This paper reviews a wide scope of unconventional thermal, mechanical, and nuclear systems and devices to determine which, if any, might be suitable within the next 10 years for application as low-pollution-potential power sources for urban vehicles. It also points out which of those found suitable are most promising for such application. It is concluded that of the many types of unconventional non-electrochemical power sources that are even conceptually possible as vehicle drives, only three appear to offer the near-future possibility of supplanting the I.C. engine in many urban automotive applications; Rankine-cycle engines (which includes steam), gas turbines, and Stirling-cycle engines. For general urban automotive application, the Rankine-cycle class, followed closely by the gas turbine, appears to be the most competitive. All of the more exotic types of systems (direct thermal-electric converters, pure mechanical and thermal energy storage, and nuclear systems) will continue to be too big, too heavy, and too expensive to be competitive.