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This paper study the case of a heavy-duty spark ignition engine fueled with LPG, for which it was demonstrated that the thermal effect of the pre-knock reactions in the end-gas occur in the presence of alkenes, one of the commercial LPG main component. In this sense, the engine was operated at full load, with different spark advances generating different levels of knock, which was characterized in terms of angle and intensity. It was developed a classical two zone thermodynamic combustion model for predicting the end-gas pressure and temperature levels, which are cycle-by-cycle variables. It was made the comparison between the cycles with knock and without and it was find that in the knocking cycles case the end-gas temperature is higher, this situation being attributed to the presence of alkenes in the fuel composition.