Results of experimental studies of the JDC (Jet Dispersion Combustion) method to stimulate burning in SI piston engines are presented. The principle of the method is to disperse combustion gases originated from the homogeneous charge flame kernel throughout whole combustion chamber by means of the gas jet introduced in the chamber from outside. Emphasis is placed on relationships between system parameters and pollutant emissions. Results of exhaust gas composition measurements are presented at various jet directions. In general, at any engine load, the external air injection tends to decrease CO and HC contents in the exhaust gas, and NOx concentration remains basically unchanged. At idling conditions, it was possible to decrease remarkably CO concentration (even below 0.2 % by volume) without increasing HC and NOx levels. The measured exhaust gas composition was strongly dependent on the injection direction. Also, some preliminary tests on the influence of the gas injection on cycle-by-cycle variations are reported.