Means to extend the range of cryogen (liquid nitrogen or liquid air) powered automobiles via burning a small amount of fossil fuel (gasoline or liquid methane) have been investigated. By utilizing both an ambient air-heat exchanger to vaporize the cryogen and a fossil fuel-fired superheater to elevate the temperature of the gaseous product, the range of the vehicle can be three times that of an ambient-heated propulsion system while not exceeding current ultra-low emission standards. Internal and external combustion power cycles using either liquid air or nitrogen as the working fluid were found to be more fuel efficient than an internal combustion engine operating on the standard Otto cycle. The fuel-cryogen operating expense for the proposed hybrid propulsion systems was found to be higher than that of the conventional automobile; however, the performance calculations were very conservative. Fossil fuel consumption per mile of dual-fuel systems are projected to be over 40% less than that of conventional gasoline automobiles, thus offering the potential of significant reductions in greenhouse gas and pollutant tail pipe emissions.