This paper examines the capability of several energy conversion processes to provide sufficient energy in a world where the non-renewable resources are getting depleted.Moreouer pollution caused by them is increasing at a rapid rate. One such efficient and non-polluting means of running the vehicles is the use of liquid nitrogen. To use liquid nitrogen as a non-polluting fuel, a multiple reheat open Rankine and a closed Brayton cycle are used. As a proof а of the principle of using liquid nitrogen as a fuel, an automobile was converted to run on liquid nitrogen in 1997. Earlier work has shown that the energy available by operating various thermodynamic cycles between atmospheric temperatures and liquid nitrogen temperatures can provide more energy per unit mass than is available from current lead acid batteries.