Our Phase I effort demonstrated that a small air/steam propulsion system could provide immediate power without a boiler, and that power could be varied by varying the ration of water and air in the propellant mix. In this proposed Phase II effort this same system will be scaled to a larger bench version for analysis of performance and efficiency. Following analysis of that data, the system will be scaled further to demonstrate its suitability for use in motive power applications, and in particular, electric hybrid systems for mass transit use. Successful development would lead to a vehicle engine retrofit with a smaller, lighter, and more fuel efficient engine in the space of a current conventional diesel engine. These goals are consistent with the subtopic narrative: Economical and durable technologies and devices for improving safety for riders and transit agency employees, reducing noise and energy consumption, or improving the rider experience. The innovations must be adaptable to existing bus and rail transit vehicles and systems.