Compared with the exhaust-powered supercharging, the secondary expansion could convert the recovered exhaust energy into torque directly without any other energy conversion.However, for an exhaust-powered supercharger, the recovered exhaust energy is used to compress the air that goes into the combustion chamber to increase engine power. Thus, the fuel economic of a gasoline engine with turbocharger is mainly improved by the engine downsizing(3). Moreover, Stobart et al. compared the reciprocating expander and turbine expander, and they indicated that the reciprocating expander is the preferred device for automotive applications for its ability to cater to fluctuating torque and velocity conditions, while the turbine expander is more suitable for converting the energy into electricity[24].