Flame developments of pilot diesel ignited hydrogen jet in an optical dual direct injection engine

This study aims to characterise the flame development for hydrogen-diesel dual direct injection (H2DDI) in an optically accessible heavy-duty engine through high-speed imaging of the natural combustion luminosity. A single hole, side mounted injector was used to inject H2 at 350 bar in addition to a centrally mounted eight-hole diesel injector providing the ignition source for the H2. The diesel pilot flame was examined without H2 to establish the combustion characteristics of the pilot flame. The pilot fuel energy was reduced from 1200J to 120J while examining the distribution of the pilot flame as the H2 energy fraction was increased. The flame distribution transitioned from an initial quasi-steady diesel flame at peak load (1200 J), to a piston bowl wall-centric flame distribution (840 J) and then to an injector centric flame (120 J). The pilot fuel quantity of 120 J was selected to investigate the ignition process of hydrogen main fuel mixtures supplying 90% energy (only 10% energy from diesel), as it is the minimum to produce repeatable diesel flames. Further, as the ignition source remains close to the diesel injector, decoupled study of the H2 flame development is permitted. The images showed that the ignition of diesel pilot fuel occurs prior to interaction between the two fuels, as the H2 requires time to penetrate to the centre of the cylinder where the diesel pilot flame forms. Prior to ignition, the H2 jet penetrates towards the ignition source whilst it is simultaneously spread clockwise by the swirl motion. Ignition of the H2 occurs after a period of interaction with the burnt products of the diesel pilot. Upon ignition, the H2 flame propagates upstream through the partially premixed H2 mixture and towards the H2 injector. Following the initial flame propagation, the combustion rate reduces as the transition into a diffusion mode occurs, with continued steady reaction zone growth, aided by the swirl motion.​