The different approaches to the design of a Stored Energy Automotive Propulsion System are evaluated. As a result, a special kind of Hydraulic System is proposed. This concept uses the reciprocating piston internal combustion engine as the prime mover, but in a radically different form. The essential difference is that the engine crank and rotary shaft are eliminated, and direct hydraulic pumping substituted for the mechanical rotary output. This results in a Free Piston Engine-Pump (FPEP). This paper outlines how the need for an FPEP results from the concept of a Stored Energy Propulsion System. The question of whether the FPEP itself is feasible will be addressed in subsequent papers.