In this paper, the effect of volume control on the melting process of phase change material (PCM) in a latent heat storage (LHTS) system used for storing the exhaust waste heat energy of a typical SI engine is explored experimentally. In the LHTS system, paraffin wax, commercially known by the code RT55, is used. A closed-loop liquid circulation system with two heat exchangers is designed with one (exhaust heat exchanger-Ex_hex) mounted to the exhaust tailpipe of the SI engine and the other (PCM heat exchanger-PCM_hex) used in the storing and melting of the RT55. The PCM_hex is designed to have a cylindrical geometry and variable internal volume and is fabricated of optical material to monitor the melting process of the RT55. The cylindrical side surface of the PCM_hex is designed as two layers for vacuum insulation application. For volume control, the movable top surface of the cylindrical PCM_hex is controlled by a linear actuator with a 1000 N capacity. Experimental studies are performed under 1/2 throttle opening and 2800 rpm constant engine speed of a propane fueled SI engine for three different PCM_hex arrangements (i.e., free volume, constant volume, and volume controlled) during 7200 s (120 min). As a result, the melting process of the RT55 with the volume controlled PCM_hex is improved, and the energy storage capacity is increased by approximately 4.57%. At the end of 7200 s for 9.136 kg solid RT55, 85.2%, 85.7%, and 87.4% liquid fractions are obtained with the free volume, constant volume, and volume controlled PCM_hex, respectively. With free volume, constant volume, and volume controlled PCM_hex, 2339.9 kJ, 2355.9 kJ, and 2446.8 kJ of energy are stored as sensible and latent heat, respectively, while 37.2%, 38%, and 39.5% PCM_hex efficiency are obtained. In addition, the rate of fuel energy stored in PCM_hex is 3.97%, 4.01%, and 4.17%, respectively.