Flame Developments of Pilot Diesel Ignited Hydrogen Jet in an Optical Dual Direct Injection Engine

<div class="section abstract"><div class="htmlview paragraph">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 35 MPa in addition to a centrally mounted eight-hole diesel injector providing the ignition source for the H2. Firstly, the diesel pilot flame was examined without H₂ to establish the combustion characteristics of the pilot flame. The pilot fuel energy was reduced from 1200 J to 120 J until the minimum repeatable diesel flame was found, which showed a flame distribution that 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 minimum pilot fuel quantity of 120 J was then used to investigate the ignition process of hydrogen main fuel mixtures supplying 90% energy and only 10% energy from diesel. The images showed three distinct stages of flame development. Firstly, 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 flow. The second stage of flame development commences as the ignition of this H₂ jet occurring 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, <i>i.e.</i> the third stage of the flame development, with continued steady reaction zone growth, aided by the swirl flow. This three-stage ignition and flame development does not change with varied diesel pilot injection timing as evidenced by the flame images with only delayed phasing for later diesel pilot injection timing. However, the diesel pilot flame merges with the newly propagating H₂ flame and thus the later diesel pilot injection timing leads to higher peak flame size and intensity.</div></div>