Combustion Analysis of Hydrogen-DDF Mode Based on OH* Chemiluminescence Images

Hydrogen–diesel dual-fuel combustion processes were visualized using an optically accessible rapid compression and expansion machine (RCEM). A hydrogen-air mixture was introduced into the combustion chamber, and a pilot injection of diesel fuel was used as the ignition source. A small amount of diesel fuel was injected as the pilot fuel at injection pressures of 40, 80, and 120 MPa using a common rail injection system. The injection amounts of diesel fuel were varied as 3, 6, and 13 mm³. The amount of hydrogen was manipulated by varying the total excess air ratio (λ_total) at 3 and 4. The RCEM was operated at a constant speed of 900 rpm, and the in-cylinder pressure and temperature at the top dead center (TDC) were set as 5 MPa and 700 K, respectively. The combustion processes were visualized via direct photography and hydroxyl (OH*) chemiluminescence photography using a high-speed camera and an image intensifier. The results indicated that the diesel mixture first ignited near the wall of the piston bowl, and the flame spread in the hydrogen–air mixture. It was also found that a reduced λ_total shortened the combustion duration, with an increased tendency of the heat release rate to shift from a dual peak to a single peak.