The low efficiency of spark-ignition engines at partial load is mainly due to non-optimum combustion. Investigation of such combustion shows that an initial phase must be distinguished from the development phase. Unacceptable combustions may be of various types such as slow and/or late burning, misfire. An experimental method based on pressure-diagram acquisition and real-time cycle-to-cycle computer processing has been developed to characterize the effects of such combustions on engine-running efficiency and smoothness. The effects of the most significant parameters --ignition, combustion-chamber geometry, mixture turbulence and composition -- have been investigated with various industrial combustion chambers, leading to guidelines for optimization. To further the study of engines, fundamental data on flame initiation have been obtained through laboratory experimentation using laser tomography.