Abstract: A heat engine system for producing work by expanding a working fluid comprising first and second components, the system comprising, an apparatus for combining the second component of the working fluid as a liquid with the first component, the first component being a gas throughout the system, a compressor for compressing the first component, a pump for compressing at least most of the second component, a heater for heating the first and second components, an expander for expanding the first and second components to produce the work, and a recuperator for transferring at least some of the energy of the working fluid from the outlet of the expander, to the working fluid from the outlet of the apparatus, wherein a substantial portion of the energy transferred in the recuperator is at least a portion of the latent heat of the second component from the outlet of the expander.