Spark-ignition engines are characterized by flame initiation near the spark location followed by flame propagation into the engine cylinder. Ansys Forte employs the G-equation model to track the propagation of fully developed, premixed or partially premixed, turbulent flames for this application. When the flame is initiated by the spark, the ignition-kernel flame has a structure that is typically smaller than the average grid size in the computational mesh. During this time, then, the kernel flame front is first tracked by a group of discrete "particles". The calculation switches from this kernel flame model to the G-equation model after the flame structure grows bigger than a characteristic flow length scale. The Kernel Flame model and the G-Equation model are described in this chapter.