This paper analyzes aprocess of combustion in a spark-ignition engine. The aim of the analysis was to determine the differences in the combustion process between the engine with a classic multipoint injection system and a system which injects fuel directly to the cylinders as well. To aid in the analysis the measurements of the high variable pressure in the engine's cylinder against the crankshaft angle were taken. This allowed us to obtain indicated diagrams for both types of engines. To eliminate distortions, both functions were approximated using combined functions. The other basic parameters were also measured: torque, RPM and instantaneous fuel consumption. The analysis of the indicated diagrams yielded an indicated mean pressure and thermal efficiency for both fuel systems. Then, a comprehensive analysis ofpressures in cylinders was performed in order to determine the way the flame spreads in the cylinder for the two types of engines. The results obtained from this analysis show that the speed of combustion is greater for the charge formed by the dual-injector fuel system. The increased speed of the combustion, especially when it reaches a 50% fraction of the exhaust gas in the cylinder, is what accounts for the increase in the indicated mean pressure and the increase in the thermal efficiency of the engine with a dual-injector fuel system. The time of the spreading of the flame, as well as the fast burn period were reduced. The increased efficiency of the combustion process in the cylinder me ans that the heat losses through the cylinder sleeve are greatly reduced. The results of the research in this paper confirm the purposefulness of using a dual-injector fuel system in a spark-ignition engine.