The four phases of a spark ignition discharge are reviewed with respect to the physics and chemistry involved. An equation based on the energy exchange at the cathode spot is presented to calculate the arc-to-glow transition. The effects of spark duration in a direct injection engine with a stratified and homogeneous injection strategy are investigated. The optimum spark duration to minimize the COV of IMEP is found to be a function of the engine speed only. The effect of energy deposition at the optimum spark duration has a negligible effect on the COV of IMEP. Plasma chemistry is developed and validated with high temperature equilibrium concentrations. Simulations prove that recombination plays an important role to sustain the high temperature of the plasma when it is cooled down. The recombination permits sustaining a higher temperature and for a longer period of time than flame chemistry only can achieve. Excessive computational time for the chemistry made me simplified the model to one dimension.