The aim of this study is to investigate the combustion process and pollutant formation in a small compression ignition engine for quadricycles. The engine is a three-cylinder, 1028 cc, with a CR injection system. It was designed to meet Euro 4 emission standard that is a future regulation for quadricycles. Two optical accesses for endoscopes were realized in the first cylinder to investigate the combustion process. Two-color pyrometry was applied to combustion images to detect both the flame temperature and the soot concentration. The engine ran with diesel fuel and both blended and pure biofuel. Operating condition at 1400 rpm, medium load was tested. A correlation between incylinder data of flame temperature and soot concentration with NOx and PM emissions at exhaust, respectively, was found. It was observed a variation of engine calibration, at fixed power output, when the engine runs with biodiesel resulting in an increase of NOx emissions