This article presents numerical and experimental results on the melting process of phase change material (PCM) in a latent heat storage (LHTES) system designed to recover the exhaust waste heat energy of a SI engine. In the LHTES system as PCM, three different paraffin waxes, commercially identified by the codes RT27, RT35, and RT55, were used. A closed-loop liquid circulation system with two heat exchangers was designed with one connected to the exhaust line of the SI engine and the other used in the melting of the PCMs. Water was used as the heat carrier fluid to melt the PCMs with hot exhaust gases. In addition, an experimental study was conducted on a single-cylinder, air-cooled SI engine fueled by gasoline with a stroke volume of 476.5 cm(3), using RT55 in the designed LHTES system. By comparing the experimental results with the analysis results, the validity of the numerical model is ensured. According to the experimental temperature results in the center of the PCM container, the error of the numerical results is approximately 7.8%, while there is a difference of 0.4 units and 2.1% in terms of PCM heat exchanger efficiency and total heat energy stored, respectively. At the end of the analyses (1440th second), performed under the same boundary conditions in the LHTES system design, the storing heat energy is as follows: 1829 kJ by 88.8% liquid fraction for RT27, 1556 kJ by 86% liquid fraction for RT35, and 1843 kJ by 83.9% liquid fraction for RT55. In addition, the PCM heat exchanger efficiency for RT27, RT35, and RT55 is 11%, 9.4%, and 11.1%, respectively.