In-cylinder expansion of internal combustion engines based on Diesel or Otto cycles cannot be completely brought down to ambient pressure, causing a 20% theoretical energy loss. Several systems have been implemented to recover and use this energy such as turbocharging, turbomechanical and turbo-electrical compounding, or the implementation of Miller cycles. In all these cases however, the amount of energy recovered is limited allowing the engine to reach an overall efficiency incremental improvement between 4% and 9%. Implementing an adequately designed expander–generator unit could efficiently recover the unexpanded exhaust gas energy and improve efficiency. In this work, the application of the expander–generator unit to a hybrid propulsion vehicle is considered, where the onboard energy storage receives power produced by an expander–generator, which could hence be employed for vehicle propulsion through an electric drivetrain. Starting from these considerations, a simple but effective modeling approach is used to evaluate the energetic potential of a spark-ignition (SI) engine electrically supercharged and equipped with an exhaust gas expander connected to an electric generator. The overall efficiency was compared to a reference turbocharged engine within a hybrid vehicle architecture. It was found that, if adequately recovered, the unexpanded gas energy could reduce engine fuel consumption and related pollutant emissions by 4–12%, depending on overall power output.