The paper reports on the optical investigation of a multiple spark ignition system carried out in a closed vessel in inert gas, and in an optical access engine in firing condition. The ignition system features a plug-top ignition coil with integrated electronics which is capable of multi-spark discharges (MSD) with short dwell time. First, the ignition system has been characterized in constant ambient conditions, at different pressure levels. The profile of the energy released by the spark and the cumulated value has been determined by measuring the fundamental electrical parameters. A high speed camera has been used to visualize the time evolution of the electric arc discharge to highlight its shape and position variability. The multiple spark system has then been mounted on an optical access engine with port fuel injection (PFI) to study the combustion characteristics in lean conditions with single and multiple discharges. The natural luminosity of the deflagration front was observed by means of the high speed camera, while correspondingly quantitative combustion analyses were performed based on the indicated pressure signal. The impact of the ignition system on the combustion stability is studied focusing on stable, near-limit and unstable operating conditions, as the relative air fuel ratio is increased. The results indicate that a λ extension of about 0.10 can be achieved using the MSD ignition. The benefit is associated with the presence of a non-homogeneous mixture preparation produced by the gasoline PFI fueling strategy. The high speed flame imaging allows to quantify cycle resolved equivalent flame radii as a function of time. The lower cycle-to-cycle variability attained with the MSD on the engine correlates to the higher ignition probability. A lower standard deviation of flame radius growth in the early stage of combustion is detected.