Indicated diagrams of a low temperature differential (LTD) Stirling engine (SE) were obtained. The evaluation of LTDSE performance was carried out using the results. The heat source temperatures were 75, 80, 85, 90 and 95 degrees Celsius. The working fluid was air with a mean pressure of atmospheric pressure. The shape of the heat exchangers was flat. The stroke volume of the power piston was 3.9 cc, and the stroke volume of the displacer was 238 cc. The LTDSE did not have any regenerators. The maximum indicated power was 3.34 mW. The polytropic exponents becomes larger than 1.4 before the displacer reached dead center, and the working fluid temperatures fluctuated. These behaviors suggest that the heat exchanger did not work effectively. The evaluation of LTDSE performance was carried out using "the maximum fluctuation of ensemble averaged working fluid temperature". The value is the fluctuation of the internal energy per the heat capacity, and has dimensions of temperature. The obtained values were from 3.2 to 4.7 degrees C. The comparison of these values with actual temperature differences suggests that the indicated work of conventional LTDSE was much lower than the thermodynamic upper limit. (c) 2015 Elsevier Ltd. All rights reserved.