The optical investigation of hydrogen enrichment effects on combustion and soot emission characteristics of CNG/diesel dual-fuel engine

Compressed natural gas-Diesel dual-fuel combustion mode, the partial replacement of diesel fuel with cleaner fuel, is one of the most important strategies to achieve clean efficient combustion. However, low substitutions conditions face unstable combustion and high emission issues. Thus, the current work investigated the effect of hydrogen blending in natural gas on the combustion and soot emission characteristics experimentally in an optical engine. An optical diagnosis study of its performance, combustion flame development, and soot concentration distribution was conducted at various hydrogen volume rates (0%, 30%) and main injection timing (13 degrees CA BTDC, 10 degrees CA BTDC, 5 degrees CA BTDC). In-cylinder flame images were captured by a high-speed camera and further processed to obtain flame characteristics as well as soot distributions. The results showed that the effect of hydrogen blending is mainly reflected in a change of ignition delay period and more ignition points at the initial combustion stage which improves combustion stability. When hydrogen is added, flame propagation speeds up, the flame distribution is more extensive especially in the first stage combustion, the heat release rate and pressure rise rate increase accordingly. Besides, soot emissions considerably decreased in the central region of the cylinder, especially during the first stages of combustion. When the injection advance angle is large, adding hydrogen results in the ignition delay period is extended, more premixed flames appear, and the maximum pressure increases. When that is small, adding hydrogen shortens the ignition delay period, but the poor diesel atomization results in diffusion combustion.