Analysis of Combustion Process in HVO-Hydrogen Dual-Fuel Operation Using Simultaneous Imaging of OH* and CH* Chemiluminescence

This study aims to clarify the effect of pilot spray ignition position and pilot flame size on combustion in hydrogen dual-fuel operation. For this purpose, the number and diameter of pilot injection nozzle holes, pilot injection quantity, and pilot injection pressure were varied in a rapid compression and expansion machine (RCEM) to visualize flame propagation through simultaneous imaging of OH* and CH* chemiluminescence. The bore and stroke of the RCEM are 102 and 105 mm, respectively. A hydrogen premixture was introduced into the cylinder, and the pilot fuel was directly injected into the cylinder using a common-rail injection system. Four pilot injectors were employed, featuring ten 0.103 mm nozzle holes, eight 0.100 mm nozzle holes, six 0.100 mm nozzle holes, and 0.123 mm nozzle holes, respectively. Hydrotreated vegetable oil (HVO) was utilized as the pilot fuel. The RCEM was operated at a constant speed of 900 rpm with a compression ratio of 15.4. The in-cylinder pressure and temperature at the top dead center were 5.8 MPa and 700 K, respectively. The injection quantities were 3.14 and 6.27 mm3, and the injection pressures were set at 40 and 80 MPa. The total excess air ratio, including HVO and hydrogen, was set at 3.0, with the hydrogen energy ratio approximately 90% under these conditions. The pilot injection timing was adjusted so that the combustion starts at 2 degrees ATDC. Results indicate that the influence of the number and diameter of nozzle holes on the heat release rate is small when the pilot injection quantity is small; however, with the larger pilot injection quantity, reducing the number of nozzle holes results in a lower peak heat release rate and a longer combustion duration. The visualization images provide insights into these differences in heat release rates.