Improving the electrical efficiency of gas-fuelled reciprocating engines through waste heat recovery is particularly attractive for distributed power generation, where conventional steam and ORC systems may become economically or technically restrictive. This study presents experimental validation of a prototype closed supercritical carbon dioxide (sCO₂) waste heat recovery system integrated with a reciprocating engine. The sCO₂ loop comprises a rotary-vane compressor and expander, an exhaust-gas heater, a recuperator and a cooler. Industrial-scale tests covered system start-up, continuous nominal-load operation, load reduction to 50%, and shutdown. During steady nominal-load operation, the sCO₂ subsystem delivered up to 42.03 kW of mechanical power at a CO₂ mass flow rate of 0.915 kg/s. Compressor inlet conditions were 75.4 bar and 35.2°C, while the discharge pressure reached 130.27 bar. With gross generator output maintained at approximately 376.9 kW, activation of the sCO₂ system reduced fuel consumption by up to 16.4 Nm³/h from an initial value of 160.9 Nm³/h. At 50% engine load, the sCO₂ subsystem still generated 29.58 kW. The experimental results demonstrate stable integration of a positive-displacement sCO₂ cycle with a reciprocating engine and confirm the potential of this technology for compact waste-heat-to-power applications where conventional turbomachinery may be uneconomic.