter combustion duration.