The current generation of automotive steam engines has demonstrated very low exhaust emissions. For the steam automotive engine to become a viable alternative, future development must now focus on improved economy, lower weight and lower cost. Improved thermodynamic cycles have the potential for superior fuel economy with lower weight and reduced cost components. The positive displacement reciprocating expander performance characteristics are modelled as a function of geometric decisions, operating pressure ratio, breathing loss, friction, and heat loss. The expander model provides a means of estimating net engine performance trade-offs as a function of the thermodynamic cycle and expander design choices. The performance potentials of an improved current cycle steam engine and an advanced reheat cycle steam engine are compared to other engine types on a cruise fuel economy basis.