Flame luminosity fields can nowadays be collected from optically accessible engines, with high spatial and temporal resolution, and constitute a very powerful investigation means for the transient combustion phenomena taking place in the engine chamber. Interpretation of the impressive amount of collected data can be quite challenging, mainly due to the variety of coupled phenomena involved. Application of Independent Component Analysis (ICA) aims here at separating spatial structures related to different combustion events, and is coupled with the analysis of the statistics of the coefficients of the independent components, and of the measured in-cylinder parameters. This paper reports on the comparison of the application of ICA to 2D images of combustion-related luminosity collected from two different optically accessible engines: Diesel and spark ignition. Independent components and their coefficients are first extracted from sets of luminosity images, and then used to identify leading structures and to study the transient behavior of the combustion process. The two components identified from the single Diesel cycle appear to be clearly related to early combustion along the fuel jets and later combustion near the bowl walls, respectively; quantitative analysis of coefficient statistics confirms the lower variability of the jet flames with respect to combustion near the chamber walls. The same can be said of the results of the analysis for SI combustion images, which are separated in early, median and final luminous combustion. The analysis is fast and reliable and can be prospectively applied to many different optical engine configurations.