Hydrogen direct injection: Optical investigation of premixed and jet-guided combustion modes

The classical approach to use hydrogen as a fuel for Internal Combustion Engines (ICEs) is premixed combustion. In order to avoid knocking and to limit NOx emissions, very lean mixtures are employed, thus resulting in a high boost pressure demand or low specific engine power. To overcome these limitations, the possibility of a diesel-like jet-guided combustion of hydrogen is explored. The approach is to ignite a directly injected hydrogen jet at its periphery by means of a conventional spark discharge, followed by a diffusion-controlled combustion while injection remains active. An optically accessible Rapid Compression Expansion Machine (RCEM) is used to investigate ignition and combustion of underexpanded hydrogen jets in air by means of simultaneous Schlieren visualization and OH chemiluminescence. Different injection and ignition timing are investigated, resulting in premixed, partially premixed and diffusion-controlled (jet-guided) combustion conditions. The possibility of ignition and combustion of the hydrogen jets in diffusion-controlled conditions is investigated for different orientations of the incoming fuel jet with respect to spark location. The combustion tests are analyzed in terms of ignition success rate, ignition delay, reacting surface and heat release rate, and an optimal orientation of the jet is assessed. The present study provides insights for optimizing hydrogen direct injection ignition and combustion for later application in ICEs.