HAJI, or Hydrogen Assisted Jet Ignition, is an ignition system which uses a hot gaseous jet to initiate and stabilise combustion. HAJI allows a dramatic reduction of cyclic variability, and an extension of the lean limit of the engine to lambda 5. Improvements in cyclic variability lead to increased power output, reduced noise, wear on components and emissions. The ability to operate ultra lean gives 25% improvements in efficiency and extremely low emissions, particularly of NOx.Combustion analysis based on the fractal dimensions of the propagating flame fronts, obtained from optical flame data, support the hypothesis of enhancement of flame speeds through the presence of active chemical species. However, the relative contributions of turbulence and active species to the mechanisms of combustion enhancement realised with HAJI are not well defined.HAJI ignition has also been simulated with a comprehensive three dimensional combustion code, KIVA3. The simulations have been calibrated against flame photographs and heat release data. The results provide evidence of the relative influence of the fluid and chemical mechanisms involved in the HAJI ignition process. The model will also prove useful in optimising future designs of HAJI device.