Small-scale energy conversion devices are being developed for a variety of applications, including portable electric power units for autonomously operated devices (such as an onboard power source for remote sensors, robots, communication stations, and various weapon applications) and propulsion units for MAV (microair vehicle) [1, 2]. Presently, batteries mainly supply this power, though their low energy density significantly limits their energy capacity per weight; the energy supply units thus represent a substantial fraction of the total load carried by the assigned platform. The highly specific energy of hydrocarbon and hydrogen fuels, as compared to other energy storing means (such as batteries, elastic elements, flywheels, pneumatics, and fuel cells) places the combustion engine as the favored candidate. In addition, the specific power (power per mass of unit) of an ICE (internal combustion engine) is much higher than that of a fuel cell, photovoltaic, and battery unit. Figure 1 shows a brief comparison of power and energy per mass basis between the various means. Here, the specific power is related to the maximum possible power produced at the start for a short period of time, while the specific energy is related to a maximum continuous power produced for long period of time (a few minutes and more). Note that the typical heat of combustion of hydrocarbons fuels is about 104 W·h/kg. If the thermal efficiency of a small ICE is around 20%, the specific useful energy of hydrocarbons is around 2×103 W·h/kg, which constitutes the upper specific energy limit for ICE. The lower limit in Figure 1