The influence of hydrogen injection timing and energy proportion on flame developments in a dual direct injection optical diesel engine

• Variation of H 2 injection timing directly controls combustion mode. • Reduction in H 2 energy proportion produces an increased initial reaction intensity. • Early H 2 injections lengthen diesel pilot ignition delay due to dilution. • Late-stage flame development dominated by H 2 jet momentum. This study shows how flame development of hydrogen-diesel dual direct injection combustion is influenced by changes in two key parameters: hydrogen injection timing and hydrogen/diesel energy ratio. High-speed imaging of the natural combustion luminosity was taken from a heavy-duty optically accessible engine. The engine was modified to include a single hole, side mounted injector for 35 MPa hydrogen direct injection into the combustion chamber. The eight-hole diesel injector remained in the original centrally mounted position, serving as a pilot flame ignition source. The results showed that reduced hydrogen energy share causes an increase in size and intensity of the diesel pilot acting to accelerate the initial combustion reaction, which is not only due to the increased diesel quantity but also the shift in diesel flame distribution. However, the combustion transitions into a near identical mixing-controlled combustion phase regardless of energy share. For hydrogen injection timing variations at fixed 90 % energy share, advanced injection was found to directly impact the hydrogen combustion mode altering the proportion of fuel injected prior to ignition of the diesel flame and the extent of mixing that has occurred. The longer residence time also increases the overlap of the two fuels prior to ignition resulting in a lengthened ignition delay due to dilution of the diesel pilot. The combustion phasing control is however preserved as the reaction was faster with a more premixed hydrogen charge at ignition.