The article examines the features of the combustion process of hydrogen-containing gas (syngas) in the working cylinder of a spark-ignition internal combustion engine. The main objective of the study is to evaluate the heat release parameters of syngas. To achieve this goal, an experimental investigation was conducted to assess the nature of syngas combustion and establish its regularity. The most significant result is the derivation of dependencies for determining the current value of the combustion characteristic index m and the combustion duration φz in Professor I.I.Vibe's heat release model for spark-ignition engines operating on syngas, with an air excess ratio α ranging from 1.0 to 2.2 and hydrogen content in the fuel composition varying between 30% and 100% by volume. The significance of the obtained results lies in the establishment of the variability of the combustion index m (ranging from 1.6 to 5.5) in Professor I.I.Vibe's semi-empirical heat release model, which more closely corresponds to the actual heat release law observed in experimental studies. The proposed dependencies for determining m and φz allow for accounting the specific features of the syngas combustion process in spark-ignition engines, thereby significantly improving the accuracy of determining the indicated pressure in the working cylinder (the relative root-mean-square error does not exceed 4.5%), which, in turn, enhances the accuracy of assessing the engine's energy and economic parameters. The results of this study can be applied in the design and construction of new spark-ignition engines running on alternative fuel.