In-cylinder reactions of several internal combustion engine configurations were investigated using a highspeed four-spectral infrared (IR) digital imaging device. The study was conducted with a greater emphasis on the preflame processes by mutually comparing results from different engine-fuel systems.The main features of the methods employed in the study include that the present multi-spectral IR imaging system permits us to capture progressively changing radiation emitted by new species produced in-cylinder fuel-air mixtures prior to being consumed by the heat-releasing reaction fronts.The study of the Diesel or compression-ignition (CI) engine reactions was performed by varying several parameters, e.g. injection pressures, intake air temperature, fuel air ratio, and the start of injection. They were investigated also for the following mixture preparation-ignition methods: (1) direct-injection of Diesel fuel; (2) homogenous charge (fumigation) ignited by pilot injection and (3) homogeneous charge self-ignition.For the spark-ignition (SI) engine processes, the reactions were investigated using different fuels including gasoline and propane, which were introduced by either the port-fuel-injection unit or carburetor.The sequential images captured of in-cylinder preflame reactions were compared with those of the subsequent combustion reactions. In particular, they are expected to serve as an overall survey of visualizing the preflame processes in the cylinder.