Alternative propulsion architecture systems are described and evaluated from the standpoint of efficiency and potential for use as mechanical energy storage systems in automobiles. Air hybrids are studied and modeling/simulation results are presented along with experimental data from a test bed to assess round-trip efficiencies of such storage systems. A possible architectural scheme is proposed for the use of compressed air for rapid energy recharge. This is followed by an assessment of hydraulic hybrids and flywheel hybrids. These systems are sized for comparable applications. Simulations are then used to objectively compare losses in these systems and to estimate operating round-trip efficiencies in energy storage applications.