Improvements to the steam engine for the purpose of small scale generation of electricity using biomass fuels and for co-generation of heat and electricity using biomass fuels in both developed and less developed countries are described. The engine is particularly well adapted to co-generation where the thermal load, as in building heating and many process applications, is extremely variable, because of its ability to operate efficiently under partial load. For the same reason, it would be suited to solar generated steam. Experiments have been conducted with steam as the working fluid. The design may in some or all respects be applied to other working fluids.