Low and medium temperature thermal energy with temperatures of 100-150 degrees C is available from renewable energy sources such as solar thermal or geothermal energy. Recent progress in flat plate solar thermal collector technology indicates that economical solutions for this temperature range are now becoming possible. Current technologies to generate mechanical energy from this temperature bracket such as Organic Ranldn Cycle machines are however complex, and therefore only economical for larger units. There is a need for a simple, cost-effective medium temperature thermal engine for small-scale applications. Recently, the atmospheric steam engine was re-evaluated for this application. The theory was extended to include a forced expansion stroke. This can increase the theoretical efficiency of the ideal engine from 6.5% to 20%. In order to assess this theory, a series of experiments was conducted at Southampton University. It was found that the isothermal expansion of steam, and its subsequent condensation, is possible. The experiments showed a maximum efficiency of 10.2% for an expansion ratio of 1:4, indicating the validity of the theory. A further increase of efficiency to approximately 17% appears possible. It was concluded that the atmospheric engine with forced expansion has development potential. (C) 2014 Elsevier Ltd. All rights reserved.