Predictive modeling of lean hydrogen combustion in directly injected internal combustion engines requires to consider effects that haven't been relevant in conventional systems. The laminar flame speed of hydrogen shows a significant sensitivity on equivalence ratio under lean conditions. Due to fuel stratification effects in direct injection engines the equivalence ratio varies locally in the flame front. To determine and predict flame propagation properly Gamma Technologies has developed different approaches to consider fuel stratification inside the cylinder with different levels of detail. A simplistic approach is used deriving probability density functions of equivalence ratios based on the available time for fuel homogenization. Additionally, a more detailed approach will be presented spatially calculating the flow field in three dimensions, based on the method of stable fluids. The resulting equivalence ratio distribution over time is then used to determine the flame speed in a zero-dimensional two-zone combustion. Besides, the flame front in lean hydrogen combustion is accelerated by thermodiffusive instabilities also for homogeneous mixtures. To take this effect into account in a zero-dimensional combustion model, a phenomenological approach is implemented and the resulting effects confirm the expected trends.