Many industrial processes and renewable energy sources produce thermal energy with temperatures below 100 degrees C. The cost-effective generation of mechanical energy from this thermal energy still constitutes an engineering problem. The atmospheric steam engine is a very simple machine which employs the steam generated by boiling water at atmospheric pressures. Its main disadvantage is the low. theoretical efficiency of 0.064. In this article, first the theory of the atmospheric steam engine is extended to show that operation for temperatures between 60 degrees C and 100 degrees C is possible although efficiencies are further reduced. Second, the addition of a forced expansion stroke, where the steam volume is increased using external energy, is shown to lead to significantly increased overall efficiencies ranging from 0.084 for a boiler temperature of T-0 = 60 degrees C to 0.25 for T-0 = 100 degrees C. The simplicity of the machine indicates cost-effectiveness. The theoretical work shows that the atmospheric steam engine still has development potential. (C) 2012 Elsevier Ltd. All rights reserved.